View Full Version : Intuitive content of Loop Gravity--Rovelli's program
In the "perche i nostri discorsi" thread, selfAdjoint gave a concise sketch of the direction that Carlo Rovelli sees in loop gravity
Originally posted by selfAdjoint
Rovelli's vision of how to move forward beyond present day physics boils down to this. Couple the standard model to the set of quantized QG loops so there will be interactions with eigenvalues of length and eigenvalues of momentum, etc. Combined states. Work on the theory non-perturbatively in that way. No infinity problems because no "classical points" - same advantage stringy physics gives, but with perhaps a more basic underpinning. The point for me is, if you could do this you could calculate numbers for the accelerators. The advantage over normal analysis would be non-perturbative calculations without infinities. The advantage over lattice would be, well it's proposed as real physics, no continuum limit required.
...
A few posts later in the thread I suggested trying to map out an intuitive description of what Loop Gravity is basically about and what makes its approach different. Perhaps it would be good to make a separate thread just to do that, without controversy as to comparative merits or any other distractions. Here is my post from the other thread proposing to do that:
-----------------------------------
Probably what we need most now is an intuitive sketch of what makes Loop Gravity different from your typical field theory----how is the backgroundlessness implemented?
Things are coming to a head. Rovelli's new book "Quantum Gravity" is major and has IMO material for a bunch of PhD dissertations just expanding on details. It also contains the "Dialog" as a final chapter. You can connect the points made about loop gravity in the Dialog to chapters and sections in the main part of the book. Also Smolin's April 2003 paper lays out what has been accomplished and what remains to do and what the prospects are for getting loop gravity tested---it is a thorough review and comparison: "How far are we from a quantum theory of gravity?"
Plus we have good accounts of loop cosmology by Bojowald
like the recent paper "Quantum Gravity and the Big Bang", and in some of Ashtekar's papers. It appears progress in cosmology has been dramatic of late. New researchers have been getting into LQG at the level of cosmology.
Plus there is this month's Berlin symposium "Strings meets Loops" which will probably generate a series of overview talks
aimed at wider audience----e.g. another cosmology overview by Bojowald, another full theory overview by Ashtekar, a spin foam overview by Rovelli, and so on.
So there is more and more accessible information than there was a year ago, about loop gravity. It looks to me as if new research possibilities are coming into focus. For example, these days I keep seeing papers about the "low energy limit" or "semi-classical limit", another place where newcomers are getting in (like those Argentine people this month---Kozameh, Gleiser, Parisi)
It seems to me to be a good time to try to say what loop gravity is about, in the simplest possible way. I am apt to make several false starts on this. If someone else has been thinking about it and wants to try, go ahead....
----------------------------------
was practically laughing just now thinking of my trying to give an intuitive explanation of loop gravity. Meteor has a copy of Three Roads, which probably has a perfectly good one, and I am too lazy to go down to the library and get a copy---never seen it.
so meteor or selfAdjoint are probably better equiped to do a sketch of loop gravity
BTW I believe that to give a good conceptual description of something often requires deeper understanding, paradoxically, than to descrbe it technically, so it is a place where one's shortcomings become evident HOWever here goes
I told you before I would make several false starts, it is inevitable, so lets get started
in loop gravity the excitations of space (or geometry or gravity allee same bizness weiss da) are polymers----essentially ball-and-stick models like of very large protein molecules
so the first analogy is a drumhead with sand sprinkled on it. you know that every different way it can vibrate is shown by the lines that appear in the sand when you excite---I hope you did this at the science museum as a kid and know what I mean: nice diagrams of lines appear on the surface and these diagrams CATALOG all the modes of vibration or the excitation modes.
the next analogy is a sink full of soapsuds---you had to wash the dishes as a kid and you can imagine the whole sink or the whole universe (whats the difference) full of soapsuds. Now in the middle of every bubble put a dot, and if one bubble contacts another bubble then connect those two dots with a line.
Now you have a network (a ball-and-stick molecule, a polymer) that fills the whole sink or universe. And we are going to give a number Q to each point (or ball, or dot, or vertex) in the network.
And a number P to each connecting line.
Whatever for? Why label each point and line with a number? (Roger Penrose thought of doing it, he is the one to ask about it) Well, using that additional information the network can tell us the VOLUME of any region----just add up the numbers attached to each point in the region, somehow-----and the AREA of any surface---just add up the numbers, in some fashion, that are attached to every line that punctures the surface.
Furthermore, if this labeled network's sticks become flexy the whole thing can be squashed flat and stuffed into your dresser drawer where you ordinarily keep undershirts.
So here is a thing which you can squash any shape and store anywhere which nevertheless tells you everything about the geometry of the universe, or the kitchen sink I forget which.
Yes, I have a copy of the book, and in it a spin network is defined as a directed graph, jointly with a series of rules that guide the evolution of the graph. To each edge of the graph is assigned a number, and the area of a surface depends on the value of the numbers of the edges that punctures the surface.Btw, the area of a surface can be computed with a formula that includes the Immirzi parameter. The volume of an object is proportional to the number of nodes of the graph thatr are inside the object.
However, in various documents on Arxiv, i've found that the edges of the graph are labeled with group representations of Lie groups, and the vertices with intertwining operators(damned if i now what's an intertwining operator!)I don't know if these spin networks are the same that the cited in the book of Smolin
Spin networks are used in the canonical approach to quantum gravity, but another approach, the sum-over-histories approach, has adopted a particular version of them, called spin foams, that are cell complexes.
selfAdjoint
Oct18-03, 10:20 AM
I don't know if these spin networks are the same that the cited in the book of Smolin
Yes they are the same. Smolin said the edges were labelled by a number in order not to confuse his readers. The closer statement would be the edges carry a spinor. This spinor is not just a label, but a genuine bit of physics. The intertwiner functions are like black boxes - deterministically relating spin reps in to spin reps out. These again are physics, somewhat like Heisenbeg's S-matrix relating momenta in to momenta out.
Originally posted by selfAdjoint
Yes they are the same. Smolin said the edges were labelled by a number in order not to confuse his readers. The closer statement would be the edges carry a spinor. This spinor is not just a label, but a genuine bit of physics. The intertwiner functions are like black boxes - deterministically relating spin reps in to spin reps out. These again are physics, somewhat like Heisenbeg's S-matrix relating momenta in to momenta out.
I'm sure you are right---what a spin network is in the literature must be the same as what Smolin describes in his book (allowing for whatever minor naming conventions differ) but I should emphasize that the picture I gave here of a "spin network" (*NOT*) bears only a faint resemblance to what defines a basis for the quantum states of geometry in the theory. Im still trying to see how to introduce the ideas in as intuitive and non-technical way possible----I may have to break down and see how Smolin did it in "Three Roads".
here's another piece of the jigsaw puzzle
in classical mechanics things move along trajectories---curved paths parametrized by time---and when you quantize the trajectories go away.
the curved paths things travel along dont exist any more, you have to erase the trajectories (or in Feynmann sum over histories you "integrate" all possible ways of getting from here to there---in any case the clear picture of a path loses reality and dissipates)
in GR, the 4D manifold is not a real thing (individual points are not events and have no physical meaning) because of gauge invariance any point will do----to define an event you need matter, like the event that two particles cross paths (at some point, but which point doesnt make any difference it is an arbitrary choice.
arbitrary choices needed to express something mathematically are called "gauge"----extra physically meaningless information that gets unavoidably mixed in as part of keeping the books.
in GR the 4D manifold is there so that you can write down the trajectory of the gravitational field. GR does not suggest that 4D spacetime exists, it is a mathematical amenity used for defining evolution of the gravitational field and the matter that goes into shaping the field.
but when GR is quantized, the trajectory goes away and one simply has a 3Dmanifold, with a space of all possible geometries defined on it
again, as in classical case, the points of the 3D manifold have no physical meaning---they are just "gauge". One can define surfaces and volumes only using matter---Rovelli and Ashtekar both use examples like by a surface I mean for instance the top of this desk.
The quantum states are functions defined on the space of all possible geometries that the 3D manifold can have. Analogous to quantizing a particle moving on the line by "wave functions" defined on the line.
The curious thing is that no one started out thinking of the "wave functions" defined on the space of geometries as spinnetworks! Nobody was looking for spin networks or asking for them! It just turned out that they appeared as the best way to CATALOG the functions defined on the space of all possible gravitational fields or all possible geometries.
At first they tried defining these functions using loops and they got a hilbertspace of loop-functions, but they couldnt get an orthogonal basis: the loops were too redundant. So they eventually borrowed spinnetworks from Penrose and they turned out to give an orthogonal basis for the space.
Also I even believe that the basis is countable and the hilbertspace is separable----technical conveniences to be sure.
So when I mentioned this polymer network thing that describes the geometry of the whole universe, but that you can stuff into the dresser drawer, it is a quantum state of geometry (a functional like a wavefunction defined on the collection of all classical geometries) and all quantum states look like this or combinations of things like this.
and matter fields must be defined on things like this
and the quantum state can evolve! At noon by some clock it can be this one in the top drawer and then at one o'clock it can be like this other one in the bottom drawer.
But it is a disconnected hopping, and there is no absolute clock you just have to choose some physical PROCESS (essentially something involving matter) to serve as a clock. This clock is part of the world and there is a correlation between what you observe the clock says and where you observe the pendulum is, or how far away the galaxies are, or whatever else. There is no one absolute time that drives the rest only correlations between different processes
processes which include, among other things, the change in the state of the 3D geometry of space.
So, when you check things out using cosmology, the evolution equation is a finite DIFFERENCE equation! It is not a differential equation. when you do loop cosmology the Friedmann equation that all cosmologists depend on becomes a step by step difference equation----e.g. Bojowald uses the scalefactor as his clock, there must be some physical process to use as clock, and correlates other stuff like curvature and inflation and density with the scalefactor. And so does everybody else that has been doing loop cosmology that I have seen. For example "Quantum Gravity and the Big Bang" the talk Bojowald gave recently.
it is interesting how the concept of time changes.
In Rovelli's textbook "Quantum Gravity" there is a philosophical section on time which I found really interesting---he finds that different branches of physics use ideas of time that are actually different from each other and also from everyday vernacular time.
he is able to distinguish around 8 or 9 different ideas of time.
Quantizing General Relativity seems to exert a strong influence on the ideas of time because both QM and GR bring insights about time which, if you try to put them together, produce something that seems radically new.
(of course one can avoid having to think about it if one throws out GR and replaces it with a lobotomized form or if one is very careful to only use GR and QM in separate situations and never together on the same problem)
exciting business
ranyart
Oct19-03, 05:53 AM
Marcus, there are a vast number of papers that continue to reveal a discrete direction, I am really glad that you take the time to post the most interesting idea's from many fields.
You may have this link allready?..but if so others may find it interesting:http://uk.arxiv.org/abs/gr-qc?0306059
Rovelli for me seems to be an architect of new thinking.
Originally posted by ranyart
Marcus, there are a vast number of papers that continue to reveal a discrete direction, I am really glad that you take the time to post the most interesting idea's from many fields.
You may have this link allready?..but if so others may find it interesting:http://uk.arxiv.org/abs/gr-qc/0306059
Rovelli for me seems to be an architect of new thinking.
thanks for calling attention to that paper, Ranyart. For some reason I had just glanced at it earlier. He probably is.
http://arxiv.org/abs/gr-qc/0306059
I will quote the last 5 sentences in (the conclusions part of) this paper and try to say why I think it is interesting
"...We have studied the propagator of our model in detail. We have shown that in the semiclassical limit it has a simple relation with the Hamilton function of the classical theory, but this relation is not a simple exponential, as one might have expected.
Instead, the propagator is real. It is the sum of two exponential
terms complex conjugate to each other, that propagate backward and forward, respectively, along the motions. Accordingly, the physical Hilbert space splits between forward and backward propagating states.
We expect this structure to be the same in quantum general relativity."
this is the kind of simple example (a system like a springbob with only a couple of degrees of freedom) that physics teachers love to use when introducing a new method---try the new approach out on the simplest thing in sight: an harmonic oscillator, a single particle in a potential well, whatever. Then the maths do not obscure the ideas.
So this is Daniele Colosi (a grad student at Marseille) and Rovelli having fun with a toy that moves in a simple ellipse. Is this your reading too? I just looked at the paper. I like it. Maybe we should make a thread about this paper or just look at it in this thread
Originally posted by marcus
thanks for calling attention to that paper, Ranyart....
http://arxiv.org/abs/gr-qc/0306059
...should make a thread about this paper or just look at it in this thread
your bringing up this paper got me looking at this and several related ones over the past day or so.
A Simple Background Independent Hamiltonian Quantum Model (Colosi/Rovelli)
Minkowski Vacuum in Background Independent Quantum Gravity
(Conrady, Doplicher, Oeckl, Rovelli, Testa)
and several related 2003 spin foam papers
recent work involving the hamiltonian seems to connect (in ways I didnt anticipate and dont fully understand) to recent spin foam work
there is the fact that in August 2002 John Baez and some others posted computer results that some spin foam numbers were not what some people expected them to be----this seems to have lead to increased interest in spin foams: something new to understand about them----several new papers with new ideas
then there is the fact that at this months symposium it is Rovelli who is talking about spin foams (and he and his associates have recently, in late 2002 and in 2003) put out several papers on spin foams
then there is the fact that several of these recent papers link up the hamiltonian and spin foam approaches----they are or seem to be trying to cure problems in both the hamiltonian and discover how to use spin foam models properly in a way that suggests some underground connection between the two
i had till now not paid attention much to spin foam quantum gravity but now because of these little hints Ive been reading in the past day or so, and because Rovelli has chosen to do the spinfoam presentation, I am beginning to pay more attention and trying to understand a little better.
BTW at the symposium the loop lineup looks like this
Ashtekar: quantum geometry and applications (this means overview and application to big bang, inflation, black holes...)
Bojowald: loop quantum cosmology (a strong run of results by him and about 10 other researchers over past 3 years, giving guidance to development of the full theory by testdriving in the cosmology case)
Rovelli: spin foams (this is the one that I cannot antipate, it will have unexpected things)
Lewandowski: the hamiltonian (this presumably will be profoundly analytic/algebraic as is the way with people from Warsaw. maybe selfAdjoint will help us understand this one [:)],
it has now been 5 years since Lewandowski found fault with Thiemann's hamiltonian and there has been a great deal of work involving hamiltonians since then! Perhaps L will summarize some of this. As befits a growing theory, the main issue here remains unresoved and people are still discovering how it should look, as for example in the paper you gave the link to )
This is merely by way of saying thanks for the link to that paper. It has given me something to do during spare moments for the past day or so
Yes they are the same. Smolin said the edges were labelled by a number in order not to confuse his readers. The closer statement would be the edges carry a spinor. This spinor is not just a label, but a genuine bit of physics. The intertwiner functions are like black boxes - deterministically relating spin reps in to spin reps out. These again are physics, somewhat like Heisenbeg's S-matrix relating momenta in to momenta out.
I've reading the paper where spin networks where introduced in the first time, "Spin networks and quantum gravity"
http://arxiv.org/abs/gr-qc/9505006
In this paper spin networks are defined like trivalent graphs with edges labeled by positive integers.A trivalent graph is a regular graph with 3 edges arriving at each node. The spin network has to follow 2 rules:
-The sum of the 3 edges that converge at a given node has to be an even number
-Each of these 3 edges can't be superior to the sum of the other 2
Do somebody know the paper where the labels passed from being numbers to group representations? Do the Lie groups have to be some specific Lie group? Are actually spin networks continued to be defined as trivalents graphs? Must the group representations be irreducible representations?
Ok, Ok, very much questions but this is interesting stuff
selfAdjoint
Oct20-03, 02:31 PM
I'm not sure, but I think gr-qc/9707010 (http://xxx.lanl.gov/abs/gr-qc/9707010) is early. See also Baez's TWF #110.
Originally posted by meteor
Do somebody know the paper where the labels passed from being numbers to group representations? Do the Lie groups have to be some specific Lie group? Are actually spin networks continued to be defined as trivalents graphs? Must the group representations be irreducible representations?
Ok, Ok, very much questions but this is interesting stuff
Meteor I hope my replying does not preclude a PF mentor or other knowledgeable person responding.
to say irreducible representation of SU2 is sort of like saying "spin" because there is one for each dimension and so roughly speaking one for each integer (or half integer if you divide each integer by two according to the quaint ancient custom of physicists)
the papers where Penrose made up "spin network" idea are not online!
however I have read about these papers and my understanding is that ALREADY AT THE BEGINNING penrose thought of the graph as labeled by "spins" that is to say either halfintegers or, what is the same, irreducible reps of SU2
as an interesting insight into human, or at least Penrose, nature, he regularly FLIPFLOPPED at the beginning between having the labels be integers which he called "colours" and dividing them all by two and calling them "spins". As a civilized mathematician he wanted to call them colours but as a savage physicist driven by brute instinct and prejudice he needed to divide them by two---as is the custom---and call them spins.
so this ambiguity of labeling has been there from the start
remember also that as children, while others are taught to skip rope and play hopscotch, physicists are taught that SU2 is the "double cover" of the rotation group, which is why an electron can turn around 720 degrees before it looks normal again. the first time it turns around it appears to have pointed teeth and is wearing a Count Dracula costume but then it turns around another 360 degrees and is its old self. But doubtless you know all this already!
ranyart
Oct20-03, 03:18 PM
Originally posted by marcus
so this ambiguity of labeling has been there from the start
remember also that as children, while others are taught to skip rope and play hopscotch, physicists are taught that SU2 is the "double cover" of the rotation group, which is why an electron can turn around 720 degrees before it looks normal again. the first time it turns around it appears to have pointed teeth and is wearing a Count Dracula costume but then it turns around another 360 degrees and is its old self. But doubtless you know all this already!
I like it!
The inside of Fort knox has a safe where the rotation of the combination/wheel-number will dictate if one opens the safe or not?
The inside cogs and wheels are dictated by the outside combination wheel, one false turn and CPT kicks in and you are forever going to be turning..and turning..the dials. Yet if one were on the inside and the back of the door had a seethrough covering, one can guide a way through any amount of infinite combinations with ease!
I'm not sure, but I think gr-qc/9707010 is early. See also Baez's TWF #110
Thanks! In Baez 110 put that spin networks can have more than 3 edges meeting at a vertex, so they are not actually considered trivalent graphs (Baez 110 was written in 1997)
I'm trying know to fathom what's the meaning of the Poisson algebra. I will post something about it
selfAdjoint
Oct21-03, 11:23 AM
Good for you if you post on the Poisson algebra. This is a missing piece in our discussions here.
Originally posted by selfAdjoint
Good for you if you post on the Poisson algebra. This is a missing piece in our discussions here.
Amen to that!
I told you I would make several false starts. Eventually there should be a non-technical description of loop gravity in only one to ten pages. Let's keep this thread going until we have one, or find one in the literature.
the basic picture of any quantum theory is you have a space of possibilities (configurations, might be simply a set of possible positions and momentums) and then you define a kind of "(not)probability" function or wave-function on that space of possibilities.
If the space of all possibilities, of whatever it is (one particle, N particles, a field, a geometry of the universe) is called A, then the the wavefunctions or quantum states or "(not)probability" functions are just complex valued functions on
A
usually there is a measure defined on called A so you can integrate these functions and they are "square integrable" which means they dont run off to infinity too much and have finite integrals
and I have to say that in mathematics this is, in a certain way, as basic as things ever get----a space, some complex-number-valued functions defined on that space---and being able to integrate or sum each of them, so each one has a finite size.
a loaf of bread, a jug of wine, and hilbert space---this is all we ask and it does not seem like a lot----the rest is trimmings.
so in a certain sense if I could just tell you how to build the configuration space called A of loop gravity and then, if I could just explain how to define a function on that space----and get a hilbertspace consisting of all the complex-number-valued functions on called A then I would be done explaining. All the rest----the selfadjoint operators on the hilbertspace, their evolution, their spectra, and all----that all "hatches" from how the original hilberspace of wavefunctions, or quantum states or whatever you call them, is defined.
So here we are down in the basement and there are not even any "spin networks" or "loops" around. I have to tell you the space called A of loop gravity.
Psssst! It is the space of "connections". A connection is one way to clothe a bare manifold with geometry if it has no geometry. The whole destiny of loop gravity, win or lose, succeed or fail, is in this one choice----the geometry of the universe shall be represented by the possible "connections" on a 3D continuum, a 3D manifold.
Until 1986 the guys like John Wheeler who were trying to quantize GR used the space of "metrics" for their called A and it gave them headaches. After 1986 almost everybody switched over to representing the geometries by connections.
Hey, the whole thing could go into another iteration if some yet other set of variables for GR were found---something that captured the essence of the shape of the world that was not a metric and not a connection---then you could have a new configuration space called A and a new hilbertspace of complex-valued functions defined on it.
It took 50 years to get from a space of metrics to a space of connections---people have tried to quantize GR for a long time. I am not betting they come up with something to replace connections, but they might.
So we are looking at the most basic question---how do you describe the shape of the world, what is a connection, how do you arrange all the possibilites to make a configuration-space, a space of all possible connections, how do you define functions on that space, that have their values in the complex number plane?
what is a connection?
Hi here,
With respect to the Poisson algebra, the only thing that I've discovered is that in 1987, Smolin and Rovelli introduced an infinite set of gauge invariant loop variables on the phase space of the theory (called then the Ashtekar phase space). These variables form a closed Poisson algebra
I don't know if Poisson algebras are anymore important in LQG since loops were substituted by spin networks
Marcus reading your anterior post, there are some ideas that have popped up in my mind, could you clarify, please? [t)]
Is it possible that this Ashtekar phase space is really the Hilbert space of LQG?. I mean, is possible that the loop variables introduced by Rovelli and Smolin are functions in this Hilbert space?
Is it possible that before the introduction of the loop variables, the functions in the Hilbert space were the connections?
Best wishes and keep fighting the stringers!
nonunitary
Oct22-03, 05:40 PM
Originally posted by meteor
Hi here,
With respect to the Poisson algebra, the only thing that I've discovered is that in 1987, Smolin and Rovelli introduced an infinite set of gauge invariant loop variables on the phase space of the theory (called then the Ashtekar phase space). These variables form a closed Poisson algebra
I don't know if Poisson algebras are anymore important in LQG since loops were substituted by spin networks
Marcus reading your anterior post, there are some ideas that have popped up in my mind, could you clarify, please? [t)]
Is it possible that this Ashtekar phase space is really the Hilbert space of LQG?. I mean, is possible that the loop variables introduced by Rovelli and Smolin are functions in this Hilbert space?
Is it possible that before the introduction of the loop variables, the functions in the Hilbert space were the connections?
Best wishes and keep fighting the stringers!
Hi there,
I am kind of new in this forum and found this tread about LQG which interests me a lot. I don´t know if I am misleading but I think that poisson algebras are still important in LQG, but they are not so much
talked about. Rovelli and Smolin indeed had a Poisson algebra that was the starting point for quantization. I think that Ashtekar and co-workers wrote a paper showing that the Rovelli-Smolin algebra was not closed and changing the poisson algebra to some other algebra I don't remenber. Recently, Sahlmann, Lewandowski and Thiemann have
taken this proposal and expanded it.
Just a few more comments. The phase space of the theory is not the same as the Hilbert space. Normaly, this is constructed out of functions of the configuration space, in this case, connections.
The loop variables are functions of the connection, but labelled by loops (or graphs in the case of spin networks).
Originally posted by marcus
what is a connection?
Gravity
Well the question to me then raises what the foundation of this whole topic is built upon? Philospohically Smolin was able to unite three roads to form algebraic topology.
The basis of the all the maths including the geometries must also follow the logic? Venn logic, and geometrical Intuitional developement? Category theory and topos(Algebraic Topology) was a integration of Smolins Three Roads?
It had to begin from supersymmetry, and from this, the gravity is understood, as well as, weak field settling to boundaries and defintion in cooling and discrete forms?
But in all of this, there is a exchange between energy/matter and the mobius can see things turn over as well as the klien bottle turning inside out. Where is that? Twisting in differential rotations?
You had to be able to see it from Kaluza and Kliens perspective and how we got there. U(1)=5d......and this includes all the covers?
Kip thorne help us to visualize, and in this great distance Ligo reads and in the quantum world how much more is this energy that continuity says, listen, things seem very smooth. But we have discrete structures, and how is the gravity revealed from the perspective of tangible objects, but in the recogniton of the distances?
Intuitively it must come to the distances? We do not disregard the structure that arises from the movement of the energy into objects(crystalization)
Sol
selfAdjoint
Oct22-03, 06:28 PM
Sol, it was a rhetorical question. Marcus is going to tell us what a connection is. I'll bet he's working on his metaphors right now.
Originally posted by selfAdjoint
Sol, it was a rhetorical question. Marcus is going to tell us what a connection is. I'll bet he's working on his metaphors right now.
Oh good:) You know how intuition can be sometimes trying to find the right words to explain the essence of things.
Sol
Hi meteor, welcome nonunitary,
everything you two said in your posts seems right to me, and in particular this:
Originally posted by nonunitary
The phase space of the theory is not the same as the Hilbert space. Normaly, this is constructed out of functions of the configuration space, in this case, connections.
The loop variables are functions of the connection, but labelled by loops (or graphs in the case of spin networks).
This is a good thing to be clear about----the configuration space is the set of possible connections (reflecting all the possible geometries there could be on the manifold). In the process of quantizing a classical theory a hilbertspace is constructed consisting of (complex number valued) functions defined on the configuration space.
Spaces of functions are typically convenient to use because they are linear---you can add two functions just like you add two vectors and so on----spaces of functions are typically infinite dimensional vectorspaces and they are handy just the way vectorspaces are. (This is hardly news to you meteor and nonunitary but might as well be said) And a hilbertspace is a type of vectorspace that commonly comes up as a set of real or complex-valued functions defined on something---in this case the classical theory's configuration space: the set of all possible geometries, as represented by the connections associated with them.
A classical theory will have a poisson algebra of various readings off of phase space corresponding to classical deterministic measurements you can make and in the process of quantizing the theory one will want to find an algebra of OPERATORS on the hilbertspace that these things correspond to.
I agree with nonunitary about the role played the poisson algebras and the algebras of quantum observables that people seem able to discover corresponding to them. Also that Sahlmann, Thiemann, Lewandowski and others have been busy with these things recently.
In effect, you are ahead of me right now and there is no need to wait for me to catch up. Anything you know about the theory that you want to explain, you should go ahead! We have no special responsibilities to anyone and no need to follow any special order.
What iterests me right now is this: how would you explain to someone with a minimum of math a way to think about connections.
A manifold is just a set equiped with coordinates around any point so that you CAN use those coordinate patches to give it geometrical shape if you so desire. But a bare manifold is devoid of geometry----there is almost nothing interesting about it unless it has some kinky topological features.
One way to put some starch in your manifold is to define a METRIC on it (which you can do because you have the kind of minimal amenities, namely coordinate patches). Once you have said what the distance between each pair of points in the manifold is, the thing snaps to attention and takes on shape.
But you can also proceed a different way. Because you have this minimal structure of coordinates you can define the tangent "plane" at every point---actually for a 3D manifold it is a tangent 3-space not a tangent plane. And the thing still looks like a ruppled shirt or wet paper bag, it just has tangentspaces stuck to it. But NOW you can decide on the infinitesimal "roll" that happens as you go from one point to the nearest neighboring points. That is, you can pick a "connection". And by continuing along a path and integrating the tiny roll at each point you get
by the end of the path a reall substantive rotation. So a "connection" is basically a contraption that tells how tangent vectors are supposed to rotate as you move along a definite path thru the manifold.
Well that bespeaks geometry too, just like a metric does. Technically it doesn't completely determine it but intuitively it goes a long ways towards defining what shape the thing is.
Now physicists learn Lie groups and Lie algebras long before they put on long pants and start shaving and having dates, so for them it is a kneejerk response that an infinitesimal "roll" is an element of su2, the Lie algebra of SU2. It is like the things you have to learn to pass your drivers test. An element of su2 is a particularly cute kind of 2x2 matrix of complex numbers.
So what is a connection? It is a program I have on my palmpilot that if you show me a point in the manifold and show me a DIRECTION in which to set out from that point, I will tell you the infinitesimal "roll" that is I will tell you a cute 2x2 matrix of complex numbers which is a member of su2.
And in a very rough sense all the possible geometries the manifold can have are reflected usefully in the set of all possible connections that can be defined on it
Ambitwistor
Oct23-03, 01:59 AM
Originally posted by meteor
Do somebody know the paper where the labels passed from being numbers to group representations? Do the Lie groups have to be some specific Lie group? Are actually spin networks continued to be defined as trivalents graphs? Must the group representations be irreducible representations?
The labels of spin network have always been representations; it's just that in the case of SU(2), representations can be simply labeled by numbers.
The Lie group can be anything, but in the connection variables, an SU(2) spatial connection is typically used, which leads to a kinematical Hilbert space of SU(2) spin networks.
Spin networks don't have to be trivalent. In fact, in LQG, trivalent spin networks have zero volume.
The group representations are irreducible, since the point of spin networks is to form an orthonormal basis of the space of connections (modulo gauge transformations); for that, you want networks labelled with irreps, as follows from the Peter-Weyl theorem; see http://arXiv.org/abs/gr-qc/9504036.
Originally posted by Ambitwistor
Spin networks don't have to be trivalent. In fact, in LQG, trivalent spin networks have zero volume.
I checked this in Rovelli's textbook and you are right: he says a node must be at least quadrivalent to have nonzero volume. I like everything in your post and look forward to many more. welcome. there is a considerable need for loop-knowledgeable people here.
I guess I need to look again at the volume formula, for some reason until I checked just now, I thought that a trivalent vertex contributed a unit of volume. I know that any vertex can be broken down by a series of surgical steps into a collection of trivalent one, I must go back and try to understand what happens to the volume.
Ambitwistor
Oct23-03, 02:34 AM
Originally posted by marcus
In the process of quantizing a classical theory a hilbertspace is constructed consisting of (complex number valued) functions defined on the configuration space.
At this level, it might be worth emphasizing that these "complex-valued functions on configuration space" are what some people might know better as "wavefunctions": the configuration space describes the system (like the position of a particle), and the value of the wavefunction (a complex number) is the probability amplitude for finding the system in that particular configuration (like the probability of finding a particle in a particular location).
So a "connection" is basically a contraption that tells how tangent vectors are supposed to rotate as you move along a definite path thru the manifold.
Well that bespeaks geometry too, just like a metric does. Technically it doesn't completely determine it but intuitively it goes a long ways towards defining what shape the thing is.
I'm not sure that I understand this side remark correctly, but if you want to speak technically, a connection does completely determine the geometry: if you have a Levi-Civita connection, then that is equivalent to having a metric. If you have some other kind of connection, then it defines a more general kind of geometry (not Riemannian) that does not arise from a metric.
Originally posted by Ambitwistor
At this level, it might be worth emphasizing that these "complex-valued functions on configuration space" are what some people might know better as "wavefunctions": the configuration space describes the system (like the position of a particle), and the value of the wavefunction (a complex number) is the probability amplitude for finding the system in that particular configuration (like the probability of finding a particle in a particular location).
absolutely right! In another thread I stressed the term "wavefunction" for this. I should always mention that as a synonym. I like to try a few alternative ways of saying things to allow for people coming to the subject from different backgrounds.
....a connection does completely determine the geometry: if you have a Levi-Civita connection, then that is equivalent to having a metric. If you have some other kind of connection, then it defines a more general kind of geometry (not Riemannian) that does not arise from a metric.
I am so glad you are on hand, ambitwistor! I will try (if the board permits it) to edit some of my posts to remove the vagueness about that in accordance with what you say.
My intuitive feel is that the connection describes the geometry, and I am puzzled that the Ashtekar variables are not simply A (the connection) but are various pairs, like [A, E] where E is the densitized triad or "electric field"(a term sometimes used depending on a possibly confusing analogy). A and E are presented as "conjugate" variables. Yet the configuration space is just the collection of all possible A's. what is the essential additional information given by E? You are probably familiar with the notation I'm using, for brevity omiting subscripts and so on.
Ambitwistor
Oct23-03, 03:00 AM
Originally posted by marcus
I guess I need to look again at the volume formula, for some reason I thought that a trivalent vertex contributed a unit of volume. And that any vertex could be broken down by a series of surgical steps into a collection of trivalent one.
As mentioned in Baez's Week 55, Loll showed that trivalent vertices do not contribute volume (http://arXiv.org/abs/gr-qc/9511030).
As for breaking down vertices into trivalent vertices, you're probably thinking of those tangle diagrams, where the spin network edges are decomposed into "virtual" nodes and edges, wired up according to recoupling theory. You can decompose spin networks into trivalent diagrams of that sort, but they aren't spin networks -- though they are equivalent to (not-necessarily trivalent) spin networks.
See http://relativity.livingreviews.org/Articles/lrr-1998-1/node17.html#TheVirtualNode
I'm impressed. In 5 minutes Ambitwistor came up with two
precisely-on-target online references---one to original work by a remarkable woman named Renata Loll and one to the exact place in Rovelli's classic LivingReviews exposition. That means Ambitwistor is a pro. Probably his/her time is too valuable to hang around much. Came thru like a big train through a small station and that was it.
Well well
I'd stay up and see what else happens tonight, but its after midnight here and time to turn in
(today selfAdjoint plugged PF, and the loop gravity threads in particular, on SPR Usenet. this could explain unexpected visits.
nice if some of these people stayed around)
selfAdjoint
Oct23-03, 09:31 AM
Marcus,
My intuitive feel is that the connection describes the geometry, and I am puzzled that the Ashtekar variables are not simply A (the connection) but are various pairs, like [A, E] where E is the densitized triad or "electric field"(a term sometimes used depending on a possibly confusing analogy). A and E are presented as "conjugate" variables. Yet the configuration space is just the collection of all possible A's. what is the essential additional information given by E? You are probably familiar with the notation I'm using, for brevity omiting subscripts and so on.
I have two thoughts on this. One, Ashtekar doesn't use all of his connection, but only the "anti-self-dual" part. Two, the Ashtekar variables specify not only a geometry but a kinematics. Thiemann's intro does a lot of degree-of-freedom counting, maybe that would be a reference on this issue.
Originally posted by selfAdjoint
Marcus,
I have two thoughts on this. One, Ashtekar doesn't use all of his connection, but only the "anti-self-dual" part. Two, the Ashtekar variables specify not only a geometry but a kinematics. Thiemann's intro does a lot of degree-of-freedom counting, maybe that would be a reference on this issue.
I was counting on being able to edit some of this expository writing, but discovered yesterday that the PF rules have changed. there is a time limit of 30 minutes afterwhich I cannot edit a post
It makes it easier to write if you can put in placeholder stuff at some point, then go research it and fix it up if necessary.
If you have more extensive editing capability (a mentor perk?) than I do, you are welcome to fix vague points in my discussion, correct errors, improve style or whatever. editing is half of writing
I was a bit sleepy when Ambitwistor passed through---not sure but I got the impression of someone who may actually do research in quantum gravity----knows work of Baez and of Renate Loll with quick exactitude.
nonunitary
Oct23-03, 11:58 AM
quote:
--------------------------------------------------------------------------------
My intuitive feel is that the connection describes the geometry, and I am puzzled that the Ashtekar variables are not simply A (the connection) but are various pairs, like [A, E] where E is the densitized triad or "electric field"(a term sometimes used depending on a possibly confusing analogy). A and E are presented as "conjugate" variables. Yet the configuration space is just the collection of all possible A's. what is the essential additional information given by E? You are probably familiar with the notation I'm using, for brevity omiting subscripts and so on.
--------------------------------------------------------------------------------
Hi there,
I think there is a confusion here. It is true that a metric and its (Levi Civita) connection carry almost the same information (up to constant re-scaling of the metric). In the basic variables of LQG the metric information of the manifold is in the triad E. The connection "A" that is known as the Ashtekar-Barbero connection has more infromation than just the metric. It also knoes about the ADM conjugate variable, namely the extrinsic curvature $K_{ab}$. Then the connection $A$ is given by $A_a^i=\Gamma^i_a- (const) K^i_a$, where the constant in the formula is the infamous Immirzi parameter.
Thus, even when the connection knows about the metric, it also has information about the extrinsic curvature, and that is why it serves as a conjugate variable for the $E$'s (that is, if the Immirzi parameter were zero, the variables would be all "configuration variables", and its Poisson bracket would vanish).
Another comment. Te self dual connections were the original variables introduced in 1986 by Ashtekar, but they were replaced in the 90's by the Ashtekar-Barbero connection with a "real" Immirzi parameter (instead of $i$ for the original self-dual case). The nice geometrical interpretation is however, lost.
Ambitwistor
Oct23-03, 12:40 PM
Originally posted by marcus
My intuitive feel is that the connection describes the geometry, and I am puzzled that the Ashtekar variables are not simply A (the connection) but are various pairs, like [A, E] where E is the densitized triad or "electric field"(a term sometimes used depending on a possibly confusing analogy). A and E are presented as "conjugate" variables. Yet the configuration space is just the collection of all possible A's. what is the essential additional information given by E? You are probably familiar with the notation I'm using, for brevity omiting subscripts and so on.
(A,E) are a conjugate pair, i.e., a point in phase space (not configuration space). It's analogous to how in QM, position and momentum (x,p) are a conjugate pair (but you only pick one of them them to form your quantum Hilbert space). In the ADM geometrodynamic variables, the 3-metric and the extrinsic curvature form the conjugate phase space variables, and you generally form your Hilbert space over the 3-metric. In the Ashtekar variables, you pick the Ashtekar connection and the densitized 3-triad.
Classically, you use the configuration space variable to describe the kinematics, and the conjugate momentum to describe the dynamics. (e.g., position in QM or the 3-metric in geometrodynamics describes the system at a given instant of time, and momentum or the extrinsic curvature describes how that state will evolve).
You can reconstruct the (densitized) 3-geometry of a spatial slice from E, not A (since the triad is basically just the "square root" of the 3-metric, as nonunitary mentioned). A itself determines a "geometry" on space, just like any connection does, but it's not the kind of metric geometry that a Levi-Civita connection defines. A actually carries information that one can use to reconstruct the spacetime geometry -- like extrinsic curvature does, and in fact A involves the extrinsic curvature (as nonunitary also pointed out).
selfAdjoint
Oct23-03, 01:55 PM
In Thiemann's derivation of the Ashtekar variables he first enlarges the phase space of the Palatini action, spanning this larger space with canonical variables K and E, K will go away but E will remain in the Ashtekar variables. He shows that the new (K,E) coincide with the Palatini (p,q) variables when a constraint is satisfied; this constraint is satisfied identically in the Palatini geometry. Only then is the connection A introduced, and it replaces the nonce variable K, and the new variables (A,E) are canonical and span the big phase space.
In general is it really true that a connection by itself specifies a geometry? Recall that in traditional Riemann you have first a metric - specified by a symmetric tensor, which restricts your choice of geometries, and then define the connection as a function of your metric (through the Christoffel symbols). This then gives you the curvature tensor and all the rest. But the contribution of the symmetric metric was important.
Ambitwistor
Oct23-03, 02:17 PM
Originally posted by selfAdjoint
In general is it really true that a connection by itself specifies a geometry? Recall that in traditional Riemann you have first a metric - specified by a symmetric tensor, which restricts your choice of geometries, and then define the connection as a function of your metric (through the Christoffel symbols). This then gives you the curvature tensor and all the rest. But the contribution of the symmetric metric was important.
Only a Levi-Civita connection specifies a Riemannian geometry, because only L-V connections are compatible with metrics. But starting at least with Klein, and certainly since Cartan, the notion of "geometry" has been expanded to include geometries other than Riemann's. You can think of a connections as giving a generalized kind of geometry, a special case of which are the Riemannian (metric) geometries.
Ambitwistor
Oct23-03, 03:33 PM
(Oops, that should be "L-C connections", i.e. "Levi-Civita".)
Ambitwistor
Oct23-03, 07:21 PM
Example: Yang-Mills gauge theories are geometric theories, even though they're not Riemannian geometries.
The gauge field A (e.g., the the scalar and vector electromagnetic potentials, together forming the 4-potential) is given by a connection, and the field strength tensor F (e.g., the electric and magnetic fields, together forming the Faraday tensor) is the curvature of that connection.
So, in addition to gravity, the fields of the Standard Model (electromagnetic, weak, strong) are also given by geometric theories, but it's not the Riemannian spacetime geometry of general relativity. The Ashtekar variables exploit this similarity by recasting general relativity in a form more similar to the geometry of other gauge theories. You can also go the other way, and try to recast the gauge theories in a form more similar to the geometry of conventional general relativity, in which case you get Kaluza-Klein theory.
Originally posted by Ambitwistor
Only a Levi-Civita connection specifies a Riemannian geometry, because only L-V connections are compatible with metrics. But starting at least with Klein, and certainly since Cartan, the notion of "geometry" has been expanded to include geometries other than Riemann's. You can think of a connections as giving a generalized kind of geometry, a special case of which are the Riemannian (metric) geometries.
At the start of the thread here I was hoping to find a way of presenting an intuitive picture of loop gravity.
Now I'm recalling the explanatory job Baez did on a variety of formalisms for GR---Palatini, Ashtekar-Sen, Barbero variation---I believe it was in TWF with references to hardcopy (the book by Ashtekar, which I have not read having been spoiled by the internet). Now I am thinking that either it is impossible to do what I had in mind. Or Baez will do it and put it on his website one of these days. Or one of the others (of several talented writers in loop gravity.) Or else....the way to go is to start with what you just said "You can think of a connections as giving a generalized kind of geometry, " and (possibly by means of dervish-like handwaving) OMIT the construction of the new GR variables but just take as given that a manifold has a space of all possible connections which reflects all its possible geometries and just go from there. *Takes a deep breath*
Was delighted by one of the other poster's (Gale's) idea of a wickedly clever third grader---which you elucidated by classical anecdote--and am wondering if that approach to quantum gravity would fly with such a third grader.
eigenguy
Oct25-03, 11:31 AM
Originally posted by marcus
At the start of the thread here I was hoping to find a way of presenting an intuitive picture of loop gravity.
I am studying the following paper
http://arxiv.org/abs/gr-qc/0207106
Abstract: A program was recently initiated to bridge the gap between the Planck scale physics described by loop quantum gravity and the familiar low energy world. We illustrate the conceptual problems and their solutions through a toy model: quantum mechanics of a point particle. Maxwell fields will be discussed in the second paper of this series which further develops the program and provides details.
Here's an excerpt:
"We will begin with the usual Weyl algebra generated by the exponentiated position and momentum operators. The standard Schrodinger representation of this algebra will play the role of the Fock representation of low energy quantum field theories and we will construct a new, unitarily inequivalent representation called the polymer particle representation in which states are mathematically analogous to the polymer-like excitations of quantum geometry. The mathematical structure of this representation mimics various features of quantum geometry quite well; in particular there are clear analogs of holonomies of connections and fluxes of electric fields, non-existence of connection operators, fundamental discreteness, spin networks, and the spaces Cyl and Cyl*. At the basic mathematical level, the two descriptions are quite distinct and, indeed, appear to be disparate. Yet, we will show that states in the standard Schrodinger Hilbert space define elements of the analog of Cyl*. As in quantum geometry, the polymer particle Cyl* does not admit a natural inner product. Nonetheless we can extract the relevant physics from elements of Cyl* by examining their shadows, which belong to the polymer particle Hilbert space HPoly. This physics is indistinguishable from that contained in Schrodinger quantum mechanics in its domain of applicability.
These results will show that, in principle, one could adopt the viewpoint that the polymer particle representation is the `fundamental one'|it incorporates the underlying discreteness of spatial geometry|and the standard Schrodinger representation corresponds only to the 'coarse-grained' sector of the fundamental theory in the continuum approximation. Indeed, this viewpoint is viable from a purely mathematical physics perspective, i.e., if the only limitation of Schrodinger quantum mechanics were its failure to take into account the discrete nature of the Riemannian geometry. In the real world, however, the corrections to non-relativistic quantum mechanics due to special relativity and quantum eld theoretic effects largely overwhelm the quantum geometry e ects, whence the above viewpoint is not physically tenable. Nonetheless, the results for this toy model illustrate why an analogous viewpoint can be viable in the full theory: Although the standard, low energy quantum field theory seems disparate from quantum geometry, it can arise, in a systematic way, as a suitable semi-classical sector of loop quantum gravity."
I'm trying to learn what the different spaces of LQG are useful for,for example I more or less know the utility of the Hilbert space, the configuration space and the phase space. But, what's the utility of the state space?
My resumee:
In LQG the two basic variables are a connection and a densitized triad field(sometimes called electric field). The connections are functions defined in the configuration space of the theory, and each connection represents a quantum state of spacetime.This configuration space is a vector space of functions
The connection and the densitized triad field form a canonical pair in the phase space of LQG, that is a infinite dimensional space
The Hilbert space of the theory is constructed of the connections defined in the configuration space. Spin network states (previously were used loop states) form the basis of this Hilbert space.
Now, is this Hilbert space the unique Hilbert space of the theory? I've read that there's something called "kinematical Hilbert space", and othe thing called "diffeomorphism invariant Hilbert space". They both refer to the same thing?
Would be good if you could clarify this: It's true that actually the complex SU(2) connection of Ashtekar is not used in LQG, but is used the real SO(3) connection introduced by Barbero?
Ambitwistor
Oct26-03, 01:40 PM
Originally posted by meteor
In LQG the two basic variables are a connection and a densitized triad field(sometimes called electric field). The connections are functions defined in the configuration space of the theory, and each connection represents a quantum state of spacetime.
Each connection (modulo an SU(2) or SO(3) gauge transformation) represents a classical state of space, not a quantum state of spacetime. (Well, not even that: it only represents space once you impose the constraints.) We haven't quantized yet.
Spin network states (previously were used loop states) form the basis of this Hilbert space. Now, is this Hilbert space the unique Hilbert space of the theory? I've read that there's something called "kinematical Hilbert space", and othe thing called "diffeomorphism invariant Hilbert space". They both refer to the same thing?
No. The kinematical Hilbert space is L^2(A/G), i.e., the (complex) Lebesgue square-integrable functions over the space of connections modulo gauge transformations. It's like saying that the configuration space of a particle is R^3 (all of space), and then saying that the space of quantum states (wavefunctions) is L^2(R^3), the space of (square-integrable) complex functions over R^3.
However, then we have to start imposing constraints. e.g., for the free particle in QM we could construct the space of states L^2(R^3), but now suppose that we really only want to quantize a particle that's constrained to move on the surface of a sphere in R^3, or something. Then we have to start chopping down the kinematical Hilbert space to get the physical Hilbert space, the wavefunctions of particles that are constrained to move on the surface of a sphere.
In loop quantum gravity, we start with the kinematical Hilbert space, which has the spin networks as a basis. It is the quantum space of states of connections (modulo gauge transformations). However, not ANY connection corresponds to a solution of Einstein's equation! Only connections which obey the Gauss, diffeomorphism, and Hamiltonian constraints are "physical", connections that represent a gravitational field. So just like we discard connections in the classical configuration space A/G that don't obey the constraints of general relativity, we have to discard states in the kinematical Hilbert space L^2(A/G) that don't obey the quantized versions of those constraints.
So, the diffeomorphism-invariant Hilbert space is what you get when you apply the diffeomorphism constraint to the kinematical Hilbert space. If you also apply the Hamiltonian constraint, you get the physical Hilbert space.
(Note: we applied the Gauss constraint before quantizing by modding out by gauge transformations to consider the space A/G, because it's easy to do that. Then we applied the other constraints after quantizing.)
See also:
http://www.lns.cornell.edu/spr/1999-05/msg0016153.html
http://www.lns.cornell.edu/spr/1999-05/msg0016258.html
Would be good if you could clarify this: It's true that actually the complex SU(2) connection of Ashtekar is not used in LQG, but is used the real SU(3) connection introduced by Barbero?
Well, there are a lot of connections floating around, actually. Some people like Ashtekar's connection. Many use Barbero's nowadays, because you don't have to deal with the reality conditions. Barbero's connection is not SU(3), it is SO(3); you can use an SU(2) connection too, but it's not the same as Ashtekar's connection.
(SU(2) and SO(3) are pretty interchangeable as far as connections are concerned, because they have the same Lie algebra. It can make a difference when global effects are concerned, but loop quantum gravity physicists are usually sloppy about such things.)
I was hoping to arrive at some posts expressing intuitive content of loop gravity. Some of us have been reading Livine's thesis and/or work co-authored with Alexandrov or with Freidel.
I find the work admirable but difficult to assimilate. It seems to me that i am gradually having to confront a more completely lorentzian fourdimensional theory----they are extending the group to the whole lorentz group and raising the dimension. How to picture this. Maybe someone else---selfAdjoint, ambitwistor, ... has ideas about how to describe this. Or is it just plain a lot more difficult and tough to describe?
I am used to having 3D connections corresponding to a 3D spatial manifold. Quantum states of 3D geometry. Operators, which presumably can evolve a bit like the Heisenberg picture but without an absolute preferred time, only one operator you choose arbitrarily to serve as clock for the other processes. This is not too bad.
but now Livine etc make us consider 4D connections corresponding to all possible geometries on some 4D manifold. The wave functions are not just functions defined on the connections but on a pair consisting of a 4D connection and a vectorfield χ
any concerns or comments about this new material
Ambitwistor
Oct27-03, 07:01 PM
Originally posted by marcus
It seems to me that i am gradually having to confront a more completely lorentzian fourdimensional theory----they are extending the group to the whole lorentz group and raising the dimension.
Well, there are many approaches floating around. The 4D approaches are more related to spin foams the usual loop quantum gravity in the canonical approach. It's probably best to start by thoroughly understanding one model, such as canonical LQG with the Ashtekar-Barbero connection, or the Barrett-Crane spin foam model, rather than trying to simultaneously learn about all the different cutting-edge approaches.
whether or not it is wise, I would like to understand the role played by this vectorfield chi, let's see how to write it
c
χ
the quantum state or wave function is defined on a pair
consisting of a connection and a vectorfield
Ψ(A, c)
as you say, Ambitwistor, the connection to spinfoam is close, but also there is a connection to the SU(2) loop gravity of the people you mentioned.
I would like to understand how this vectorfield seems to serve as a bridge between the SU(2) and the covariant (i.e. SL(2,C) or lorentzian) approaces
eigenguy
Oct27-03, 07:41 PM
Originally posted by Ambitwistor
Well, there are many approaches floating around. The 4D approaches are more related to spin foams the usual loop quantum gravity in the canonical approach. It's probably best to start by thoroughly understanding one model, such as canonical LQG with the Ashtekar-Barbero connection, or the Barrett-Crane spin foam model, rather than trying to simultaneously learn about all the different cutting-edge approaches.
I agree, this is excellent advice ambitwistor.
Marcus, if you want your understanding of LQG to advance beyond the impressionistic level it's on now, you really need to commit to just one or two papers on a specific topic and really go over them with a fine tooth comb, proving every intermediate result you can (if you can).
I was advised, quite wisely as it turns out, to look just at the issue of relating polymer and fock states beginning with the pedagogically effective paper I referred you to. You should listen to ambitwistor and jeff. (I must say I'm having an increasingly hard time understanding how you managed that physics expert award thing. Maybe your true calling is politics?[6)])
the role of c[
see page 98 of the thesis
a bridge player is discussing the taking of a particular trick
right after equation (8.30) he says
"With the help of a gauge transformation, one notices that it's always possible to rotate a given c(x) to be the same fixed one eg. (1,0,0,0). So an invariant function is completely determined by its section at c=c0"
and he defines a restricted wavefunction that now depends only on the connection
instead of f(A,c) we are now looking at
fχ = χ0(A), which I will just call f(A) for the moment
"let us remark that f(A) has a residual SU(2) invariance.
Thus we are in the process of studying functions of a lorentz connection, effectively not invariant under SL(2,C) but simply
under the compact group SU(2)!"
Livine's italics and exclamation point. so this is one of the things this vectorfield chi does.
Originally posted by marcus
the role of c[
see page 98 of the thesis...
he calls it the "time normal" and makes it one of the configuration variables along with the connection.
he gives some more idea of how he thinks of it right there on page 98, before the part I quoted, before (8.30)
it's a vectorfield with values in the quotient SL(2,C)/SU(2)
that you can think of as a normal to the hypersurface
and he gives a reference to maybe the best article on this
chi "boost" gadget, "time normal" "internal time direction"
the reference is to
http://arxiv.org/gr-qc/0207084
Projected Spin Networks for Lorentz connection: Linking spin foams and loop gravity.
it is dated 12 April, 2003 tho the number suggests earlier.
this 15 page paper (along with the Alexandrov/Livine one we were reading earlier today gr-qc/0205109) might be the best
auxilliary reading to have handy when looking over the thesis. but the thesis is fairly self-contained as such go
selfAdjoint
Oct28-03, 11:15 AM
Well in the last paper you cite he says the chi field determines the imbedding of the 3-d space Σ in the 4-d spacetime. Which I can see, a field of little vectors normal to that hypersurface and by their direction determining just which shape it takes in 4-space. Then he goes to the network and only keeps the chis at the vertices. And by this he reduces the group action on them from Poincare SO(1,3) (he calls it Lorentz) to a product of rotation groups SO(3) over the vertices. So far so good, it seems to me. If you really want to see the origin of the chis spelled out I guess you would have to go back to Holst's paper (Red Queen, Red Queen!) or the earlier papers by Livine that he cites.
Originally posted by selfAdjoint
...you would have to go back to Holst's paper (Red Queen, Red Queen!)
even to stay in one place, in other words, I have to run faster than a bandersnatch, but hey no problem we are always doing that!
Getting back to the original purpose of the thread, how to introduce loop gravity and spin foams in plain terms---minimum of technicality.
In the "spin foam models" thread a new poster came in today and expressed some curiosity about what was being talked about and I said I would try to do this. Thing is quantizing geometry---in other words general relativity (a theory of changeable geometry that has tested out well)----is on the agenda and amounts to "putting QM and GR together".
Want to say how spin networks and spin foams work in this context.
Originally posted by marcus
...amounts to "putting QM and GR together".
Want to say how spin networks and spin foams work in this context.
Space in GR is dynamic--it can change as matter or energy flows around, it can undulate, effects can ripple outwards carrying the news of events that effected the shape of space in some locale. So in quantizing GR one needs a way of describing geometry and the change in geometry.
It will turn out that networks can be used to get a handle on geometries-----to assign probability-like "amplitudes" to the various possible geometries that space can have
a network is basically fairly simple---something like a net or a large ball and stick molecular model---Ashtekar likes to call networks "polymers". A network can have thousands of individual links and vertices---or balls and sticks---or trillions and jillions, so in that sense it is complicated. But in another sense it is simple because made up of simple elements.
have to go, but will get back to this later
...a network is...something like a net or a large...molecular model---Ashtekar likes to call networks "polymers". A network can have thousands of individual links and vertices...so in that sense it is complicated. But in another sense it is simple because made up of simple elements.
there are several different ways to tell the story, here's one. A network by itself is rather amorphous---it lacks geometry. all it is is a bunch of points with a list of which pairs of points are connected (in which direction, to make later work simpler there is a preferred direction, the links are one-way-street type)
it doesnt have fixed angles or lengths written onto its links so it doesnt define shape by itself, but you can get a whole lot of different possible geometries to live on it, by assigning data to the links, like angles but not exactly angles.
What's done is to choose a GROUP of ways to twist and turn as you run along any link from point A to point B. This group is called G and it is usually some group of 2x2 matrices---you can write down all kinds of rotations with a mere 2x2 matrix and even other simple actions like expansions and contractions etc. There are several different groups of 2x2 matrices some larger with more varied action and some smaller and there is some range of choice in doing the theory.
Think of the matrices in the group as rotations and imagine that you go all thru the network and label each link with a group element that describes a "rotation" or something more general that happens when you run along that link.
That assignment of one group element to each link in the web can be called a (discrete) "connection".
Intuitively it connects how things are oriented around one node in the web with how they are oriented at the neighbor node just down the street.
A discrete connection is a stripped-down version of a much fancier bigger machine that lives on a smooth manifold, a continuum. If you were to plunge our finite network into a continuum, where an official bigtime connection was living, then IT would induce a discrete connection on our network which would be a kind of meager skeleton or no-frills diagrammatic sketch of the original. But let's not bother to define exactly what a manifold is or what the usual idea of connection is in differential geometry. Our stripped-down finite skeletal idea is workable.
(Indeed I got the discrete connection idea from E.R.Livine's thesis and some of the articles around it, it may actually turn out to work better for some things!)
Imagine a web with E links and V nodes (the E stands the word "edge" which is sometimes used for the links in a network, and V for vertex). A connection, at least before it comes to live on the graph, is just a list (g1, g2,....gE) of group elements. So if you like cartesian product set-notation the space of all possible connections which could be chosen for this particular web is
GxGx....xG = GE
I am telling you this set of connections because the theory defines its core hilbertspace on it. That is the basic thing in any quantum theory---that space and the operators on it. But first, since that is a bit technical, can you see how that in some sense this collection of all possible connections (each one telling a specific way things twist and turn as you run thru the network) is tantamount to the range of possible geometries?
Things keep happening in quantum gravity and I dont have a sticky here where I can keep the useful source material handy or post recent
developments. So I'll try using this thread.
Rovelli just posted a new draft of his book "Quantum Gravity". It is the November 25 draft and is quite a bit changed from the August draft some of us were reading earlier. the contract for publication has been signed with Cambridge University Press.
We were discussing stuff from Livine's thesis in this and another thread.
http://arxiv.org/gr-qc/0309028
Girelli and Livine have come out with a paper about quantizing speed.
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032
Ichiro Oda has posted "A Relation Between Topological Quantum Field Theory and the Kodama State"
http://arxiv.org/hep-th/0311149
The last sentence of the "Discussion" section at the end of the paper reads: "Of course, one of the big problems in future is to clarify whether the Lorentzian Kodama state is normalizable under an appropriate inner product or not." It is clear that he is specifically interested in applications to general relativity---he mentions loop quantum gravity in the first paragraph and refers to gravity/GR at several points in the paper. This paper can be seen as Oda's careful response to an earlier paper by Witten gr-qc/0306083.
Witten said Kodama was not normalizable (in whatever inner product Witten thought was appropriate) and Oda does not buy this and says politely that the question is still open---is, in fact, the "big problem in future".
Daniele Oriti's thesis is out
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"
Smolin and Starodubtsev have posted a brief paper which writes the actions for Palatini GR and Ashtekar GR and BF topological QFT and also another (FΛF) type of TQFT all in the same formula. There is a dynamic variable which as it changes seems to make the system change smoothly from one theory to another
"General Relativity with a topological phase: an action principle"
http://arxiv.org/hep-th/0311163
I found some family resemblance between this paper and Oda's--but both are quite recent and neither cites the other.
The cosmological constant occurs in a number of recent quantum gravity papers. The one by Girelli/Livine is one of the most recent. One of the most basic--perhaps a landmark---is Karim Noui and Philippe Roche "Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
A paper by a couple of New Zealanders
http://www.arxiv.org/abs/gr-qc/0311086
points out a discrepancy between two ways that people have used
to calculate BH quasi-normal modes ("ringing frequencies")
and attempts to resolve the discrepancy by "critically re-assessing"
the approach used by Motl and Neitzke
the issues are still shifting around the area operator in loop gravity
and the Immirzi parameter which occurs in some versions
but not all (e.g. the covariant version used by some people such as Livine/Alexandrov/Noui/others?) what happens to the spectrum of the area operator in covariant loop gravity and how does that compare with the results (themselves not yet conclusive) for BH vibration modes?
lumidek
Nov28-03, 08:23 PM
Dear Marcus,
I always appreciate your interest and your qualified comments about physics - and not only the technical ones. [;)] Let me just say a couple of words about your new explanation. The current paper
http://www.arxiv.org/abs/gr-qc/0311086
is pretty far from the original proposals by Shahar Hod and Olaf Dreyer because no results - except for the four-dimensional Schwarzschild black holes in the infinite space - provide us with evidence of Hod's and Dreyer's conjectures. Everything else seems to contradict the general predictions by Hod and Dreyer (about the asymptotic real part of the quasinormal modes).
When you wrote that the authors "critically re-assess the approach used by Motl and Neitzke", you are twisting the words to get a very different meaning. The New Zealanders do not claim that there is anything wrong with our monodromy calculation done with Andy Neitzke! They are saying that something is probably wrong with a paper by Castello-Branco and Abdalla who did not use our methods carefully.
To claim - today - that our results with Andy are wrong would not be the most reasonable thing to do because essentially all of our results have been confirmed numerically - for example, our prediction for the Reissner-Nordstrom black holes was confirmed beautifully by colleagues like Berti, Kokkotas, Cardoso, Lemos and others (after our paper).
I and Andy used the so-called monodromy method, and as far as I know everyone agrees that it works correctly in the contexts that we studied in our paper. Of course, not only our results, but also the method can be useful for other people and other problems. However we can't guarantee that the results obtained by other people, using our machinery, will be correct.
I found some of the results in the Schwarzschild-de-Sitter context a bit counter-intuitive, and there have been many papers about the Schwarzschild-de-Sitter black hole. (Our paper with Andy was never about de Sitter, all the black holes that we studied were in empty space.)
The New Zealanders mostly criticize the paper by Castello-Branco and Abdalla [19]
http://arxiv.org/abs/gr-qc/0309090
Yes, I also think that they used our method a little bit blindly. Their results looked too similar to the case of the flat space. The New Zealanders claim that the method can't be used - at least not in this way - if two horizons coincide, and I tend to agree with that. Moreover, there are many questions about the choice of the boundary conditions.
The quasinormal modes remain a lively topic. You can observe a rather complete list of the developments if you look which papers cite e.g. my first paper:
http://www.slac.stanford.edu/spires/find/hep/www?c=00203,6,1135
Once again, be sure that none argues that my results or our results with Andy (and the methods used to derive them in our context) are incorrect. The newer papers study more complicated cases where some errors have been done - but not by me and Andy. [6)]
Best wishes
Luboš
selfAdjoint
Nov28-03, 09:21 PM
Welcome to the Physics Forums, Lubos. We're honored to have you here.
Originally posted by selfAdjoint
Welcome to the Physics Forums, Lubos. We're honored to have you here.
Indeed so! and thanks for making the distinction between your work and the use of a similar method by others which, you say, the New Zealanders were examining critically. Any more explication you can give would, I suspect, be much appreciated.
best wishes,
marcus
Things keep happening in quantum gravity and there is no sticky here to keep handy links to source material about loop gravity developments. So I continue trying to use this thread.
The term "Loop Gravity" is used for want of a better one for a broad range of research approaches to quantizing general relativity.
Rovelli briefly discusses "the name of the theory" on page (xvi) of his new book. The name "loop" is something of an accident because current approaches are not so much concerned with loops. But no one has come up with a good alternative.
The main things the new approaches seem to have in common is that they emerge from General Relativity (rather than Particle Physics) and that they aren't string/brane theories.
A kind of merging among topological QFT ("TQFT") and non-commutative geometry (especially because of the Cosmological Constant) and spinfoams and (Lorentzian spin network-based) LQG seems to be in process. Some people seem to have found a way to do spin network analysis with non-compact groups---so they can use SL(2,C) for gauge in place of SU(2)---hep-th/0205268. Some of this may matter or may not, seems too early to judge. But it might help to keep some of the links handy for reference.
Today Lubos Motl posted a message to Non-unitary ("somewhere in the tropics") containing one link. This link was offered as a characterization of LQG. It was to a 5-year old 11-page paper by Rovelli and Upadhya which was intended as a quick into. They call it a "Primer" to the subject. It does not really characterize the field of loop gravity but Lubos might appreciate it if I include the link in this "sticky" list of links so here it is:
Rovelli/Upahya 5-year-old brief 11-page "primer" to the subject
http://arxiv.org/abs/gr-qc/9806079
Rovelli just posted a new draft of his book "Quantum Gravity". It is the November 25 draft and is quite a bit changed from the August draft some of us were reading earlier. the contract for publication has been signed with Cambridge University Press. The PDF file for Rovelli's book is at his homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
It takes about 10 minutes to download and convert so that it can appear on the screen. It is 300 plus pages long.
The SPIRES database on citations is often handy. There is a topcited list for the smaller series GR-QG (general relativity and quantum gravity) as well as for the huge series HEP-TH.
http://www.slac.stanford.edu/library/topcites/topcites.review.2002.html
We were discussing stuff from Livine's thesis in this and another thread. Here is Livine's thesis. He does a lot with explicitly covariant---SL(2,C)-style---spin networks and makes an explicit bridge from LQG to Lorentzian spinfoams.
http://arxiv.org/gr-qc/0309028
Girelli and Livine have come out with a paper about quantizing speed.
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032
Ichiro Oda has posted "A Relation Between Topological Quantum Field Theory and the Kodama State"
http://arxiv.org/hep-th/0311149
The last sentence of the "Discussion" section at the end of the paper reads: "Of course, one of the big problems in future is to clarify whether the Lorentzian Kodama state is normalizable under an appropriate inner product or not." This paper can be seen as Oda's careful response to an earlier paper by Witten gr-qc/0306083.
Witten said Kodama was not normalizable (in whatever inner product Witten thought was appropriate). Apparently Oda does not buy this and says politely that the question is still open---is, in fact, the "big problem in future".
Daniele Oriti's thesis is out
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"
Smolin and Starodubtsev have posted a brief paper which writes the actions for Palatini GR and Ashtekar GR and BF topological QFT and also another (FΛF) type of TQFT all in the same formula. There is a dynamic variable which as it changes seems to make the system change smoothly from one theory to another
"General Relativity with a topological phase: an action principle"
http://arxiv.org/hep-th/0311163
I found some family resemblance between this paper and Oda's--but both are quite recent and neither cites the other.
The cosmological constant occurs in a number of recent quantum gravity papers. The one by Girelli/Livine is one of the most recent. One of the most basic--perhaps a landmark---is Karim Noui and Philippe Roche "Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
this is the substitute for a sticky to keep links to current
Quantum Gravity resources, so I will update it from time to time.
QG Phenomenology seems to be attracting research interest (as prospects emerge for testing planck-scale effects)
the best and most recent survey of it that I have found is a November article by Giovanni Amelino-Camelia called
"Quantum Gravity Phenomenology"
http://arxiv.org/physics/0311037
It is 8 pages and was prepared in tandem with an article he wrote for the November 2003 issue of "Physics World". This issue was devoted to the current state of affairs in Quantum Gravity and had 3 invited survey articles, one on phenomenology (prospects for testing the theories) by Giovanni A-C, one on LQG by Carlo Rovelli, and one on string by Leonard Susskind.
There is a major issue within LQG about how energy and momentum transform under the Lorentz group. The two different points of view are exemplified by
Rovelli and Speciale "Reconcile Planck-scale discreteness and the Lorentz-Fitzgerald contraction"
http://arxiv.org/gr-qc/0205108
this was published in 2003 in Physics Review D.
Magueijo and Smolin "Generalized Lorentz invariance with an invariant energy scale"
http://arxiv.org/gr-qc/0207085
this was also published in 2003 in Physics Review D.
The two versions yield different predictions about dispersion in arrival time of gammaray bursts. It is barely possible that GLAST may be able to distinguish between the two approaches to building loop quantum gravity theory when it starts up in 2006.
The nub of the issue is that the planck length (or area, or energy) plays a crucial role as marking where quantum gravity effects become dominant. But relatively moving observers see lengths and areas differently! How can one reconcile having a theory locally embodying the principle of relativity (no preferred frame, all observers equal) that nevertheless has a certain length (or energy) as an important landmark?
Intuitively, just as in 1905 SR where the speed of light is the same for all observers, this other landmark the planck energy should be the same for all observers! But how can one manage this? So there is this internal debate in LQG on how to accomodate this and maybe observational data will help settle it
[edit: I checked on the launch date for Gammaray Large Area Space Telescope, GLAST, and it is not until 2006]
According to Loren B's post on another thread, the January 2004
issue of Scientific American has an article that (at least in part)
deals with Loop Gravity. I haven't seen it. Don't know if there is a web reference.
It seems like it might be a useful article to add to the list here, as a recent survey for wide audience. Does anyone have more information? The article is by Lee Smolin
[edit: I found a link to a two-paragraph teaser for the article
http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=00012BDE-E7EA-1FD3-A7EA83414B7F012C
more than that, and the crafty editors of digital SciAm want us to pay for it]
nonunitary
Dec16-03, 06:02 PM
The Sciam issue seems to be interesting. Too bad one cannot get it online!
By the way, there is a link in the General Relativity Hyperspace
where forthcoming events are announced:
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference
There is also a workshop on Loop Quantum Gravity announced there, apart from the preliminary programme for GR17.
Originally posted by nonunitary
By the way, there is a link in the General Relativity Hyperspace
where forthcoming events are announced:
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference
There is also a workshop on Loop Quantum Gravity announced there, apart from the preliminary programme for GR17.
Thanks! I followed the link you gave to:
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference
and saw the announcement of "Loops and Spinfoam" conference in May 2004 in Marseille, organized by Rovelli, Freidel, and Roche.
and also the program you mentioned, for the 17th International Conference on General Relativity at:
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?03Dec.8
to be held in Dublin on 18-24 July 2004.
nonunitary
Dec17-03, 12:45 PM
Hi,
One can also see the most recent announcements at:
http://www.maths.qmul.ac.uk/wbin/GRnewslist/conference?10
where another LQG event is advertised.
I followed the links at the site of the organizers and found the
page:
http://www.nuclecu.unam.mx/~corichi/lqg.htm
Bye
Originally posted by nonunitary
...
http://www.nuclecu.unam.mx/~corichi/lqg.htm
...
Thanks again! this Mexico City symposium on loop and spinfoam
is really interesting news. I like the list of people
who have already said they would attend. One could learn more about quantum gravity, maybe, in Mexico Jan30-Feb1 next year than one might in Dublin in July with the 17th International Conference. I will copy the program planned for this long-weekend loop/foam symposium, from your link:
1. Spin foam models
(to include 2+1 and 3+1 theories; limitations of the Barrett-Crane
model; relation to the canonical approach to dynamics; the role of
the Barbero-Immirzi parameter; canonical and spin foam geometries; issue
of the `continuum limit', renormalization group flows)
2. Status of the Hamiltonian constraint
(to include 2+1 and 3+1 theories; spin-foam and Thiemann-type
approaches; quantum cosmology; Semi-classical corrections to
Einstein equations; factor ordering; too many solutions?
issue of finding solutions and inner product, the "phoenix project").
3. Semi-classical issues
(to include relation between kinematical and dynamical semi-classical
states; quantum field theory on quantum geometry; quantum cosmology;
Minkowski coherent state and Minkowski spin foam)
4. Loop quantum phenomenology
(to include Lorentz invariance;`Double special relativity'; quantum
cosmology; Kodama state and de Sitter background)
5. Conceptual issues
(observables through matter coupling, string theory in polymer
representation; matter couplings on semi-classical states of
geometry and string theory; issue of time; meaning of histories used
in spin foam, role of quantum groups is LQG)
Originally posted by marcus
4. Loop quantum phenomenology
(to include Lorentz invariance;`Double special relativity'; quantum
cosmology; Kodama state and de Sitter background)
the programme for the Mexico City symposium is a good "weather-vane" to point out what the interesting questions are now.
Loop quantum phenomenology seems to have some important issues.
Especially around DSR and GLAST and possible variations in photon time of flight over cosmological distances.
the person who has the most to say about this for me now is
in Wroclaw Poland, name of Jurek (Jerzy) Kowalski-Glikman
"Velocity of particles in Doubly Special Relativity"
http://arxiv.org./abs/hep-th/0304027
"Doubly Special Relativity and quantum gravity phenomenology"
http://arxiv.org./abs/hep-th/0312140
http://www.ift.uni.wroc.pl/JK-G/
Jerzy K-G has published (in 2001) with Amelino-Camelia and he has published (in 2003) with Freidel and Smolin. I think he understands DSR more as a mathematician, more rigorously and clearly than Amelino-Camelia who initiated the research into it and originally conceived of it!! I am impressed by these two papers. It looks to me as if he basically just takes control of DSR and contradicts Smolin and Amelino-Camelia. the way Jerzy K-G develops DSR it does what it is supposed to--there are two observer-independent scales one of speed and one of energy--and also the speed of a gammaray photon does NOT change with energy: it stays c for all photons, indeed all massless particles. This "Polish" version of DSR will be destroyed if it turns out in 2006 that GLAST does after all see any speed-variation in high-energy photons, so the Polish version of the theory is firmly and directly falsifiable. Have to say I like it. Alejandro Corichi should make sure K-G comes to the Mexico City symposium for topic 4 "Loop quantum phenomenology" [:)]
selfAdjoint
Dec17-03, 10:48 PM
Say, Marcus, it really looks like we PFers are pretty up to speed on the stuff they are going to have on that program. Thanks mostly to you, I might add. Great going!
Originally posted by selfAdjoint
Say, Marcus, it really looks like we PFers are pretty up to speed on the stuff they are going to have on that program. Thanks mostly to you, I might add. Great going!
by "you" you have got to mean Meteor, Nonunitary, yourself, Ambitwistor as well, to mention only the first that come to mind. Thanks to all. I didnt realize until today what a key role Jerzy K-G is playing. Look at the program from the Wroczlaw Institute of Theoretical Physics for the "40th Winter School in Theoretical Physics" for Feb 4-14, 2004. Each year since 1964 they have had a winterschool gathering a dozen or so worldclass people at this
ski-resort. this year the topic chosen is "Quantum Gravity Phenom."
Steve Carlip is one of the organizers, and so is (you guessed it) Jerzy Kowalski-Glikman.
Speakers:
E. Alvarez Quantum Gravity
G. Amelino-Camelia Introduction to quantum gravity phenomenology
P. De Bernardis Cosmology with BOOMERANG, WMAP
A. Grillo Planck-scale kinematics and the Pierre Auger Observatory
T. Jacobson Astrophysical bounds on Planck-supressed Lorentz violation
J. Kowalski-Glikman Introduction to doubly special relativity
C. Laemmerzahl Tests of Lorentz symmetry in space and interferometry
P. Lipari Ultra-high-energy cosmic-rays
J. Martin Trans-Planckian cosmology
N. Mavromatos PCT symmetry and quantum gravity phenomenology
T. Piran Gamma-ray bursts
J. Pullin Canonical quantum gravity phenomenology
L. Smolin Cosmological constant in Quantum Gravity
The names Ted Jacobson and Nick Mavromatos especially ring a bell as they've been writing papers about QG phenomenology, various kinds of astronomical tests, that other people cite regularly. IIRC Mavromatos co-authors with Ellis. But I guess so does Pullin and Lipari and several others, ring a bell I mean
So much is going on! end-January Mexico City (loop/foam symposium)
first half of February Karpasz School (QG phenomenology)
then Freidel and Rovelli's Loop Gravity/Spinfoam conference at Marseille May 3-May 7
then the July thing at Dublin--17th International General Relativity conference
Good thing nonunitary provided the links to these things. Yours truly had not registered all the activity.
Well, given that my knowledge in differential geometry is rather poor, I've ordered the book "Differential geometry" of Schaum to Amazon. Hope that will be a good book, like all the other Schaum books that I've read
A question: Then the SO(2) connection used like a variable in Ashtekar's general relativity is a real connection or a complex connection? There are papers that say that is real and others that is complex. I'm dying in the doubt
I've just read that loop quantum gravity violates the "weak energy condition" at short distances, when the granularity of spacetime becomes significant. I've don't have the foggiest idea of what is the weak energy condition, so I'm going to read about it right now
Hi Meteor, I started a thread with your two questions
1. about Ashtekar's new variables
2. about "weak energy condition"
Both could lead to discussion and I am trying to save this thread as a kind of "sticky" for useful links, source material, conference news, and so on. Hope it is OK for me to make a separate thread for what you asked about. You will see it.
I called it "Loop gravity---two questions"
several people have expressed interest in the SciAm January 2004 article by Lee Smolin, "Atoms of Space and Time"
The complete article is probably worth a visit to your local public library. It is written for general audience but manages to give a fairly clear picture of the field and how it developed. Here is an exerpt, as a sample, from the section where Smolin is describing how he and some others got started:
-------quote page 68----
...In the mid-1980s a few of us...Ashtekar...Jacobson...Rovelli...decided to reexamine the question of whether quantum mechanics could be combined consistently with general relativity using the standard techniques. We knew that the negative results from the 1970s had an important loophole. Those calculations assumed that the geometry of space is continuous and smooth, no matter how minutely we examine it, just as people had expected matter to be before the discovery of atoms.
Some of our teachers and mentors had pointed out that if this assumption was wrong, the old calculation would not be reliable.
So we began searching for a way to do calculations without assuming that space is smooth and continuous. We insisted on not making any assumptions beyond the experimentally well tested principles of general relativity and quantum theory. In particular we kept two key principles of general relativity at the heart of our calculations.
The first is known as background independence. This principle says that the geometry of spacetime is not fixed. Instead the geometry is an evolving, dynamical quantity. To find the geometry, one has to solve certain equations that include all the effects of matter and energy. Incidentally, string theory, as currently formulated, is not background independent; the equations describing the strings are set up in a predetermined classical (that is, nonquantum) spacetime.
The second principle, known by the imposing name of diffeomorphism invariance, is closely related to background independence. This principle implies that, unlike theories prior to general relativity, one is free to choose any set of coordinates to map spacetime and express the equations. A point in spacetime is defined only by what physically happens at it, not by its location according to some special set of coordinates...
...By carefully combining these two principles with the standard techniques of quantum mechanics, we developed....[the means]...to do a calculation...
That calculation revealed, to our delight, that space is quantized. We had laid the foundations of...loop quantum gravity...
------end of exerpt-----
ranyart
Dec24-03, 04:35 AM
Amazing..
http://uk.arxiv.org/PS_cache/gr-qc/pdf/0312/0312103.pdf
Originally posted by ranyart
Amazing..
http://uk.arxiv.org/PS_cache/gr-qc/pdf/0312/0312103.pdf
thanks for the lead, ranyart! a Loop Quantum Cosmology
article. I will have a look. In case anyone wants the abstract:
http://uk.arxiv.org./abs/gr-qc/0312103
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology and Boundary Proposals"
invited talk at the 10th Marcel Grossman meeting July 2003
18 pages, 5 figures
edit: this turned out to be more than the title suggests.
there are 45 references (it's a mini-survey article)
a thumbnail sketch of LQG and quick review of current work
in the general theory, not limited to cosmology
followed by another concise review of current Loop cosmology
developments
then, on pages 10-12, they present their results
relating to cosmological boundary conditions
(citing and comparing work of Hartle/Hawking and of Vilenkin)
finally, pages 12-15, they discuss open questions having to do with behavior around the cosmological singularity or bounce, and graph some results of calculation around the bounce.
it is an interesting paper from several standpoints---
for instance what they choose to emphasize in the overview of the general field: on page 4 at the top, they cite two papers by Sahlmann, another two by Sahlmann/Thiemann, and one by Lewandowski/Okolow.
the view of Loop gravity is on the abstract side, through the window of "representations of the classical algebra"
the paper connects to history by citing Hartle and Hawking "Wave Function of the Universe" (1983) and Vilenkin "Quantum Creation of Universes" (1984) and pointing out the central long-standing concern with cosmological boundary conditions in the Wheeler-DeWitt quantum cosmology model.
they use the algebraic representation-theory raised earlier, carried over and specialized to cosmology, to say how and why Loop quantum cosmology differs from vintage 1980s (Wheeler-DeWitt) quantum cosmology-----different Hilbertspace, different operators, discrete spectra instead of continuous---references to the Bohr compactification and the Stone-von Neumann theorem at bottom of page 6.
then starting on page 7 they focus on the dynamics of loop quantum cosmology---the Hamiltonian constraint and difference equation that determines evolution around the bounce---and
show how the loop model matches up with Wheeler-DeWitt: in effect has the right limiting behavior (see for example Figure 1).
They also discuss ways the modern theory differs from the vintage model
(eliminates the singularity, provides for varying degrees of inflation depending on assumptions, and gives rise to somewhat different boundary conditions, or to similar ones in a different way)
this paper ties a number of threads together.
the other research currents it draws on and connects to
are as significant as the research results
it relates current research in loop cosmology with the historical antecedents (connects it to Wheeler/DeWitt/Hawking/Hartle/Vilenkin) by addressing issues that were traditionally central to earlier work
and it points up linkage between the specialized field of loop cosmology and the algebraic approach to the broader field of LQG associated with Ashtekar/Lewandowski/Thiemann/Sahlmann
the bibliography is extensive and up to date, as you might want from a "mini-survey". For example, the Husain/Winkler "On Singularity Resolution" paper that ranyart just found posted a couple of days ago on arxiv (gr-qc/0312094) is their reference 25.
an earlier brief overview of loop cosmology this year
http://arxiv.org./abs/astro-ph/0309478
is only 6 pages and less abstract
that is Bojowald's
"Quantum Gravity and the Big Bang"
it is less hilbert spacey but gives a quick idea of what
the field is about and how the calculations are done
both papers are good, just different introductions to the same thing
ranyart
Dec24-03, 04:05 PM
Marcus I know someone asked for a good link for loop Quantum Gravity?
I presume you have this link somewhere?..if so I can always delete it:
http://arxiv.org./abs/gr-qc/0306008
I will post just to the Abstract as I automatically link directly to pre-print papers, but maybe the abstract is more usual practice.
Originally posted by ranyart
I presume you have this link somewhere?..if so I can always delete it:
http://arxiv.org./abs/gr-qc/0306008
"Cosmological applications of loop quantum gravity"
I know the paper but I didnt have the link handy, not handy to this thread anyway. Thanks for mentioning it. Bojowald co-authored that with Hugo Morales-Tecotl, in Mexico City. It's good because it is introductory, part of a loop gravity seminar taught for undergrad and grad students. I read somewhere that Rovelli was Morales-Tecotl's thesis advisor, which makes me think that Morales-Tecotl is also a young person too, like Bojowald----recent PhD or recent postdoc.
I will post just to the Abstract as I automatically link directly to pre-print papers, but maybe the abstract is more usual practice.
Whichever you prefer! We can provide links to either abstract and full text. I don't know that one is more useful or usual than the other. I always look at the abstract first because a long PDF download ties up my computer and the abstract tells me how many pages.
Right now I feel a bit to lazy to bother but probably all these links should be gathered in a list-----or two lists: one for the full theory and one for the specific application to cosmology. It gets tedious playing librarian but it is actually easier than having to go through piles of paper on my desk. Thanks again for contributing these good links!
Loop gravity is a planck-scale theory and planck units have a special place in it. This month the National Institute of Standards and Technology (NIST) posted new values for the basic planck units
http://physics.nist.gov/cuu/Constants/
choose "universal" from the menu to find (among other things) the 2002 CODATA recommended values of
planck mass
planck length
planck time
planck temperature
the uncertainties have been reduced by an order of magnitude since
the values of planck units were posted in the 1998 CODATA set.
Maybe this is no big deal but it is nice that the natural units for Loop Gravity are gradually beginning to look more like a recognized system of units
A good article on timekeeping, discussing GR effects on the GPS
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
Giovanni Amelino-Camelia, Jerzy Kowalski-Glikman, and two others
"Phenomenology of Doubly Special Relativity"
dated 30 December 2003 (recent)
about 22 pages
http://arxiv.org/gr-qc/0312124
Giovanni A-C is the most eminent person in quantum gravity phenomenology and I believe the fastest riser is Jerzy K-G.
QG Phenomenology is a hot field with a lot of recent papers---both theoretical and observational. The theoretical part says what are the various quantum gravity models and what (in the case of those models that actually predict and can be tested by possible observation) do they predict and how---with planned space observatories etc---can they be tested.
Like pruning a tree, the observationalists can do the loop gravity/spin foam theorists a favor by chopping off the bad branches
(that actually make testable predictions but the predictions are wrong). So there is growing interest and visibility for this QG Phenomenology business.
And so when Giovanni A-C and Jerzy K-G get together on a paper and give the latest word on the subject it is apt to be worth paying some attention. So I posted it. I thought it was.
It is probably time to gather the links in this "surrogate sticky" thread into a single post----there are enough links now so they are too spread out
Loop Gravity looks like it going to have an active year in 2004,
getting progressively more visible and well-established, with even some recognizeable prospects for testing (phenomenology).
Here are some straws in the wind for 2004.
Smolin's January "Scientific American" article
Rovelli's book "Quantum Gravity" now at Cambridge University Press (but the 30 December 2003 draft is still online)
a spate of conferences and symposia:
Mexico City in January (loop/foam)
Polish Winterschool in February (quantum gravity phenomenology, DSR)
Marseille in May (loop/foam)
Dublin in July (the whole range of gen rel)
How broad a range should "Loop Gravity" cover?
It is actually a fleet of theories being developed which share
two key characteristics
These were underscored in Smolin's article and can be abbreviated DI-BI (diffeomorphism invariance-background independence)
All these formulations of quantum gravity attempt to quantize General Relativity and preserve these two key features of the original 1915 theory. I will quote Smolin's short description of DI and BI in a moment.
The distinction between loop and spin foam approaches has become somewhat artificial. They have always been two parts of a single enterprise, and Livine's 2003 thesis (Boucles et Mousses de Spin en Gravite Quantique) showed how to bridge the formal divide. As can be seen from the titles and programs at the various conferences, one no longer discusses loop as distinct from foam---instead there is apt to be a unified "loop/foam" conference, or loop/foam survey talk, or session of talks.
From a historical perspective, the main thing Loop Gravity does (and and stringy theories do not) is to actually quantize the theory of General Relativity itself and, in doing so, retain the essential features DI and BI, which stringy models lack. Accordingly the main criticism of the Loop Gravity approach(es) by string folk is to insist that attempts to retain the Background Independence and Diffeo-Invariance features of Relativity are doomed to fail (in essence because quantizing GR has so-far proven difficult.) The essential features of the classical 1915 theory are declared to be too radical---General Relativity must be somehow wrong and in need of replacement by a theory which on the one hand predicts the same numbers but on the other hand has room for absolute space and time---a fixed background, a uniform flow of time.
DI and BI actually imply that continuous time does not exist at planck scale. In Bojowald's Loop Quantum Cosmology papers, for instance, there is no time coordinate. The (quantized) scale-factor of the universe is used as a clock----its eigenvalues are the ticks of the clock---it is meaningless to ask if they are "uniformly spaced"----the progress of the big bang or bounce expansion proceeds in quantized steps--the size of the universe is its own clock.
Quantizing GR means doing physics without time. Or at least with a quantized time as Bojowald does cosmology. This indeed is radical and apparently has many people in a state of denial if not outright horror ("This can't be right!")
As Rovelli says, his book is as much about time as about gravity. The quantization of time (or disappearance of continuous time coordinates at the quantum level) was not something anyone "put in by hand". It arose on its own accord from taking GR seriously and quantizing it by standard approaches.
Well, I should quote Smolin about what DI and BI mean. I will post this now and edit in the quotes later.
---------Smolin SciAm quote----
In particular we kept two key principles of general relativity at the heart of our calculations.
The first is known as background independence. This principle says that the geometry of spacetime is not fixed. Instead the geometry is an evolving, dynamical quantity. To find the geometry, one has to solve certain equations that include all the effects of matter and energy. Incidentally, string theory, as currently formulated, is not background independent; the equations describing the strings are set up in a predetermined classical (that is, nonquantum) spacetime.
The second principle, known by the imposing name of diffeomorphism invariance, is closely related to background independence. This principle implies that, unlike theories prior to general relativity, one is free to choose any set of coordinates to map spacetime and express the equations. A point in spacetime is defined only by what physically happens at it, not by its location according to some special set of coordinates...
...By carefully combining these two principles with the standard techniques of quantum mechanics, we developed....[the means]...to do a calculation...
That calculation revealed, to our delight, that space is quantized. We had laid the foundations of...loop quantum gravity...
------------end quote----------------
In an earlier post on this thread I gave the program list for the 40th annual Polish Winterschool of Theoretical Physics, but I apparently didnt give a link
http://www.ws2004.ift.uni.wroc.pl/html.html
The tradition is every winter to choose a topic in Theoretical Physics and get together the world's top people at a Polish ski resort for a couple of weeks of tutorials, seminars, and talks on new research.
It is at Ladek Zdroi, a spa in SW Poland on the Czech border in the Sudeten mountain range.
The first winterschool was in 1964. I guess detente was part of the aim of getting scientists together from east and west, or maybe just good science.
This year the topic chosen is "Quantum Gravity Phenomenology".
That mostly means loop/spin foam/doubly special relativity stuff. Does not seem to be much in way of stringy phenomenology because there doesnt seem to be much testable stringy prediction.
But notice that E. Alvarez is on the program. He is a string theorist who presented a oft-cited wake-up paper "Loops versus Strings" at a conference of string (and other HEP) people a couple of years ago.
Also notice the central role of Jerzy Kowalski-Glikman, who is covering Doubly Special Relativity. Involving a bending of Lorentz symmetry, DSR seems to be taking a prominent place in quantum gravity phenomenology.
What is the key idea in DSR? I will try a separate post on that.
The key idea of DSR is to repeat the success of 1905
in an analogous situation
Lorentz transformations of 1905 SR look just like square old Galilean frame transformations but "bent" slightly by a factor which is ordinarily very close to one except at very high speeds.
So you take the block of numbers you would have used in a Galilean and tweak slightly by a factor sqrt(1 - beta2), and then it turns out that a certain physical quantity (c, the Planck unit of speed) is the same in all frames, that is, is unchanged by the new "deformed" Galilean transformations.
In DSR you tweak the transformation matrix even a bit more and you get that TWO physical quantities are unchanged, not only the Planck speed unit, c, is invariant but also the planck energy unit.
We have to look at the situations in 1905 and now about 100 years later. In 1905 they had square Galilean frame change matrices and they noticed that Maxwell equations predicted a definite speed for EM radiation. So they had two choices
1. there was a preferred frame ("aether") that the equations worked in and they didnt work in other frames ("moving observers")
2. there was no preferred frame ("Galilean relativity") and Maxwell worked in all the frames you could transform to with a straight Galilean framechange. But then there was something that should be traveling the same speed in all frames!!!! Paradox. So they tweaked, or bent, or "deformed" the Galilean symmetries matrices slightly in a way that wouldnt be noticeable at small boosts, in other words with small everyday speed changes.
Lorentz and Poincare saw how, but failed to take it seriously, so Einstein eventually did and gets the credit.
Now in 2004 we have Lorentz framechange matrices---they look like Galilean in the everyday cases and are "deformed" or maybe one should say "subtley adjusted" so that the planck unit speed is invariant.
And we have realized that there are other natural units BESIDES the natural unit of speed, namely there is a natural unit of energy Ep. This is a gateway to planck scale where physics is apt to be a good deal different and there is a growing realization that all observers should probably see the same Ep. Or wait, there is still the preferred frame or "aether" possibility.
1. there could be a preferred frame "breaking Lorentz symmetry" as they say, then we dont have to adjust the matrices, and whatever is true is only really true in the universe's preferred frame---what you see from other perspectives deviates more or less from what you would be seeing in the one true frame. This is boring.
(but when people do "observational tests of quantum gravity" it is this Lorentz symmetry breaking hypothesis that they are really testing nowadays)
2. DSR is the other alternative, where there is no preferred frame, no "Lorentz symmetry breaking". You subtley adjust the transformations so they leave TWO rather than only one of the planck units invariant.
Basically here I am just repeating the account given in this latest DSR paper
"Phenomenology of Doubly Special Relativity"
by Giovanni A-C, Jerzy K-G, and two other people
http://arxiv.org/gr-qc/0312124
There is no sticky for links to loop gravity source material. So this thread can serve as a surrogate. This post gathers links from several earlier posts, and shortens the comments. These links include some that I found and some that other PF posters have contributed, to whom thanks!
The term "Loop Gravity" is used for a broad range of background-independent approaches to quantizing general relativity. Rovelli briefly discusses "the name of the theory" on page (xvi) of his new book. The name "loop" is something of an accident because current approaches are not so much concerned with loops. But no one has come up with a designation that includes spin foams and the various models based on spin networks and is any more convenient.
The main things the new approaches seem to have in common is that they emerge from General Relativity (rather than Particle Physics) and that they aren't string/brane theories.
A kind of merging among topological quantum field theory ("TQFT") and non-commutative geometry (especially because of the Cosmological Constant) and spinfoams and (Lorentzian spin network-based) Loop Gravity seems to be in progress. In another direction Loop Gravity seems to be connecting up with Doubly Special Relativity (DSR). A way has been found to do spin network analysis with non-compact groups---using SL(2,C) for gauge instead of SU(2). It seems too early to judge which of these trends are significant in the long term, but it may help to keep some of the links handy for reference.
Rovelli just posted the 30 December 2003 draft of his book "Quantum Gravity". The PDF file is at his homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html.
The book is around 350 pages long and takes a few (like ten?) minutes to download and convert.
To download the 30 December 2003 draft of the book directly:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf
The SPIRES database on citations is often handy. There is a topcited list for the smaller series GR-QG (general relativity and quantum gravity)here:
http://www.slac.stanford.edu/library/topcites/top40.2002.E.html
And the more extensive series HEP-TH here:
http://www.slac.stanford.edu/library/topcites/topcites.review.2002.html
We were discussing stuff from Livine's thesis in this and another thread. Here is Livine's thesis. He does a lot with explicitly covariant---SL(2,C)-style---spin networks and makes an explicit bridge from LQG to Lorentzian spinfoams.
http://arxiv.org/gr-qc/0309028
Girelli and Livine have come out with a paper about quantizing speed.
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032
Ichiro Oda has posted "A Relation Between Topological Quantum Field Theory and the Kodama State"
http://arxiv.org/hep-th/0311149
Daniele Oriti's thesis is out
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"
Smolin and Starodubtsev
"General Relativity with a topological phase: an action principle"
http://arxiv.org/hep-th/0311163
Karim Noui and Philippe Roche
"Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
The cosmological constant occurs in a number of recent quantum gravity papers, for instance the one by Girelli/Livine.
-------Quantum Gravity Phenomenology---------
two recent papers:
Giovanni Amelino-Camelia, Jerzy Kowalski-Glikman, Gianlucca Mandanici, and Andrea Procaccini
"Phenomenology of Doubly Special Relativity"
http://arxiv.org/gr-qc/0312124
dated 30 December 2003
Jerzy Kowalski-Glikman
"Doubly Special Relativity and quantum gravity phenomenology"
http://arxiv.org/hep-th/0312140
dated 12 December 2003
other fairly recent ones:
Jerzy Kowalski-Glikman and Sebastian Nowak
"Doubly Special Relativity and de Sitter space"
http://arxiv.org/hep-th/0304101
dated 11 October 2003
M. Daszkiewicz, K. Imilkowska, J. Kowalski-Glikman
"Velocity of particles in Doubly Special Relativity"
http://arxiv.org/hep-th/0304027
dated 3 April 2003
---------Loop Quantum Cosmology-------
as a background reference for classical (non-quantum) cosmology:
Charles Lineweaver
"Inflation and the Cosmic Microwave Background"
http://arxiv.org/astro-ph/0305179
dated 12 May 2003
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology and Boundary Proposals"
http://arxiv.org/gr-qc/0312103
dated 23 December 2003
Martin Bojowald
"Quantum Gravity and the Big Bang"
http://arxiv.org./astro-ph/0309478
dated 17 September 2003, briefly summarizes how
LQG can serve to cure the big bang singularity and
motivate inflationary expansion. Short and less technical
than the other two papers.
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology, Boundary Proposals, and Inflation"
http://arxiv.org/gr-qc/0303072
dated 19 March 2003
-------recent conferences------
Strings meet Loops (Albert Einstein Institute, MPI-Potsdam) October 2003
http://www.aei-potsdam.mpg.de/events/stringloop.html
Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm
--------upcoming conferences--------
Quantum Gravity Phenomenology, (40th annual Polish Winterschool in Theoretical Physics) February 2004
http://www.ws2004.ift.uni.wroc.pl/html.html
Loop/SpinFoam Conference (Marseille) May 2004
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?03Aug.1
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?10
General Relativity Conference (Dublin) July 2004
more annoucements at
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference
----------fundamental constants, planck units, time-keeping-------
In December 2003, the National Institute of Standards and Technology (NIST) posted new CODATA recommended values for the basic planck units
http://physics.nist.gov/cuu/Constants/
choose "universal" from the menu to find (among other things) the recommended values of
planck mass
planck length
planck time
planck temperature
A 1997 article on timekeeping, discussing GR effects allowed-for in the GPS
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
------projected observational means for testing quantum gravity------
Floyd Stecker
"Cosmic Physics: the High Energy Frontier
http://arxiv.org/astro-ph/0309027
dated September 2003
Floyd Stecker is at the NASA Goddard Laboratory for High Energy Astrophysics and something of a world-class expert on gamma-ray bursts and cosmic ray research. It seems that man-made accelerators are not big or powerful enough to be very effective in providing empirical guidance to quantum gravity theory. So what is apt to take the place of accelerators is high energy astrophysics. Stecker discusses the various earth-based and orbital instruments, currently operating, or under construction, or planned, or proposed, and the kind of data becoming available. Among many other things he discusses GLAST, planned to start operating 2006, which, if there are tiny energy-dependent differences in speed among gamma-ray-burst photons, may be able to detect same. Also discusses neutrino observation.
Loop Gravity is a theory under construction, so Rovelli's
Chapter 7 "Dynamics and Matter", pages 199-212, is describing
work in progress. Section 7.1 discusses the hamiltonian and
7.2 the inclusion of matter.
Table 7,1 on page 208 gives "Quantum numbers of the spin network states for gravity and matter."
The graph \Gamma with N nodes and L links, is like a big quantum number describing adjacency. Nodes correspond to regions or chunks of space and links to the surfaces between those volumes
\Gamma adjacency
i_n volume of node n
j_l area of surface l
F_n number of fermions at node n
S_n number of scalars at node n
w_n field strength at node n
k_l electric flux across surface l
In section 7.2.4 "The quantum states of space and matter", notation is given for |s> a quantum state of space and matter.
As one has come to expect, quantities like volume/area, fieldstrength/flux appear as irreducible representations/intertwiners.
More details about this on pages 208 and 209.
"thus we can write
|s> = |\Gamma,i_n,j_l,F_n,S_n,w_n,k_l>
This state describes a quantum excitation of the system that has a simple interpretation as follows. There are N regions n, that have volume and where fermions and Higgs scalars can be located. These are separated by L surfaces l, that have area and are crossed by flux of the (electric) gauge field. The quantum numbers are related to observable quantities as in Table 7.1. This completes the definition of the kinematics of the coupled gravity+matter system."
the next section, 7.3 "Matter: dynamics and finiteness" writes the hamiltonian compounded of four pieces.
H = H_{Einstein} + H_{YangMills} + H_{Dirac} + H_{Higgs}
One brief exerpt from Section 7.3, "...The fact that the total hamiltonian turns out to be finite is extremely remarkable. It is perhaps the major payoff of the background independent quantization strategy on which LQG is based..."
For the finiteness result Rovelli cites "Lectures on Quantum Gravity"
http://arxiv.org/gr-qc/0210094
these are notes at the grad student level prepared by Thomas Thiemann, which are to appear in a textbook series called
"Lecture Notes in Physics" (Springer, Berlin)
and also "Quantum Gravity as the Natural Regulator of the Hamiltonian Constraint of Matter Quantum Field Theories"
http://arxiv.org/gr-qc/9705019
It is a bit of luck that quantizing space makes the ordinary infinities of QFT go away:"...the ultraviolet divergences of ordinary quantum field theory can be directly interpreted as a consequence of the approximation that disregards the quantized, discrete, nature of quantum geometry..."
Labguy posted this in the Astronomy forum, under the heading "Albert is Still Looking Good". It bears on Loop Gravity so I'll copy it and add it to our links.
-----------
Recent release, passed through Ned Wright's Cosmology site:
http://www.astro.ucla.edu/~wright/cosmolog.htm#04Dec03
"A Double Radio Pulsar.
9 Jan 2004 - Lyne et al. (2004, Science in press) gives the details about PSR J0737-3039 A&B, the double radio pulsar binary with a relativistic orbit, previously reported as a single pulsar in a binary system on 4 Dec 2003. The mass of the 23 millisecond pulsar (A) is 1.337+/-0.005 M(sun) while the mass of the 2.8 second pulsar (B) is 1.250+/-0.005 M(sun). There are now 6 measured constraints on (MA,MB) and the values given above are consistent with all 6 constraints, providing a stringent test of General Relativity which GR passes with flying colors".
And:
"An amazing binary pulsar.
4 Dec 03 - Nature today published a paper (Burgay et al. 2003, Nature, 426, 531-533) about a newly announced millisecond pulsar, PSR J0737-3039, in a relativistic binary system. Radio pulsars are neutron stars (NS) which have a mass of about 1.4 solar masses and a radius of 10 km, magnetic fields billions to trillions of times larger than the Earth's magnetic field, and spin periods from 1.6 milliseconds to several seconds. PSR J0737-3039 is orbiting another neutron star every 2.4 hours and the two stars will merge in 85 Myr due to gravitational radiation. Hence LIGO will have many more detectable NS+NS merger events based on the statistics of two objects instead of the previous estimate based solely on the one merging binary pulsar PSR B1913+16 known earlier.
...the relative motion of the two stars is 14,000 km in 22 seconds,..."
-----------
The relevance of continued observational confirmation of GR to Loop Gravity is that the theory is distinguished by treating General Relativity (with its basic assumptions of background independence and diff-invariance) as a fundamental theory to be quantized.
By contrast certain alternatives to Loop Gravity do not treat GR as fundamental. Instead it is treated as the low-energy limit of some hoped-for but still unknown theory not requiring GR's basic assumptions (such as background independence).
The Nature article also points out that having found another binary system due to merge in the (astronomically) near future----85 million years in this case---is suggestive that the merger of a pair of neutron stars (the kind of thing LIGO would like to detect gravity waves from) may be a more frequent event than was estimated earlier. LIGO will itself be testing, and potentially offering further confirmation of, General Relativity.
The source article for the binary pulsar was posted a couple of
days ago, 7 January 2004.
http://arxiv.org/astro-ph/0401086
it is 21 pages
and discusses the implications for testing GR in detail
the article is information rich, with plenty of
tables (of orbit parameters etc.) and figures.
6 binary pulsars are known
this one was discovered by an Australian dish (Parkes)
in 2003
several different tests (testing several different predictions)
of GR are possible as more observations of the binary system
accumulate
It is a lucky find.
In another thread the question of demographics came up again.
Numbers of papers, or even numbers of "blockbuster" papers that get lots of follow-up citations, dont necessarily mean all that much but the issue gets raised now and then so we should have some kind of objective data. There is a small and increasing output of papers
in Loop Gravity:
I just went to arxiv.org "Search Physics Archives" page and
put in [ABS = loop quantum gravity]OR[ABS = spin foam]OR[ABS = loop quantum cosmology] since 2000 and it gave me
these numbers of papers:
2000 46
2001 48
2002 64
2003 70
These are the preprints at the archive that have somewhere in their ABSTRACTS either the words loop quantum gravity, or the words spin foam, or the words loop quantum cosmology.
--------------
Although I'm not especially interested in string/brane theories, some people seem interested in comparisons so here's the same numbers for
[ABS = string]OR[ABS = brane]OR[ABS = M-theory]
2000 1457
2001 1496
2002 1500
2003 1265
That is, those where the abstract summary of the paper has in it somewhere the word string, or the word brane, or the word M-theory.
The numbers speak for themselves. There's more to say about the current state of research in Loop Gravity---hope to get back to this later today.
A guy at Stanford-SLAC puts out a list of the top-cited papers each year in each archived category. He hasnt done it for 2003 yet, so his most recent list is as of end 2002.
I was interested in the most-cited papers in the category gr-qc
"General Relativity and Quantum Gravity"
http://www.slac.stanford.edu/library/topcites/2002.gr-qc.1.shtml
and, in particular, in RECENT papers (dated 2000, 2001 and 2002) in that category. So here's the "Top Ten" list for that category, with the older (pre-2000) papers winnowed out. Of course most string papers are over in the hep-th, high energy physics-theory, category. I am focussing just on gr-qc here.
---------------------
57 citations
RELATIVITY IN SPACE-TIMES WITH SHORT DISTANCE STRUCTURE GOVERNED BY AN OBSERVER INDEPENDENT (PLANCKIAN) LENGTH SCALE
By Giovanni Amelino-Camelia (Rome U.).
Published in Int.J.Mod.Phys.D11:35-60,2002 [PS file for arXiv: gr-qc/0012051]
54 citations
CLASSICAL BLACK HOLE PRODUCTION IN HIGH-ENERGY COLLISIONS
By Douglas M. Eardley, Steven B. Giddings (UC, Santa Barbara).
Published in Phys.Rev.D66:044011,2002 [PS file for arXiv: gr-qc/0201034]
36 citations
INTRODUCTION TO MODERN CANONICAL QUANTUM GENERAL RELATIVITY
By Thomas Thiemann (Potsdam, Max Planck Inst.). [PS file for arXiv: gr-qc/0110034]
31 citations
THE CONFRONTATION BETWEEN GENERAL RELATIVITY AND EXPERIMENT
By Clifford M. Will (Washington U., St. Louis).
Published in Living Rev.Rel.4:4,2001 [PS file for arXiv: gr-qc/0103036]
28 citations
EXTENDING THE LIFETIME OF 3-D BLACK HOLE COMPUTATIONS WITH A NEW HYPERBOLIC SYSTEM OF EVOLUTION EQUATIONS
By Lawrence E. Kidder, Mark A. Scheel, Saul A. Teukolsky (Cornell U., Radio. Space Res. Ctr.).
Published in Phys.Rev.D64:064017,2001 [PS file for arXiv: gr-qc/0105031]
28 citations
GEOMETRY AND DYNAMICS OF THE BRANE WORLD
By Roy Maartens (Portsmouth U.). [PS file for arXiv: gr-qc/0101059]
27 citations
AN ALTERNATIVE TO QUINTESSENCE
By Alexander Yu. Kamenshchik (Landau Inst. & Landau Network Centro Volta), Ugo Moschella (Insubria U., Como & INFN, Milan), Vincent Pasquier (Saclay).
Published in Phys.Lett.B511:265-268,2001 [PS file for arXiv: gr-qc/0103004]
23 citations
QUANTUM GRAVITY: A PROGRESS REPORT
By S. Carlip (UC, Davis).
Published in Rept.Prog.Phys.64:885,2001 [PS file for arXiv: gr-qc/0108040]
21 citations
QUANTUM GEOMETRY OF ISOLATED HORIZONS AND BLACK HOLE ENTROPY
By A. Ashtekar (Penn State U. & Santa Barbara, KITP), John C. Baez (UC, Riverside & Penn State U.), Kiriil Krasnov (UC, Santa Barbara & Santa Barbara, KITP).
Published in Adv.Theor.Math.Phys.4:1-94,2000 [PS file for arXiv: gr-qc/0005126]
19 citations
GENERALIZED LORENTZ INVARIANCE WITH AN INVARIANT ENERGY SCALE
By Joao Magueijo (Imperial Coll., London), Lee Smolin (Perimeter Inst. Theor. Phys. & Waterloo U.).
Published in Phys.Rev.D67:044017,2003 [PS file for arXiv: gr-qc/0207085]
---------------------
obviously there are different ways of gauging activity and what the
most-cited papers in a field are and what the cut-off should be for "recent" work, but here is one measure: the number of other papers that cited the given paper in their references. At least it gives an idea of who the people are that write the papers that get most often cited in this division of the archive (and some slight indication as to what topics are of lively current interest)
Loop Gravity-watchers will be familiar with Etera Livine, Laurent Freidel, and David Louapre. David has been a sometimes poster at PF and both Livine and Freidel are among those whose Lorentzian spinfoam papers we discussed. Freidel and Louapre (with help from Livine) are preparing a series of papers on 2+1 dimensional quantum gravity of which the first is now available.
The ancestor of this series is
Laurent Freidel, “ A Ponzano-Regge model of Lorentzian 3-Dimensional gravity ”, Nucl. Phys. Proc. Suppl. 88 (2000) 237-240 , gr-qc/0102098.
The announced series of four papers, of which the first was just posted:
L.Freidel and D. Louapre,"Ponzano-Regge model revisited I: Gauge fixing, observables and interacting spinning particles"
http://arxiv.org./hep-th/0401076
L.Freidel and D. Louapre, “Ponzano-Regge model revisited II: Mathematical aspects; relation with Chern-Simons theory, DSU(2) quantum group and link invariant". To appear.
L.Freidel, E. Livine and D. Louapre, “Ponzano-Regge model revisited III: The Field Theory limit”. To appear.
L.Freidel and D. Louapre, “Ponzano-Regge model revisited IV: Lorentzian 3D Quantum Geometry”. To appear.
------other papers of possible related interest-------
L. Freidel and D. Louapre, “Diffeomorphisms and spin foam models,” Nucl. Phys. B 662, 279
http://arxiv.org/gr-qc/0212001
K. Noui and A. Perez “Three dimensional loop gravity coupled to point particles”, to appear.
----------miscellaneous-----
Links to a couple of things at PF here:
interesting post from "notevenwrong" about testability
http://www.physicsforums.com/showthread.php?s=&postid=128657#post128657
I dont think there's any need for any of us to justify
or explain why we are interested in the kinds of physics that
we're interested in.
So I dont normally bother to explain why I like to
report current developments in Loop Gravity or discuss
new papers with anyone who might be interested. But
here I was being challenged to explain my excitement and
I talked about why it's interesting to me personally
(doesnt have to be to you, but you may be enthused by
some of the same things). It's a ramble though.
http://www.physicsforums.com/showthread.php?s=&postid=128783#post128783
The MIT Theoretical Physicist Frank Wilczek
gave the keynote address at a December 2003
Conference on space and particle physics
"SpacePart03"
the text and one slide from his address has just been posted
http://www.arxiv.org/abs/astro-ph/0401347
He sketches the current situation. Wilczek is always
being asked to deliver Particle Physics overviews and keynotes (at the opening of a new accelerator or the closing ceremony for an old one), probably because he is most clearsighted senior theoretician they've got.
His 3 article series "Scaling Mount Planck" in Physics Today
issues back in 2001 and 2002 was a masterful survey of
big questions in theoretical physics (well, I was impressed).
Here he is doing it again, so its worth a look.
Basically it is a simple message. There are now two "standard models"
The one of particle physics and the one of cosmology.
Each has a certain number of exogenous parameters
IIRC he identifies four independent inputs to standard model
comology. And he describes progress and prospects as regards
particle physics explaining certain of the inputs to cosmology.
You can guess a lot of what the elder statesman is going to say
but no other voice has the clarity and authoritiy AFAIK.
I note he puts General Relativity in a good light (something particle physicists may eventually have to believe in on its own terms, in spite of the different underlying spacetime concept). He specifically mentions Diffeomorphism Invariance (calling it by Einstein's name for it: "general covariance")
"General relativity manifestly provides a beautiful, conceptually driven theory of gravity. It has scored many triumphs, both qualitative (big bang cosmology, black hole physics) and quantitative (precession of Mercury, binary pulsar).
The low-energy effective theory of gravity and the other interactions is defined algorithmically by the minimal coupling prescription, or equivalently by restricting to low-dimension operators. In this context, “low” means compared to the Planck energy scale, so this effective theory is very effective indeed.
As in the gauge sector, symmetry---here, general covariance---greatly constrains the possible couplings, bringing us down to just two relevant parameters. Almost all the observed phenomena of gravity are described using only one of these parameters, namely Newton’s gravitational constant. We are just now coming to accept that the other parameter, the value of the cosmological term, plays an important role in describing late-time cosmology..."
This is just a brief exerpt and his forte is his sense of proportion so you have to read the whole thing to get a feel for the relative importance he gives to things.
Earlier articles (June, Novemember 2001, August 2002)
http://www.physicstoday.org/pt/vol-54/iss-6/p12.html
http://www.physicstoday.org/pt/vol-54/iss-11/p12.html
http://www.if.ufrgs.br/~jgallas/wilczek.html
--------marginal note-----
At the same December 2003 space and particle physics conference where Wilczek gave the keynote there were a couple of talks on "Lorentz Symmetry Violation".
a paper on the subject by one of the partcipants
http://arxiv.org/hep-th/0312310
have to follow up on this later, got to run now
(Lorentz violation active research topic, relates to DSR)
Four Koreans I never heard of before just posted a DSR paper
http://arxiv.org/gr-qc/0401078
"Propagation of Light in Doubly Special Relativity"
It draws a on work by Jerzy Kowalski-Glikman and by Joao Magueijo, in particular on a paper they call KMM (for Kimberly, Magueijo, Medeiros)
http://arxiv.org/gr-qc/0303067
The authors are Kim, Kim, Rim, and Yee.
They are from various universities in Korea.
The timing of this is remarkable.
There is a Loop Gravity conference in Mexico City scheduled for next week and Thiemann brings out a Loop-String paper
doing string theory related algebra in the background independent LQG fashion.
Thiemann is scheduled to give a talk at that conference.
Here is the program for the upcoming Mexico City conference
Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm
Here is the link for Thiemann's new paper, and exerpts from the abstract
http://arxiv.org/hep-th/0401172
The LQG -- String: Loop Quantum Gravity Quantization of String Theory I. Flat Target Space
We combine I. background independent Loop Quantum Gravity (LQG) quantization techniques, II. the mathematically rigorous framework of Algebraic Quantum Field Theory (AQFT) and III. the theory of integrable systems resulting in the invariant Pohlmeyer Charges in order to set up the general representation theory (superselection theory) for the closed bosonic quantum string on flat target space.
....solve some of the major puzzles of string theory such as the cosmological constant problem. The solution presented in this paper exploits the flatness of the target space in several important ways. In a companion paper we treat the more complicated case of curved target spaces.
I was not aware until recently that this year Cambridge University Press is expected to bring out two graduate-level LQG texts.
I know Rovelli's 350-page "Quantum Gravity" is at press and expected out this year.
But it also seems Cambridge is publishing a 300+ page book by Thiemann called "Modern Canonical Quantum General Relativity". This is listed as "at press" in the references of a recent paper by the author, with projected publication date 2004.
I believe that a draft of Thiemann's "Modern...Quantum...Relativity" may be online at
http://arxiv.org/gr-qc/0110034
In draft, the title also had the word "Introduction" but that may have now been dropped for the sake of brevity.
Rovelli has an online draft of "Quantum Gravity" at his website---the link was given in several earlier posts.
These two books would together form the basis of a hefty year course in grad school. They complement each other to some extent: Rovelli's is more conceptual and discursive. It discusses philosophical foundations and presents many of the ideas historically. I like the well-chosen graphic examples used to illustrate the ideas. By comparison, the exposition in Thiemann's book is predominantly abstract mathematics, going into mathematical detail with thoroughness and rigor.
I should mention that the Berlin publisher Springer Verlag has also brought out a Loop Gravity textbook as part of its "Lecture Notes" series. This is a shorter, and more introductory, set of notes by Thiemann called
"Lectures on Loop Quantum Gravity".
A draft of this is online at
http://arxiv.org/gr-qc/0210094
As far as I know these three books, all appearing at about the same time, are the first hardcopy textbooks for the new field of Loop Gravity. In that sense 2004 looks like a landmark year in the development of the subject.
Back in the middle of December I posted the organizer's list of topics (for talks and discussion) for the Loop symposium being held Jan 30 thru Feb 1 in Mexico City.
At that time I did not have a list of the talks or the people giving them. You might be interested. These are most of the people active in Loop Gravity research and many of the talks and the discussion sections center around delicate or key unresolved issues. It is clearly a symposium for people inside the field to share their latest ideas and results, "in house" so to speak. So here is the program:
[I made topics bold or caps for emphasis, so its easier to scan]
--------
friday
Welcome: Alejandro Corichi 1:00-1:10
Session: PHENOMENOLOGY
Chair: Hugo Morales
Sudarsky <End of New Ether>
Discussion
Freidel <Imprints of Planck scale structures and DSR>
Discussion
Session: SEMI-CLASSICAL Issues
Chair: Luca Bombelli
Sahlmann: <Brief survey of available frameworks>
Discussion
Ashtekar:<Physical Applications>
saturday
Session: SPIN FOAMS
Chair: Jose Antonio Zapata
Perez <Status Report>
Discussion
Crane <Beyond Barrett-Crane models>
Discussion
Session: HAMILTONIAN CONSTRAINT
Chair: Karim Noui
Freidel<Relation between spin-foams and canonical gravity>
Discussion
Ashtekar <viability of the Thiemann scheme>
Discussion
Thiemann <The Phoenix project>
Discussion
sunday
Conceptual Issues
Chair: Alejandro Corichi
Jorge Pullin <Future directions that will maximize impact>
Possible topics of discussion:
*Prospects for discrete formulations
*Status of non-compact gauge groups: Integration theory
*Role of supersymmetry
*Observables from particles in 2+1 gravity
*Quantum cosmology and observations
*Where is physics in spin-foams?
*Is Kodama state viable?
*Causality in quantum gravity
* ...
* ...
Abhay Ashtekar <Closing remarks>
LIST OF PARTICIPANTS:
A. Ashtekar (Penn State)
L. Bombelli (Mississippi)
L. Crane (Kansas State U.)
A. Corichi (UNAM. Mexico)
L. Freidel (Perimeter)
J.M. Garcia-Islas (CIMAT, Mexico)
F. Girelli (Perimeter)
E. Livine (Perimeter)
H. Morales-Técotl (UAM-I, México)
K. Nuoi (Penn State)
A. Perez (Penn State)
J. Pullin (Louisiana)
H. Sahlmann (Penn State)
D. Sudarsky (UNAM, Mexico)
T. Thiemann. (Perimeter)
J.A. Zapata (UNAM, Mexico)
---------
Given that marcus wants this thread like a sticky for new papers, I will post this one:
"Comparison of area spectra in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0401110
Abstract:
We compare two area spectra proposed in loop quantum gravity in different approaches to compute the entropy of the Schwarzschild black hole. We describe the black hole in general microcanonical and canonical area ensembles for these spectra. For one of these spectra - the equally-spaced spectrum - we show in light of a proposed connection of the black hole area spectrum to the quasinormal mode spectrum that this spectrum is completely consistent with this connection. This follows {\em without} requiring a change in the gauge group of the spin degrees of freedom in this formalism from SU(2) to SO(3).
Originally posted by meteor
Given that marcus wants this thread like a sticky for new papers, I will post this one:
"Comparison of area spectra in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0401110
One of the authors, G. Gour, has co-authored with Jacob Bekenstein and the approach to the area spectrum taken here goes back to
a paper of Bekenstein's:
http://arxiv.org/abs/hep-th/0107045
----------quote from abstract------------
The Case for Discrete Energy Levels of a Black Hole
Authors: Jacob D. Bekenstein
Comments: Invited talk at "2001: A Spacetime Odyssey", inaugural conference of the Michigan Center for Theoretical Physics, May 22-25, 2001, 11 pages, to appear in the proceedings published by World Scientific Publishing
Journal-ref: 2001: A Spacetime Odyssey, eds. M. J. Duff and J. T. Liu, (World Scientific Publishing, Singapore 2002), pp. 21-31
The adiabatic invariant nature of black hole horizon area in classical gravity suggests that in quantum theory the corresponding operator has a discrete spectrum. I here develop further an algebraic approach to black hole quantization which starts from very elementary assumptions, and proceeds by exploiting symmetry. It predicts a uniformly spaced area spectrum for all charges and angular momenta. Area eigenvalues are degenerate; correspondence with black hole entropy then dictates a precise value for the interval between eigenvalues.
--------end quote-----
it would be extremely interesting if the equal-spaced area spectrum gains credibility. this is a wonderful paper to have on our
Loop reference shelf "surrogate sticky".
at present measurement cannot distinguish between this version and the unequally-spaced spectrum, it would seem, so theorists can pursue either
if this equal-spaced (ES) notion of the spectrum is right then the Immirzi parameter is
ln 3/3pi
just for fun I will write it in tex.
\gamma_{ES} = \frac{ln 3}{3 \pi}
this paper by G. Gour and his buddy has potential for stirring up a controversy, I suspect
as you well know, a commonly assumed value is
\gamma = \frac{ln 2}{\sqrt{3} \pi}
lumidek
Jan29-04, 11:14 AM
Dear marcus,
I am not sure why you exactly think that the idea of the discrete area (or even energy) spectrum of the black hole should be gaining credibility. It contradicts all important things that we know about the black hole, for example the thermal character of the radiation and other semiclassical calculations. These well-established insights, initiated by Hawking in the 1970s, have been amazingly confirmed by the developments in string theory in the 1990s, so there is really little doubt that they're correct.
The energy spacing comparable to the Hawking temperature does not sound too reasonable because it would essentially imply that the energy that the black hole must emit must be an integer multiple of the Hawking temperature. The real spectrum copies the nice and smooth curve by Planck.
There is no serious support for the idea of this hugely discretized spectrum - perhaps some children's toy models and many overly speculative papers (which is the polite way to describe the crackpots). The precise numbers that should determine the scaling are totally unscientific, too. You only get a log of an integer in these formulae because some people who propose it are imagining that everything in the physical world is made from bits (or "trits") - simply because their imagination does not go beyond it.
All the best
Luboš
Originally posted by lumidek
Dear marcus,
I am not sure why you exactly think that the idea of the discrete area (or even energy) spectrum of the black hole should be gaining credibility.
Dear Lubos, of course I remember what you had to say in your very first QNM paper about evenly spaced area spectrum. Seems a long time ago now.
In a PF context we need to be careful not to say "discrete area spectrum" (which includes the unequal spacing case) when we mean the "evenly spaced spectrum".
There seems to be wide agreement with the notion that discrete area spectrum is compatible with the themal character of the Hawking radiation. (Indeed I had the impression that you were also in agreement with this.)
However you and many other people have argued against the evenly spaced area spectrum for precisely the reason you mention. I believe you are in agreement here with Loop authorities like Ashtekar, Rovelli, Smolin. Back in (when was it?) 1995 when Rovelli and Smolin derived the discrete area spectrum, it was not equally spaced----spacing between eigenv. got closer and closer as area got larger.
No doubt you find it reassuring to be in such good company (on the same side of the fence as Rovelli and Smolin) on this issue[;)]
-------
But my personal view is that Jacob Bekenstein is a brilliant and original mind-----he deserves credit at a fundamental level for black hole thermodynamics, Hawking radiation, BH temperature, entropy and all that good stuff that started in the 1970s. He kicked it off.
If he says to keep an open mind and think about equal-spaced (ES) spectrum then I respect him and I am going to keep an open mind about it. Surprises sometimes happen. What if Gilad Gour (also very smart) and Bekenstein are right and there is something nobody has thought of yet that makes it compatible. Have to run, but back later.
lumidek
Jan29-04, 12:09 PM
Dear Marcus,
it's great if we agree that the evenly spaced spectrum does not seem too good.
Concerning the discrete, non-evenly-spaced spectrum, it is not a physical question whether the exponentially dense spectrum of the black hole energy is discrete or not. Of course, if the number of states (exponential of entropy) is finite, it must be discrete "in some sense".
The density of the states is increasing exponentially with a power of the mass. It is naive to think that you can determine the black hole energy with the precision comparable to this expected exponentially small spacing - simply because the width of the black hole itself is exponentially bigger (the width behaves as a power of the mass, without any exponentiating). The width essentially tells you the minimal error with which you can measure the spectrum, and this minimal error is much bigger than the required resolution to determine the character of the exponentially fine spacing.
Once you admit that the gaps in the spectrum are not huge (such as the power law gaps of the evenly spaced spectrum), there is no other physical question to ask about the character of the spectrum.
All the models that give you the super-exact energies must neglect the interactions and Hawking radiation because Hawking radiation makes black holes unstable and energies undetermined (width). Quite generally, you may be worried that neglecting Hawking radiation means neglecting quantum mechanics, and therefore all the conclusions about the quantization are internally inconsistent.
The only exception are extremal, BPS black holes that don't decay (temperature vanishes), and their energy is determined exactly by the BPS bound.
All the best
Luboš
Originally posted by lumidek
Dear Marcus,
it's great if we agree that the evenly spaced spectrum does not seem too good...
We dont agree Lubos. You indicate that you have a firm opinion that evenly spaced (ES) area spectrum doesnt seem good.
I am trying to keep an open mind.
Originally posted by lumidek
The energy spacing comparable to the Hawking temperature does not sound too reasonable because it would essentially imply that the energy that the black hole must emit must be an integer multiple of the Hawking temperature. The real spectrum copies the nice and smooth curve by Planck.
There is no serious support for the idea of this hugely discretized spectrum -
We were talking about area eigenvalue spacing.
The Gilad Gour paper does not consider "huge" gaps in the area spectrum but, on the contrary, gaps comparable in size to
the Planck area---in other words very tiny.
You may be jumping to conclusions because I mentioned Bekenstein as one who has argued for considering ES area spectrum.
See equation (3) of the Gour Suneeta paper.
Evenly spaced does not imply huge gaps, or gap-size dependent (as you suggest) on the Hawking temp. of the hole.
By your own argument in the your preceeding post if the gaps are
small like in Rovelli-Smolin spectrum (Planck area size) then one would not be able to physically tell the difference from smooth black body. The same applies to Gour-Suneeta because similarly small gaps.
lumidek
Jan29-04, 01:38 PM
Dear Marcus,
I see, sorry. I made a too quick conclusion that you share the only approach that I consider reasonable in this question.
It's great to keep an open mind, but if Hawking is correct, a lot of ideas can evaporate from a mind that is too open. [6)]
All the best
Luboš
Originally posted by meteor
... I will post this one:
"Comparison of area spectra in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0401110
Abstract:
We compare two area spectra proposed in loop quantum gravity in different approaches to compute the entropy of the Schwarzschild black hole. We describe the black hole in general microcanonical and canonical area ensembles for these spectra. For one of these spectra - the equally-spaced spectrum - we show in light of a proposed connection of the black hole area spectrum to the quasinormal mode spectrum that this spectrum is completely consistent with this connection. This follows without requiring a change in the gauge group of the spin degrees of freedom in this formalism from SU(2) to SO(3).
This is the paper by Gilad Gour and V. Suneeta (both at U Alberta
Edmunton) that Meteor called to our attention.
An objection raised earlier seems not to apply since, while the area spectrum is discrete (as usual in LQG) and evenly spaced, the spacing is microscopic (planck-scale) and not fundamentally very different from the un-evenly spaced. This paper could be important and needs a more careful look. So let's look at Gour/Suneeta equations (2) and (3)
These are the two competing formulas for the area of a surface S.
the first is in the non-evenly spaced (NS) case
A_S = 8\pi l_P^2 \gamma \Sigma \sqrt{j_n(j_n + 1)}
the second is in the evenly spaced (ES) case
A_S = 8\pi l_P^2 \gamma \Sigma (j_n + 1/2)
We are talking about a spin network state and a physical surface, S, defined by some material object. The spin network state has N edges which intersect the surface and each edge contributes a bit of area to the sum. The intersecting edges are indexed n = 1,....,N.
Oh yeah, lP2 is the planck unit of area, the square of the planck length
Equation (2) is the standard 1994 result of Rovelli and Smolin. A priori one wonders how Gour/Suneeta could possibly be challenging this by proposing a different formula for the area, namely equation (3).
Furthermore their proposal may make some people nervous since it results in a value of ln 3/3pi for the Immirzi parameter. As it happens this doesnt bother me. Some versions of LQG dont even have an Immirzi parameter: Loop theories are under construction and there is still room for variation in the parameters and even some surprises. So I am not going to dismiss this out of hand just because of some number being an unfamiliar ln 3/3 pi instead of the more usual value that has been around longer.
Anyway Meteor posted the link to this a few days ago so lets see what it is about.
common sense suggests there may be pitfalls in Gour/Suneeta approach.
the other formula goes back to 1994. why would Rovelli/Smolin have chosen their more complicated formula unless there were some reason?
but so far so good.
anybody (Meteor?) who wants to help read thru this short (7 page)
paper is invited
a couple of more links on the area issue:
Alexios Polychronakos
Area spectrum and quasinormal modes of black holes
http://arxiv.org/hep-th/0304135
Alekseev, Polychronakos, Smedbaeck
On the area and entropy of a black hole
http://arxiv.org/hep-th/0004036
Alexkseev and Smedbaeck are at the Institute for Theoretical Physics, Upsala University, in Sweden
Polychronakos is at the University of Ioannina in Greece and also the
CUNY Physics Department in the USA.
These seem to be the dangerous people we were warned about earlier. They think that, just as an atom while it is in a heat bath in equilibrium at a given temperature can radiate with a black body spectrum but by itself radiates a line spectrum, so a black hole not surrounded by radiation and therefore not in equilibrium with its surroundings may in fact radiate a line spectrum. Over much of the spectrum it would strongly resemble black body but at low frequencies it would deviate and become more obviously liney.
Polychronakos says, in "Area spectrum and...", for instance)
-------quote from page 9----------
We do not feel that this is damning. The high-frequency exponential part of the spectrum is accurately reproduced, the discreteness there being inconsequential. This is the energy range in which the photons (or other emitted particles) behave essentially like classical particles, whose scattering properties are expected to be accurately reproduced by the classical black hole metric. For frequencies close to the thermal frequency, however, the wavelength of the photons becomes comparable to the size of the black hole and they sense global properties of its geometry. Backreaction due to geometry change at emission and absorption of such photons is expected to be important, the energy of these photons being of the same order as
the energy spacing of the black hole. A deviation from ideal black-body spectrum, which assumes a fixed metric and ignores back-reaction, would seem reasonable.
-----end quote----------
If my opinion counts, (I'm only an amateur, not a high-level physicist) I don't like the idea of the equally-spaced area spectrum, because it means that practically all the contributions to the black hole area comes from edges carrying a representation of 1. Would be desirable to know why the representations=1/2 are excluded from puncture the black hole area
Originally posted by meteor
If my opinion counts..
you found the paper and put it in for discussion
so your opinion doesnt count???
Gour/Suneeta cite Polychronakos (hep-th/0304135)
which I am finding the easiest to read and the
most helpful. (the subject is new to me, maybe to
you as well)
On page 8 Polychronakos says
"To summarize...if the standard counting formula for states (6) is
assumed, then the equidistant area spectrum as proposed in [14]
naturally explains the domination of spin-1 links
and reproduces the ringing mode properties of black holes, without
the need to eliminate half-integer spins..."
You say "...would be desirable to know why spin-1/2" are excluded. It is a good question but, according to him, they are not eliminated arbitrarily or "by hand".
He says there is no need to exclude them, because statistically the spin-1 punctures are more prevalent.
Why? this is what I am wondering. He says there is a natural explanation for why spin-1 is more prevalent and predominates over the other. The explanation is on page 5, I think. Around equations (12) and (13). It is very brief and does not seem hard.
It seems to depend on the semiclassical result of Hod!
Hod made an important contribution to Loop gravity with his number
4ln3, which Motl, fortunately enough, was able to confirm by a direct calculation. It has added interest to the field and given them something to work on, and here it is showing up again on page 5 of Polychronakos.
Another paper:
"Beyond space and time"
http://arxiv.org/abs/physics/0401128
Author: Ruediger Vaas
Abstract:
"The secret network of the universe: How quantum geometry might complete Einstein's dream. - An informal introduction to quantum geometry (loop quantum gravity), spin networks, quantum black holes, and the work of Abhay Ashtekar, Carlo Rovelli, Lee Smolin and others."
After a quick inspection, I'm amazed: You will not find a single equation in the text!
An interesting phrase of the text: "If one could observe nature with maximum possible enlargement, space and time would dissolve and the granular mesh of the spin network would come to light ( or more precisely: the quantum physical superposition of all possible configurations of these entities)"
Originally posted by meteor
Another paper:
"Beyond space and time"
http://arxiv.org/abs/physics/0401128
Author: Ruediger Vaas
My suspicion about that article is that
Rudy Vaas is a science-journalist, and it was
written for the German public to run in
the popular press. In English it sounds "over-the-top"
in places, but a good editor could fix that.
Amitabha Sen translated it into English.
Sen is a reputable theoretical physicist.
In 1982 he started the line of development that
resulted in Ashtekar's 1986 "new variables"
approach. Maybe the new variables should
be called "Ashtekar-Sen"
It could be a good popularization article if
it were edited to tone down the journalistic excess.
Originally posted by meteor
"Comparison of area spectra in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0401110
Abstract:
We compare two area spectra proposed in loop quantum gravity in different approaches to compute the entropy of the Schwarzschild black hole....
I've come to the conclusion that we should have a thread about the "equidistant spectrum" version LQG area.
this is about the reasoned opposition of ideas
the majority Loop people say the Rovelli/Smolin spectrum
which is not equidistant or "evenly spaced".
But a few people say to look at the evenly spaced spectrum
and they seem to have a case to make
the papers by Polychronakos
and by him and Alekseev etc
seem pretty clear and interesting too
as well as the one Meteor brought.
here are those other two links:
Alexios Polychronakos
Area spectrum and quasinormal modes of black holes
http://arxiv.org/hep-th/0304135
Alekseev, Polychronakos, Smedbaeck
On the area and entropy of a black hole
http://arxiv.org/hep-th/0004036
http://xxx.soton.ac.uk/abs/gr-qc/0401122
The Pauli Exclusion Principle and SU(2) Versus SO(3) in Loop Quantum Gravity
Author: John Swain
Abstract:
"Recent attempts to resolve the ambiguity in the loop quantum gravity description of the quantization of area has led to the idea that j=1 edges of spin-networks dominate in their contribution to black hole areas as opposed to j=1/2 which would naively be expected. This suggests that the true gauge group involved might be SO(3) rather than SU(2) with attendant difficulties. We argue that the assumption that a version of the Pauli principle is present in loop quantum gravity allows one to maintain SU(2) as the gauge group while still naturally achieving the desired suppression of spin-1/2 punctures. Areas come from j=1 punctures rather than j=1/2 punctures for much the same reason that photons lead to macroscopic classically observable fields while electrons do not."
In this paper, Swain tries to incorporate some kind of Pauli exclusion principle to LQG: If only at most two edges of j=1/2 are allowed to puncture a given surface, then the predominance of edges j=1 puncturing the horizon of a BH is naturally explained
Meteor thanks for the John Swain article. It seems as if there are a number of interesting possible reasons that the spin 1 edges play such a prominent role in the black hole horizon area.
We are now at page 10 of this thread and many of the links were gathered back on page 7, so I will update things and bring the earlier links forward:
--------------------------------
So far there is no sticky for Loop Gravity reference links. And this thread is serving as a surrogate "reference library". Thanks to all who have contributed so far!
The term "Loop Gravity" is used for a broad range of background-independent approaches to quantizing general relativity. Rovelli briefly discusses "the name of the theory" on page (xvi) of his new book. The name "loop" is something of an historical accident---current approaches are not so much involved with loops as with spin network states. But no one has come up with a collective designation that includes spin foams and is more convenient.
The main things the new approaches seem to have in common is that they emerge from General Relativity (rather than Particle Physics) and that they aren't string/brane theories.
-------Loop Gravity texts--------
Rovelli posted the 30 December 2003 draft of his book "Quantum Gravity", to be published this year by Cambridge University Press.
The PDF file is at his homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html.
The book is around 350 pages long and takes a few (like ten?) minutes to download and convert.
To download the 30 December 2003 draft of the book directly:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf
Here are Thiemann's Lecture Notes (they have been published in Berlin by Springer Verlag)
"Lectures on Loop Quantum Gravity".
A draft is online at
http://arxiv.org/gr-qc/0210094
---------some current work------
We were discussing stuff from Livine's thesis in this and another thread. Here is Livine's thesis. He does a lot with explicitly covariant---SL(2,C)-style---spin networks and makes an explicit bridge from LQG to Lorentzian spinfoams.
http://arxiv.org/gr-qc/0309028
Girelli and Livine have come out with a paper about quantizing speed.
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032
Ichiro Oda has posted "A Relation Between Topological Quantum Field Theory and the Kodama State"
http://arxiv.org/hep-th/0311149
Daniele Oriti's thesis is out
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"
Smolin and Magueijo
"Gravity's Rainbow"
http://www.arxiv.org/abs/gr-qc/0305055
(this was revised and reposted 3 February 2004,
I haven't read the revised version yet)
Smolin and Starodubtsev
"General Relativity with a topological phase: an action principle"
http://arxiv.org/hep-th/0311163
Karim Noui and Philippe Roche
"Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
The cosmological constant occurs in a number of recent quantum gravity papers, for instance the one by Girelli/Livine.
-------Quantum Gravity Phenomenology---------
three recent papers:
Giovanni Amelino-Camelia
"A perspective on quantum gravity phenomenology"
http://www.arxiv.org/abs/gr-qc/0402009
dated 2 February 2004
Giovanni Amelino-Camelia, Jerzy Kowalski-Glikman, Gianlucca Mandanici, and Andrea Procaccini
"Phenomenology of Doubly Special Relativity"
http://arxiv.org/gr-qc/0312124
dated 30 December 2003
Jerzy Kowalski-Glikman
"Doubly Special Relativity and quantum gravity phenomenology"
http://arxiv.org/hep-th/0312140
dated 12 December 2003
Jerzy Lukierski
"Relation between quantum κ-Poincare framework and doubly special relativity"
http://arxiv.org./hep-th/0402117
dated 18 February 2004
other fairly recent ones:
Jerzy Kowalski-Glikman and Sebastian Nowak
"Doubly Special Relativity and de Sitter space"
http://arxiv.org/hep-th/0304101
dated 11 October 2003
M. Daszkiewicz, K. Imilkowska, J. Kowalski-Glikman
"Velocity of particles in Doubly Special Relativity"
http://arxiv.org/hep-th/0304027
dated 3 April 2003
---------Loop Quantum Cosmology-------
as a background reference for classical (non-quantum) cosmology:
Charles Lineweaver
"Inflation and the Cosmic Microwave Background"
http://arxiv.org/astro-ph/0305179
dated 12 May 2003
Martin Bojowald
"Loop Quantum Cosmology: Recent Progress"
http://arxiv.org/gr-qc/0402053
One of the invited plenary talks at the January 2004 ICGC
conference (see list of recent conferences)
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology and Boundary Proposals"
http://arxiv.org/gr-qc/0312103
dated 23 December 2003
Martin Bojowald
"Quantum Gravity and the Big Bang"
http://arxiv.org./astro-ph/0309478
dated 17 September 2003, briefly summarizes how
LQG can serve to cure the big bang singularity and
motivate inflationary expansion. Short and less technical
than the other two papers.
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology, Boundary Proposals, and Inflation"
http://arxiv.org/gr-qc/0303072
dated 19 March 2003
Shinji Tsujikawa, Parampreet Singh, Roy Maartens
"Loop quantum gravity effects on inflation and the CMB"
http://arxiv.org/astro-ph/0311015
from the Tsujikawa/Singh/Maartens abstract:
"In loop quantum cosmology, the universe avoids a big bang singularity and undergoes an early kinetic-dominated super-inflation phase, with a quantum-corrected Friedmann equation. As a result, an inflaton field is driven up its potential hill, thus setting the initial conditions for standard inflation. We show that this effect can raise the inflaton high enough to achieve sufficient e-foldings in the standard inflation era. We analyze the cosmological perturbations and show that loop quantum effects can leave a signature on the largest scales in the CMB, with some loss of power and running of the spectral index."
-------recent conferences------
Strings meet Loops (Albert Einstein Institute, MPI-Potsdam) October 2003
http://www.aei-potsdam.mpg.de/events/stringloop.html
Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm
International Conference on Gravity and Cosmology (India) January 2004
http://www.cusat.ac.in/icgc04/
Quantum Gravity Phenomenology, (40th annual Polish Winterschool in Theoretical Physics) February 2004
http://www.ws2004.ift.uni.wroc.pl/html.html
--------upcoming conferences--------
Loop/SpinFoam Conference (Marseille) May 2004
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?03Aug.1
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?10
General Relativity Conference (Dublin) July 2004
more annoucements at
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference
----------fundamental constants, planck units, time-keeping-------
In December 2003, the National Institute of Standards and Technology (NIST) posted new CODATA recommended values for the basic planck units
http://physics.nist.gov/cuu/Constants/
choose "universal" from the menu to find (among other things) the recommended values of
planck mass
planck length
planck time
planck temperature
A 1997 article on timekeeping, discussing GR effects allowed-for in the GPS
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
------projected observational means for testing quantum gravity------
Floyd Stecker
"Cosmic Physics: the High Energy Frontier
http://arxiv.org/astro-ph/0309027
dated September 2003
Stecker discusses the various earth-based and orbital instruments, currently operating, or under construction, or planned, or proposed, and the kind of data becoming available. Among many other things he discusses GLAST, planned to start operating 2007, which, if there are tiny energy-dependent differences in speed among gamma-ray-burst photons, may be able to detect same. Also discusses neutrino observation.
========
simply to have this link handy:
http://www.physicsforums.com/misc/howtolatex.pdf
In another thread the question of "majority consensus" (or one might say "research demographics") came up again.
Numbers of papers, or even numbers of blockbuster papers that get lots of follow-up citations, dont necessarily mean all that much but the issue gets raised now and then so we should have some kind of objective data. There is a small and increasing output of papers in Loop Gravity:
Curious about quantifying this, I went to arxiv.org "Search Physics Archives" page and put in [ABS = loop quantum gravity]OR[ABS = spin foam]OR[ABS = loop quantum cosmology] since 2000. The engine found
these numbers of papers:
2000 46
2001 48
2002 64
2003 70
Y(2/11) 73*
These are the preprints at the archive that have somewhere in their ABSTRACTS either the words loop quantum gravity, or the words spin foam, or the words loop quantum cosmology.
--------------
Although I'm not especially interested in string/brane theories, some people seem interested in comparisons so here's the same numbers for
[ABS = string]OR[ABS = brane]OR[ABS = M-theory]
2000 1457
2001 1496
2002 1500
2003 1265
Y(2/11) 911*
That is, those where the abstract summary of the paper has in it somewhere the word string, or the word brane, or the word M-theory.
*The search engine also has a "Past Year" option which gives the papers posted in the year-to-date: the preceding 12 months. On February 11 I ran the same check for year-to-date and got corresponding numbers for the "Year to 2/11" which I've listed here separately as Y(2/11).
Y(2/11) reflects activity in part of the calendar year 2003 and in part of the calendar year 2004.
Meteor started us collecting recent work bearing on
the vibration of black holes and the Loop Gravty area spectrum.
Gilad Gour and V. Suneeta
"Comparison of area spectra in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0401110
Alexios Polychronakos
Area spectrum and quasinormal modes of black holes
http://arxiv.org/hep-th/0304135
Alekseev, Polychronakos, Smedbaeck
On the area and entropy of a black hole
http://arxiv.org/hep-th/0004036
Gour and Suneeta are at the University of Alberta.
Alexkseev and Smedbaeck are at the Institute for Theoretical Physics, Upsala University, in Sweden
Polychronakos is at the University of Ioannina in Greece and also the
CUNY Physics Department in the USA.
Setare and Vagenas
"Area Spectrum of Kerr and extremal Kerr Black Holes from Quasinormal Modes"
http://arxiv.org/hep-th/0401187
Berti, Cardoso, Yoshida
"Highly Damped Quasinormal Modes of Kerr Black Holes: A Complete Numerical Investigation"
http://arxiv.org/gr-qc/0401052
Setare is in Iran, Vagenas at Barcelona.
Berti is in St. Louis, Cardosoo and Yoshida are in Portugal.
Here is the link for this post:
http://www.physicsforums.com/showthread.php?s=&postid=140731#post140731
selfAdjoint
Feb3-04, 08:51 PM
Didn't Lubos write or cowrite a paper on this subject? I seem to remember it being mentioned some months ago on s.p.r.
Originally posted by selfAdjoint
Didn't Lubos write or cowrite a paper on this subject? I seem to remember it being mentioned some months ago on s.p.r.
Sure did. One solo and one with Andy Neitzke that I know of.
would you like the links?
why dont I give them in any case:
Lubos' paper:
http://arxiv.org/gr-qc/0212096
Lubos and Andy's paper:
http://arxiv.org/hep-th/0301173
------------------------
what's exciting about the more recent paper by Gour and Suneeta
is that they challenge the Area operator spectrum derived by Rovelli and Smolin in 1994 and propose a quantum correction in the area.
this resolves a long-standing difficulty and produces some nice
results.
Meteor brought the paper in. Ive been reading it and like it quite a bit. (it is however "revisionist" in a sense)
afterthought edit: selfAdjoint, I put a longer discussion of the LQG area spectrum in the thread called "Loop Quantum Gravity". Didnt want to take extra space in this thread which is serving as a link-basket.
A couple of days ago Rovelli posted the program for the May 2004 Conference (at Luminy on the Mediterranean)
the organizers:
Laurent Freidel
Philippe Roche
Carlo Rovelli
----exerpted material in no particular order----
A tentative list of morning speakers, still to be confirmed, is as
follows
Loops:
Abhay Ashtekar (quantum geometry)
Thomas Thiemann (dynamics and low energy)
Lee Smolin (overall results)
Ted Jacobson (devil's advocate)
Applications:
Martin Bojowald (loop cosmology)
Alejandro Corichi (black holes)
Daniel Sudarsky (phenomenology)
Spin foams:
John Baez (spinfoams)
Laurent Freidel (GFT, sum over complexes)
John Barrett (BC model)
Alejandro Perez (spinfoams)
Related approaches:
Jorge Pullin (consistent discretization)
Peter Forcacs or Max Neidermair (fixed point)
Ian Ambjorn or Renate Loll (dynamical triangulations)
John Klauder (general covariant dynamics)
....
....
6. SCIENTIFIC PROGRAM
Aim of the conference is to make the point on where we are in the
loop/spinfoam approach to quantum gravity. In particular:
evaluate the results obtained so far, point out open problems,
and discuss the directions of development that appear to be most
promising. The conference is therefore mostly (but not
exclusively) addressed to our community. The four days will
focus on 1) Loops, 2) Applications, 3) Spinfoams, 4) Related
approaches.
The conference will be articulated in:
- morning talks of approximately 30 minutes, meant to summarize the
present state of the different aspects of the field, followed by
ample discussion time.
- afternoon presentations of novel results. The duration of
these will be decided dividing the available time by the
number of communications accepted.
- A panel session, followed by a general discussion, on the last
day.
----end of exerpts----
Philippe Roche at the University of Montpellier has this webpage about the conference:
http://w3.lpm.univ-montp2.fr/~philippe/quantumgravitywebsite/
Other conferences: Here are some recent and upcoming ones mentioned in a previous post in this thread.
....
....
-------recent conferences------
Strings meet Loops (Albert Einstein Institute, MPI-Potsdam) October 2003
http://www.aei-potsdam.mpg.de/events/stringloop.html
Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm
International Conference on Gravity and Cosmology (India) January 2004
http://www.cusat.ac.in/icgc04/
Quantum Gravity Phenomenology, (40th annual Polish Winterschool in Theoretical Physics) February 2004
http://www.ws2004.ift.uni.wroc.pl/html.html
--------upcoming conferences--------
Loop/SpinFoam Conference (Marseille) May 2004
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?03Aug.1
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?10
General Relativity Conference (Dublin) July 2004
more annoucements at
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference
...
...
"Big crunch avoidance in k=1 loop quantum cosmology"
http://arxiv.org/abs/gr-qc/0312110
Authors: Parampreet Singh, Alexey Toporensky
Abstract:
"It is well known that a closed universe with a minimally coupled massive scalar field always collapses to a singularity unless the initial conditions are extremely fine tuned. We show that the corrections to the equations of motion for the massive scalar field, given by loop quantum gravity in high curvature regime, always lead to a bounce independently of the initial conditions. In contrast to the previous works in loop quantum cosmology, we note that the singularity can be avoided even at the semi-classical level of effective dynamical equations with quantum modifications, without using a discrete quantum evolution"
k refers to curvature. In the case k=1 it represents a flat universe. In the text there's discussion about the geometrical density operator (never heard of this operator before). The effective Friedmann equation for LQC is presented, also the Raychaudhuri equation
Originally posted by meteor
"Big crunch avoidance in k=1 loop quantum cosmology"
http://arxiv.org/abs/gr-qc/0312110
Authors: Parampreet Singh, Alexey Toporensky
k refers to curvature. In the case k=1 it represents a flat universe. In the text there's discussion about the geometrical density operator (never heard of this operator before). The effective Friedmann equation for LQC is presented, also the Raychaudhuri equation
Meteor, Bojowald recently gave a survey and cited this article. It was a plenary talk on loop quantum cosmology at a January 5-10
conference called "ICGC 2004"
So he must think it is a good paper. I have not read it. I think the flat case is k=0 and the k=1 case is positive curvature----which would normally lead to a big crunch (at least in the sort of ordinary cosmology we used to have before there was a cosmological constant)
Bojowald's survey talk at the conference is online at arxiv and it has links to lots of recent LQC papers which saves us trouble in that department. One link for many:
Bojowald
Loop Quantum Cosmology: Recent Progress
http://arxiv.org/gr-qc/0402053
There is an event going on now (Feb 4-14) in Poland
about Quantum Gravity Phenomenology
Here is the latest speaker list I could find:
--------quote from program-------
Speakers:
E. Alvarez---Quantum Gravity
G. Amelino-Camelia---Introduction to quantum gravity phenomenology
P. De Bernardis---Cosmology with BOOMERANG, WMAP
A. Grillo---Planck-scale kinematics and the Pierre Auger Observatory
T. Jacobson---Astrophysical bounds on Planck-supressed Lorentz violation
J. Kowalski-Glikman---Introduction to doubly special relativity
C. Laemmerzahl---Tests of Lorentz symmetry in space and interferometry
P. Lipari---Ultra-high-energy cosmic-rays
J. Martin---Trans-Planckian cosmology
N. Mavromatos---PCT symmetry and quantum gravity phenomenology
T. Piran---Gamma-ray bursts
J. Pullin---Canonical quantum gravity phenomenology
L. Smolin---Cosmological constant in Quantum Gravity
....to gather together world-leading scientists working on the field of quantum gravity, astrophysics, and cosmology along with a number of post-graduate students and young post-docs and to offer young scientists the opportunity to learn about recent developments in the theoretical investigation of Planck-scale physics that might be tested experimentally in the near future. The lectures presented at the School would provide a broad coverage of subjects relevant for this field, including models of the fate of Lorentz invariance in quantum space-time, loop quantum gravity and string theory, cosmology and astrophysics.
-----------end quote----------
the so-called "Bohr compactification" of the real line
is named after Harald Bohr (b.1877, Niels younger bro)
Hans Halvorson at Princeton is a quality mathematician-cum-philosopher and I like the way he writes. he seems to think philosophically about quantum mechanics but also do functional analysis and topology as well. dont know any more about him.
"Complementarity of representations in quantum mechanics"
http://arxiv.org/quant-ph/0110102
AFAIK no one here has already noted Halvorson or this paper.
The Bohr compactification is important to cosmology and not much is on line about it, but Halvorson's paper is online, and see bottom of page 9, around equation (11) for a brief discussion
**************************
To see how the Bohr compactification fits into quantum cosmology see
Viqar Husain and Oliver Winkler
"On singularity resolution in quantum gravity"
http://arxiv.org/gr-qc/0312094
Best kind of confirmation of Bojowald's work because derives similar results (removal of BB singularity) entirely outside of the Loop Gravity framework.
Bohr compactification enters at top of page 6, right after they introduce the almost periodic functions on the Reals:
"It is well-known that AP(R) is naturally isomorphic to C(RBohr), the algebra of continuous functions on the so-called Bohr-compactification of R. As the name suggests, RBohr is a compact group which can be obtained as the dual group of Rdiscr, the real line endowed with the discrete topology. This suggests that taking
L2(RBohr,dµ0),
where µ0 is Haar measure on RBohr, as the
Hilbert space for our theory is a viable option. This is the decisive point where we depart from the traditional approach in geometrodynamics, where the Hilbert space is the conventional Schroedinger space L2(R, dx). Once we adopt this new choice, basis states in our Hilbert space are..."
they dont use the Ashtekar variables! they dont use the connection! they use the same ADM variables that Wheeler and DeWitt tried to use!!!
but they still manage to remove the Big Bang singularity.
the key thing turns out not to be cooking down LQG to get LQC
but something about an idea Harald Bohr had about "almost periodic functions" and something about topology.
Like what they say on page 10:
"Our main result is that there is an alternative to the Schroedinger quantization of the FRW cosmology in the standard ADM geometrodynamical variables. This quantization leads to conclusions
qualitatively similar to those obtained in loop quantum cosmology starting from the connection triad variables: (i) the Hamiltonian constraint acts like a difference operator, and (ii) the inverse scale factor can be represented as a densely defined operator.
Thus it is the representation space and the realizations of the basic observables rather than the nature of the classical variables that are responsible for the similar conclusions for this model."
their italics, my bolding
thanks to Ranyart for calling Viqar Husain/Oliver Winkler's paper to my attention.
Two new papers were just posted on Doubly Special Relativity
http://arxiv.org/gr-qc/0402092
Giovanni Amelino-Camelia
"Some encouraging and some cautionary remarks on Double Special Relativity in Quantum Gravity"
dated 22 February
(based on a talk given at the 10th Marcel Grossmann meeting on GR)
http://arxiv.org/hep-th/0402117
Jerzy Lukierski
"Relation between quantum kappa-Poincare framework and Doubly Special Relativity"
dated 18 February
Jose Mourao has co-authored numerously with such folk as Ashtekar, Lewandowski, Marolf, Thiemann, Renate Loll, and was thesis advisor of Jose Manuel Velhinho
here is a picture of Mourao and some bio stuff
http://www.math.ist.utl.pt/~jmourao/textojm.html
Velhinho's work on disertation with Mourao covered 1995-2001
This looks to me like a good new paper by Velhinho
http://arxiv.org/math-ph/0402060
It presents a summary of how the development of LQG is going
and the style is efficient, not cumbersome. He writes as
a mathematician.
On page 19 is treated the issue of spatial diff invariance---which was the basis of the "nonstandardness" discussion with Urs and others in the TT Loop-String thread, or so I gather from Urs' recent posts.
it is an interesting issue and Velhinho provides a concise overview in a couple of pages.
Velhinho is at University of Beira in Portugal
Murao is in the Mathematics Department at Lisbon Tech (the Inst. Sup. Tech)
LQG connects with semiclassical study of black holes
(through the BH entropy formula, Hawking radiation and QN modes)
Here are two interesting papers about Hawking radiation
(predicting that it is not perfectly thermal when quantum effects are adjusted for, and describing the mechanism)
Frank Wilczek and Maulik Parikh
"Hawking Radiation as Tunneling"
http://arxiv.org/hep-th/9907001
Maulik Parikh
"Energy Conservation and Hawking Radiation"
http://arxiv.org/hep-th/0402166
6 pages, dated 23 February
"Dynamics of loop quantum gravity and spin foam models in three dimensions":
http://arxiv.org/abs/gr-qc/0402112
Authors: Karim Noui, Alejandro Perez
Abstract:
"We present a rigorous regularization of Rovellis's generalized projection operator in canonical 2+1 gravity. This work establishes a clear-cut connection between loop quantum gravity and the spin foam approach in this simplified setting. The point of view adopted here provides a new perspective to tackle the problem of dynamics in the physically relevant
3+1 case."
I just printed it out :)
ranyart
Feb26-04, 03:26 AM
Originally posted by meteor
"Dynamics of loop quantum gravity and spin foam models in three dimensions":
http://arxiv.org/abs/gr-qc/0402112
Authors: Karim Noui, Alejandro Perez
Abstract:
"We present a rigorous regularization of Rovellis's generalized projection operator in canonical 2+1 gravity. This work establishes a clear-cut connection between loop quantum gravity and the spin foam approach in this simplified setting. The point of view adopted here provides a new perspective to tackle the problem of dynamics in the physically relevant
3+1 case."
I just printed it out :)
Likewise here, but are you aware there are a number of other compatable papers? all preceeding and very relevant:
http://uk.arxiv.org/abs/gr-qc/0402110
http://uk.arxiv.org/abs/gr-qc/0402111
http://uk.arxiv.org/abs/gr-qc/0402112
http://uk.arxiv.org/PS_cache/gr-qc/pdf/0402/0402113.pdf
Just as an after thought I belive this can go here to?
http://uk.arxiv.org/abs/hep-th/0311030
Or maybe Marcus can place it into a relevant forum?
[a)]
selfAdjoint
Feb26-04, 10:25 AM
Working through their paper gr-qc/0402112, I see that Noui and Perez do a standard quantization, regulating to get a finite sum and then showing that the regulator can be eliminated in quantizing. They then show that their physical Hilbert space is the same as the one obtained in the LQG approach.
All of this depends crucially on the fact that they can represent their 2+1 geometry as the product of a Riemann surface and a line. It's not clear how, or if, this quantization could be extended to 3+1 spacetime. On the other hand, the fact that in this case the standard type of quantization agrees with the Ashtekar et al quantization is supportive for the LQG folks.
We are now at page 11 of the thread, so I will update things and bring the earlier links forward. So far there is no sticky thread for Loop Gravity reference links, and this thread is serving as a surrogate sticky "reference library". Thanks to all who have contributed so far!
-------Loop Gravity texts--------
Rovelli posted the 30 December 2003 draft of his book "Quantum Gravity", to be published this year by Cambridge University Press.
The PDF file is at his homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
The book is around 350 pages long and takes a few (like ten?) minutes to download and convert.
To download the 30 December 2003 draft of the book directly:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf
Here are Thiemann's Lecture Notes (they have been published in Berlin by Springer Verlag)
"Lectures on Loop Quantum Gravity".
A draft is online at
http://arxiv.org/gr-qc/0210094
-------Quantum Gravity Phenomenology and DSR---------
some recent phenomenology and DSR papers:
Giovanni Amelino-Camelia
"A perspective on quantum gravity phenomenology"
http://www.arxiv.org/abs/gr-qc/0402009
dated 2 February 2004
Giovanni Amelino-Camelia, Jerzy Kowalski-Glikman, Gianlucca Mandanici, and Andrea Procaccini
"Phenomenology of Doubly Special Relativity"
http://arxiv.org/gr-qc/0312124
dated 30 December 2003
Jerzy Kowalski-Glikman
"Doubly Special Relativity and quantum gravity phenomenology"
http://arxiv.org/hep-th/0312140
dated 12 December 2003
Jerzy Lukierski
"Relation between quantum κ-Poincare framework and doubly special relativity"
http://arxiv.org./hep-th/0402117
dated 18 February 2004
other less recent ones:
Jerzy Kowalski-Glikman and Sebastian Nowak
"Doubly Special Relativity and de Sitter space"
http://arxiv.org/hep-th/0304101
dated 11 October 2003
M. Daszkiewicz, K. Imilkowska, J. Kowalski-Glikman
"Velocity of particles in Doubly Special Relativity"
http://arxiv.org/hep-th/0304027
dated 3 April 2003
---------Loop Quantum Cosmology-------
Martin Bojowald
"Loop Quantum Cosmology: Recent Progress"
http://arxiv.org/gr-qc/0402053
One of the invited plenary talks at the January 2004 ICGC
conference (see list of recent conferences)
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology and Boundary Proposals"
http://arxiv.org/gr-qc/0312103
dated 23 December 2003
Martin Bojowald
"Quantum Gravity and the Big Bang"
http://arxiv.org./astro-ph/0309478
dated 17 September 2003, briefly summarizes how
LQG can serve to cure the big bang singularity and
motivate inflationary expansion. Short and less technical
than the other two papers.
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology, Boundary Proposals, and Inflation"
http://arxiv.org/gr-qc/0303072
dated 19 March 2003
Shinji Tsujikawa, Parampreet Singh, Roy Maartens
"Loop quantum gravity effects on inflation and the CMB"
http://arxiv.org/astro-ph/0311015
from the Tsujikawa/Singh/Maartens abstract:
"In loop quantum cosmology, the universe avoids a big bang singularity and undergoes an early kinetic-dominated super-inflation phase, with a quantum-corrected Friedmann equation. As a result, an inflaton field is driven up its potential hill, thus setting the initial conditions for standard inflation. We show that this effect can raise the inflaton high enough to achieve sufficient e-foldings in the standard inflation era. We analyze the cosmological perturbations and show that loop quantum effects can leave a signature on the largest scales in the CMB, with some loss of power and running of the spectral index."
Viqar Husain and Oliver Winkler "On singularity resolution in quantum gravity"
http://arxiv.org/gr-qc/0312094
this is especially interesting because they duplicate LQC results (for example by Bojowald) using the older version of quantum gravity, ADM variables, quantized metric. Shows that the removal of the big bang singularity is "robust"---doesnt depend on using a particular formalism.
as a background reference for classical (non-quantum) cosmology:
Charles Lineweaver
"Inflation and the Cosmic Microwave Background"
http://arxiv.org/astro-ph/0305179
dated 12 May 2003
-------recent conferences------
Strings meet Loops (Albert Einstein Institute, MPI-Potsdam) October 2003
http://www.aei-potsdam.mpg.de/events/stringloop.html
Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm
International Conference on Gravity and Cosmology (India) January 2004
http://www.cusat.ac.in/icgc04/
Quantum Gravity Phenomenology, (40th annual Polish Winterschool in Theoretical Physics) February 2004
http://www.ws2004.ift.uni.wroc.pl/html.html
--------upcoming conferences--------
Loop/SpinFoam Conference (Marseille) May 2004
http://w3.lpm.univ-montp2.fr/~philippe/quantumgravitywebsite/
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?03Aug.1
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?04Feb.2
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?10
General Relativity Conference (Dublin) July 2004
more annoucements at
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference
----------fundamental constants, planck units, time-keeping-------
Historical source for Planck units, the 1899 paper (thanks arivero!)
http://www.bbaw.de/bibliothek/digital/struktur/10-sitz/1899-1/jpg-0600/00000494.htm
In December 2003, the National Institute of Standards and Technology (NIST) posted new CODATA recommended values for the basic planck units
http://physics.nist.gov/cuu/Constants/
choose "universal" from the menu to find (among other things) the recommended values of
planck mass
planck length
planck time
planck temperature
A 1997 article on timekeeping, discussing GR effects allowed-for in the GPS
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
------observational means for testing quantum gravity------
Floyd Stecker
"Cosmic Physics: the High Energy Frontier
http://arxiv.org/astro-ph/0309027
dated September 2003
Stecker discusses the various earth-based and orbital instruments, currently operating, or under construction, or planned, or proposed, and the kind of data becoming available. Among many other things he discusses GLAST, planned to start operating 2007, which, if there are tiny energy-dependent differences in speed among gamma-ray-burst photons, may be able to detect same. Also discusses neutrino observation.
------links to an unselective assortment of current work------
Livine's thesis
http://arxiv.org/gr-qc/0309028
Girelli and Livine
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032
Oriti's thesis
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"
Karim Noui and Philippe Roche
"Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
The cosmological constant occurs in a number of recent quantum gravity papers, for instance the one by Girelli/Livine.
Velhinho "On the structure of the space of generalized connections"
http://arxiv.org/math-ph/0402060
Noui and Perez "Three dimensional loop quantum gravity: physical scalar product and spin foam models"
http://arxiv.org/gr-qc/0402110
Noui and Perez "Three dimensional loop quantum gravity: coupling to point particles"
http://arxiv.org/gr-qc/0402111
Noui and Perez "Dynamics of Loop Quantum Gravity and Spin Foam Models in Three Dimensions"
http://arxiv.org/gr-qc/0402112
Noui and Perez "Observability and Geometry in Three Dimensional Quantum Gravity"
http://arxiv.org/gr-qc/0402113
Freidel and Louapre "Ponzano-Regge model revisited, I."
http://arxiv.org/hep-th/0401076
Gambini and Pullin "Canonical Quantum Gravity..."
http://arxiv.org/gr-qc/0402062
Buffenoir, Henneaux, Noui, Roche
Hamiltonian Analysis of Plebanski Theory
http://arxiv.org./gr-qc/0404041
(spin foam, BF)
========
simply to have this link on LaTex handy:
http://www.physicsforums.com/misc/howtolatex.pdf
"Separable Hilbert space in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0403047
By Carlo Rovelli and Winston Fairbairn
Abstract:"We study the separability of the state space of loop quantum gravity. In the standard construction, the kinematical Hilbert space of the diffeomorphism-invariant states is nonseparable. This is a consequence of the fact that the knot-space of the equivalence classes of graphs under diffeomorphisms is noncountable. However, the continuous moduli labeling these classes do not appear to affect the physics of the theory. We investigate the possibility that these moduli could be only the consequence of a poor choice in the fine-tuning of the mathematical setting. We show that by simply choosing a minor extension of the functional class of the classical fields and coordinates, the moduli disappear, the knot classes become countable, and the kinematical Hilbert space of loop quantum gravity becomes separable"
The Spring 2004 issue of Jorge Pullin's newsletter is out.
"Matters of Gravity"
http://arxiv.org./abs/gr-qc/0403051
It has a number of QG conference reports
by Bojowald
Date
Corichi
and others
selfAdjoint
Mar15-04, 10:04 AM
Thank you Marcus, for pointing to this. John Baez used to keep us up to date on this important publication, but he seems to have moved out of QG. The reports were very interesting.
"The Duel: Strings versus loops"
http://arxiv.org/abs/physics/0403112
Janitor
Mar25-04, 12:08 AM
All I can ever remember seeing at J.B.'s website is QG and category theory. Is he now down to just category theory?
Many of the QG pioneers like Ashtekar, Lewandowski, Baez are not publishing much and there is a new crop (many of whom were unknown in the Nineties.)
Baez has not published in QG for several years, or only negligibly.
Ashtekar likewise. But both play an important role. Ashtekar is a big presence at conferences. Baez will be a principle figure at the Marseille QG conference in May and the Dublin conference in July.
I guess the people who started up LQG in the 1990s are already
over 40, maybe pushing 50. Ashtekar must be over 50. So with some exceptions it really seems to be the next wave of young LQGists that is making the field progress
maybe the term is "mathopause"----it gets mathematicians
you could have direct knowledge yourself so why elaborate
Janitor
Mar25-04, 12:35 AM
Yeah, I guess I'm at that age. Of course, I never had the right stuff in the first place!
Originally posted by Janitor
Yeah, I guess I'm at that age...
argh! bummer! confession is the pits.
I actually think Baez might blossom again, just in a different field. he is a remarkable guy.
and if he can maybe there is hope for the rest of us.
meanwhile there are up-and-coming people in LQG to watch
here are a few names off the top of my head in no
particular order
Etera Livine
Laurent Freidel
Phillipe Roche
Martin Bojowald
Karim Noui
Hanno Sahlmann
Velhinho
Kowalski-Glikman
selfAdjoint
Mar25-04, 04:34 PM
My thesis advisor, Ed Fadell, published important research in the later 1990's. He was pushing 40 when I knew him in the early 1960's, so probably he was 70 when he did that. Don't ever count anybody out.
My thesis advisor, Ed Fadell, published important research in the later 1990's. He was pushing 40 when I knew him in the early 1960's, so probably he was 70 when he did that. Don't ever count anybody out.
Algebraic topology, impressive, maybe abstract algebra too?
Had a student by the name of Bob Brown whom I may have met but am not sure (he did abstract algebra IIRC, was teaching galois theory)
just happened on this:
E. Fadell, Homotopy groups of configuration spaces and the string problem of Dirac, Duke Math.J. 29 (1962), 231-242.
Edward Fadell apparently had several topnotch students which is another
dimension---the vitality that goes into that, as well as research. It is possible you made a real good choice of advisor
genes, character
we should all have it whatever it is that never stops
Are grand old men more the norm in physics?
Hans Bethe
did you ever look at the "mathematical geneology" website
it is like the Begats in the bible
I looked up Marc Rieffel not long ago
George Mackey advised Rieffel
and so and so advised Mackey and...
back to Birkhoff at Harvard around 1905 if I remember right
selfAdjoint
Mar25-04, 08:47 PM
You met Bob Brown?! He and I studied together - in fact I was a fixture at his and his wife Brenda's apartment, since I had no significant other of my own then. Bob's had a standout career at UCLA.
You met Bob Brown?! He and I studied together - in fact I was a fixture at his and his wife Brenda's apartment, since I had no significant other of my own then. Bob's had a standout career at UCLA.
Im not sure it was the same Bob Brown
algebraist (I formed a high opinion of him as a teacher and person)
could be the same, I'll write you a PM later
this just out
http://arxiv.org./abs/gr-qc/0403106
"Inflationary Cosmology and Quantization Ambiguities in Semi-Classical Loop Quantum Gravity"
Martin Bojowald, James E. Lidsey, David J. Mulryne, Parampreet Singh, Reza Tavakol
15 pages, 8 figures
"Loop quantum gravity (LQG) or quantum geometry is
at present the main background independent and non–
perturbative candidate for a quantum theory of gravity
(see for example [1, 2]). Key successes of this approach
have been the prediction of discrete spectra for geometrical
operators [3], the existence of well defined operators
for the matter Hamiltonians which provides a cure for
the ultraviolet divergences [4], and the derivation of the
Bekenstein–Hawking entropy formula [5].
Given that LQG effects are likely to have important
consequences in high energy and high curvature regimes,
early universe cosmology provides a natural environment
to test these new features....
From the loop quantum cosmology (LQC) perspective,
the evolution of the universe is comprised of the three distinct
phases. Initially, there is a truly discrete quantum
phase which is described by a difference equation [9, 10].
A key consequence of this discretization is the removal
of the initial singularity [9]. As its volume increases,
the universe enters an intermediate semi–classical phase
in which the evolution equations take a continuous form
but include modifications due to non–perturbative quantization
effects [12]. Finally, there is the classical phase
in which the usual continuous ODE/PDE cosmological
equations are recovered and quantum effects vanish..."
also just out, Clifford Will's paper, could be the same paper he gave last week at Ulm to the German Physical Society
http://arxiv.org./abs/gr-qc/0403100
"Testing Alternative Theories of Gravity Using LISA"
-----quote from abstract-------
We investigate the possible bounds which could be placed on alternative
theories of gravity using gravitational wave detection from inspiralling compact binaries with the proposed LISA space interferometer. Specifically, we estimate lower bounds on the coupling parameter ω of scalar-tensor theories of the Brans-Dicke type and on the Compton wavelength of the graviton λ_g in hypothetical massive graviton theories.
In these theories, modifications of the gravitational radiation damping formulae or of the propagation of the waves translate into a change in the phase evolution of the observed gravitational waveform. We obtain the bounds through the technique of matched filtering, employing the LISA Sensitivity Curve Generator (SCG), available online. For a neutron star inspiralling into a 103M⊙ black hole in the Virgo Cluster,
in a two-year integration, we find a lower bound ω > 3 × 10^5. For lower-mass black holes, the bound could be as large as 2 × 10^6. The bound is independent of LISA arm length, but is inversely proportional to the LISA position noise error. Lower bounds on the graviton Compton wavelength ranging from 10^15 km to 5 × 10^16 km can be obtained from one-year observations of massive binary black hole inspirals at cosmological distances (3 Gpc), for masses ranging from 10^4 to 10^7M⊙. For the
highest-mass systems (10^7M⊙), the bound is proportional to (LISA arm length)1/2 and to (LISA acceleration noise)^−1/2. For the others, the bound is independent of these parameters because of the dominance of white-dwarf confusion noise in the relevant part of the frequency spectrum. These bounds improve and extend earlier work which used analytic formulae for the noise curves.
---------end quote-------
Initially, there is a truly discrete quantum
phase which is described by a difference equation [9, 10].
A key consequence of this discretization is the removal
of the initial singularity [9]. As its volume increases,
the universe enters an intermediate semi–classical phase
in which the evolution equations take a continuous form
but include modifications due to non–perturbative quantization
effects [12].
Is there any paper that can says when exactly in time the evolution of the universe change to be described by a difference equation to be described by differential equations?
Is there any paper that can says when exactly in time the evolution of the universe change to be described by a difference equation to be described by differential equations?
IIRC Ashtekar's paper "Quantum Geometry in Action: Big Bang and Black Holes"
gives an estimate of several hundred steps (of the difference equation) to converge to the semi-classical model
it is the usual sort of limiting process
the quantum regime converges to the semiclassical (after a very short period on the order of 100 planck time units)
and the semiclassical converges thereafter more gradually to
the ordinary or partial differential equation model
but as with other kinds of convergence one cannot say with precision the exact moment when
the discrete model stops being appropriate and the semiclassical model
begins to apply
there is a transition period when both are giving approximately the same answer
So what one needs is a rough order of magnitude idea of when the transition between models happens. If it is not in that Ashtekar paper then I must be thinking of one by Ashtekar, Bojowald, Lewandowski called
"Mathematical Structure of Loop Quantum Cosmology"
I will try to get a link and page reference for the several hundred planck time units or DiffEq timesteps---it's in one or the other or both papers. May be other places as well so someone else could come up with yet another link.
------------LONG LAPSE OF TIME-----
I forgot to get the references, however the one I mentioned first has something.
See page 10 of gr-qc/0202008, last paragraph of section 3.1 "Big Bang".
Ashtekar says there that the semiclassical model (Wheeler-DeWitt) is recovered when the scale of the universe is a few hundred planck lengths. that is, very soon.
Also next to last paragraph on page 8.
I would like to find a more recent and more precise paper, in answer to your question. At the moment I dont have one. Perhaps someone else out there does.
"3.3 Chern-Simons perturbation theory.
Setting
\frac{3}{4} = \frac{2}{3}
our Lagrangian becomes the Chern-Simons-functional..."
there is a mathematician named Dror Bar-Natan
on page 19 of this paper
q-alg/9702009
"The Fundamental Theorem of Vassiliev Invariants"
he claims to prove something
by setting 3/4 equal to 2/3.
His paper is about the "Fundamental Theorem of Vassiliev Invariants"
and it is divided into four sections with four different ways of proving
the fundamental theory and at the end of each section he has
a concluding paragraph entitled
"Why are we not happy?"
This shows a philosophical concern with the problem of human happiness.
Also he proves the theorem by algebra, by physics (the oldest way, already almost 10 years old), by geometry, by topology. and he finds something always unsatisfying or wrong. in the middle of the proof by physics he says
"This is of course silly."
Dror Bar-Natan has an unusual expository style. Or at least I hope that it is unusual.
BTW he calls the topological method "combinatorial-topological" because doubtless he thinks of combinatorics and topology as very close neighbors or almost joined at the hip
He cites V.I.Arnold a russian mathematician. Fairbairn and Rovelli also cited a book by V.I. Arnold. It would be possible to suspect that something is going on with knot theory and Vassiliev invariants. the quirky Bar-Natan tone of voice even encourages this suspicion.
Perhaps it will be necessary to classify knots-with-nodes and I cannot at the moment visualize how this would be done.
I will get the LQG paper by Gambini and Pullin that cites this Bar-Nathan.
Nonunitary gave this link in another thread.
------quote from nonunitary post in "chunkymorphism" thread---
...As far as I know the first paper about the invariants was
gr-qc/9803018
but you are right about the chunkymorphisms. The are a new invention of Rovelli. I haven't read the paper so I can not comment.
-----endquote----
the Gambini/Pullin paper
http://arxiv.org/gr-qc/9803018
is called
"Vassiliev invariants: a new framework for quantum gravity"
selfAdjoint
Mar30-04, 10:40 PM
How do you get to those q_alg papers in the arxiv? I have been trying every trick in the book for half an hour now and nothing! Click on mathematics and get, but can't ge q-alg or QA. Click on 1997 and search, nothing.
How do you get to those q_alg papers in the arxiv? I have been trying every trick in the book for half an hour now and nothing! Click on mathematics and get, but can't ge q-alg or QA. Click on 1997 and search, nothing.
only have a minute to reply but try
http://arxiv.org/PS_cache/q-alg/pdf/9702/9702009.pdf
will get back in a few minutes and check that this works
Im back.
this should get the abstract:
http://arxiv.org/q-alg/9702009
now I understand. the problem is to use the search engine
to find a paper like this one, but hopefully more recent
------------------
go to arxiv
don't click on search immediately
because right beside the button that says "search" there is
a menu box where you can select "math"
select "math" and then click on "search"
you then get a form where you can type in Author and Keyword
I typed in Bar-Natan and knot
and got many QA papers including this sample
3. math.QA/0201043 [abs, ps, pdf, other] :
Title: On Khovanov's categorification of the Jones polynomial
Authors: Dror Bar-Natan
Comments: Published by Algebraic and Geometric Topology at this http URL, 34 pages with many figures, source contains associated program and data file
Subj-class: Quantum Algebra; Geometric Topology
MSC-class: 57M25
Journal-ref: Algebraic and Geometric Topology 2 (2002) 337-370
selfAdjoint
Mar31-04, 10:08 AM
Thanks. Using your hint, I fooled around and found it with"Vassiliev invariants" which is what I was interested in anyway. Bar-Natan's motive for "why are we not happy" is perfectly clear; he wants to impose on his students a careful understanding of what it means to have a "proved theorem" which you can use to prove other things, and what it does NOT mean - which is the status of what he calls the fundamental theorem of Vassiliev invariants.
Hi selfAdjoint, I concur with your description of Bar-Natan's
serious and commendable motive but I also delight immoderately in
his sense of humor
which he uses to the hilt in implementing his serious idea
thanks to nonunitary for this, I never would have seen the paper if
he had not referred to that one by Gambini and Pullin about LQG and
the Vassiliev invariants
you know diff manifolds are in a deep sense just gussied up Rn
and it just shows you what an enormously rich thing Euclidean space is
that you can have all these different variations on that theme
the theme of the continuum
the theme of the coordinate patch and the metric
all fundamentally Rn at the root
can knots and networks be comparably rich
why is there all this interest in them just now
well this is not purely a rhetorical question although it
sounds like it, I was actually wondering, but not expecting to
be able to get an answer
it was clever of you to study algebraic topology in grad school
maybe it will be useful after lo these many years
You might be interested to have a look at Frieder Lenz's
lecture notes on
"Topological concepts in gauge theories"
http://arxiv.org./hep-th/0403286
the whole thing is 83 pages
http://arxiv.org./PS_cache/hep-th/pdf/0403/0403286.pdf
They just came out.
he has a good historical sense and begins with a story about something that happened in 1833 involving Carl Gauss and a magnetic monopole :-)
these notes strike me as student-friendly
by someone who is considerate and puts in some nice pictures
Getting ready for a Brahms Req rehearsal tonite.
Up to you to decide if Frieder Lenz's notes are good or not and for what.
selfAdjoint
Apr1-04, 09:17 AM
Thanks for the links, Marcus.
You know, reading Bar Natan's account of the topological proof of his "fundamental theorem" and its defects, I couldn't help thinking here's a natural arena for spectral sequences. That's only because I'm reading A User's Guide to Spectral Sequences at the same time, but seriously there are his filtered graded algebra and all - by a theorem, there is guaranteed to be a spectral sequence with the 1-page E^{p,q}_1 isomorphic to the homology of the algebra. But that's no good unless you can compute the limiting page E^{p,q}_{\infty} . The differentials of the sequence encode non trivial information about the algebra. I can't believe somebody hasn't tried this.
Bolen, Bombelli, Corichi
http://arxiv.org./abs/gr-qc/0404004
"Semiclassical States in Quantum Cosmology: Bianchi I Coherent States"
"We study coherent states for Bianchi type I cosmological models, as examples of semiclassical states for time-reparametrization invariant systems. This simple model allows us to study explicitly the relationship between exact semiclassical states in the kinematical Hilbert space and corresponding ones in the physical Hilbert space, which we construct here using the group averaging technique. We find that it is possible to construct good semiclassical physical states by such a procedure in this model; we also discuss the sense in which the original kinematical states may be a good approximation to the physical ones, and the situations in which this is the case. In addition, these models can be deparametrized in a natural way, and we study the effect of time evolution on an "intrinsic" coherent state in the reduced phase space, in order to estimate the time for this state to spread significantly."
john baez
Apr3-04, 12:00 AM
Baez has not published in QG for several years, or only negligibly.
Well, I don't think my paper with Christensen and Egan on asymptotics of 10j symbols was negligible - it contained the results of literally billions of calculations, and it was the first detailed analysis of a spin foam model of quantum gravity. And that was back in August of 2002, which isn't several years yet, just a couple! "Several" means at least 3! :smile:
But, you're right in perceiving that I'm mainly interested in other things
these days.
I found out about this thread from Carlo Rovelli, who sent me an email teasing me about it. I couldn't resist replying to an article entitled "the great John Baez burnout"! I'll take it as a compliment, since it suggests there was a flame flickering there for a while.
Here's how I replied to Rovelli's email:
Dear Carlo -
Hi! I hadn't seen these... thanks. It's pretty funny.
You know you're getting old when you start getting emails
with subject headers like this.
I am in fact rather fed up with quantum gravity. One reason is that
nobody knows a spin foam model that approximates GR in the classical
limit, and I don't see how to get one, despite a lot of work. But
there's another, equally important *positive* reason: these days, work
on n-categories is really revolutionizing mathematics! The subject
is packed with incredible suprises; it goes all the way down to
the foundations of how we think, and there are huge wide-open fields
of fruit ripe for the picking. I can't help but wanting to spend
my time doing this: it's as cool almost as quantum gravity, but I *know*
it will work.
But I might switch back to quantum gravity if and when spin foam
models seem to start working... because I really love the *physical*
universe, and the most mysterious and exciting aspect of math
to me is how it lets us understand the physical universe.
It will be fun to see everyone in Marseilles and see what their
mood is. Probably rather different from mine!
Best,
jb
Just so nobody gets the wrong idea: while I'm tired of trying to find a spin foam model with something like GR as its classical limit, I don't see any reason this should be impossible. Christensen, Egan and I just looked at a few versions of the Barrett-Crane model, and we didn't even succeed in ruling those out, just showing that they were far stranger than anyone expected.
I'm even *more* pessimistic about string theory and M-theory - otherwise I might switch to that.
But really, what got me off quantum gravity was the knowledge that I won't live forever. I have a choice of working on quantum gravity, where nobody knows for sure what's right and what's not, and working on mathematics, where I'm *sure* what I'm doing is right. I spent about a decade working on the former; now I want to do more of the latter.
maybe the term is "mathopause"----it gets mathematicians.
Actually, the idea that mathematicians burn out early is a bit of a myth. Sure, some of them *die* early, like Abel and Galois and Riemann. But the ones who keep living often keep doing good stuff - although lots of them get tired of publishing and spend more time just thinking and talking to people, because it's easier and more fun. For example, take Dennis Sullivan, or Erdos (who got other people to do the writing).
In case anyone is interested, I have a new paper called "Quantum Quandaries: A Category-Theoretic Perspective", in which I argue that a lot of the puzzling things about quantum mechanics will become less puzzling when it becomes part of a theory of quantum gravity, because the category of Hilbert spaces is a lot like a category where the morphisms are spacetimes:
http://math.ucr.edu/home/baez/quantum.ps
This will appear in a volume edited by Steven French, Dean Rickles and Juha Saatsi, probably to be entitled "Structural Foundations of Quantum Gravity".
So, I'm not *completely* fed up with quantum gravity.
I'm also working a lot on the foundations of quantum theory:
http://math.ucr.edu/home/baez/qg-fall2003/
http://math.ucr.edu/home/baez/qg-winter2004/
http://math.ucr.edu/home/baez/qg-spring2004/
So, please don't count me out yet! :smile:
But, it's true that there's a nice new crop of people working on loop quantum gravity and spin foam models.
I won't live forever
Do you have the proof for that? :biggrin:
http://arxiv.org./abs/gr-qc/0404018
Background Independent Quantum Gravity: a Status Report
125 pages
Ashtekar and Lewandowski
cragwolf
Apr6-04, 04:39 AM
Just call me a Baez fanboy. I have spent countless hours at his web site undergoing significant neural rewiring. Because of him I've been inspired to learn mathematics (I mean really learn it, beyond the "mathematical methods for physics" course I took way back in my undergraduate years). Baez is on the cutting edge of physics and mathematics, but he kindly and humbly devotes some of his time to helping us lesser beings learn something about the wonders of these subjects. His web site is a pedagogical paradise.
"Flat spacetime vacuum in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0404021
Authors: A. Mikovic
Comments: 20 pages, 6 figures
"We construct a state in the loop quantum gravity theory with zero cosmological constant, which should correspond to the flat spacetime vacuum solution. This is done by defining the loop transform coefficients of a flat connection wavefunction in the holomorphic representation which satisfies all the constraints of quantum General Relativity and it is peaked around the flat space triads. The loop transform coefficients are defined as spin foam state sum invariants of the spin networks embedded in the spatial manifold for the SU(2) quantum group. We also obtain an expression for the vacuum wavefunction in the triad represntation, by defining the corresponding spin networks functional integrals as SU(2) quantum group state sums"
Looking at the text, he mentions something called "spin network invariants". Never heard of this before (though I'm familiar with things like knot invariants or manifold invariants)
john baez
Apr7-04, 11:49 PM
Looking at the text, he mentions something called "spin network invariants". Never heard of this before (though I'm familiar with things like knot invariants or manifold invariants)
A spin network invariant is a function that assigns a complex number to each spin network embedded in space, where the number doesn't change when you apply a diffeomorphism of space to your spin network. (Here "space" is some 3-dimensional manifold.)
In loop quantum gravity, quantum states are commonly taken to be spin network invariants. You can think of such a state as a big fat linear combination of spin networks, where the coefficients are the aforementioned complex numbers.
If you attach a spin 0, 1/2, 1,... to a knot, you get a spin network of a specially simple kind. So, any spin network invariant gives an infinite sequence of knot invariants. But it has more information.
selfAdjoint
Apr8-04, 10:50 AM
By the way Dr. Baez, I have printed of and am studying your new Quantum Quandries paper. (http://arxiv.org/abs/quant-ph/0404040). How neat! From a sufficiently high perspective, quantum physics and general relativity are more like each other than either of them is like set theory. I am always glad to see set theory marginalized, because of my prejudice for Tarski's theorem and the BSS-machine results.
I've pretty much decided that there isn't a Santa Claus, but is there really a John Baez? :biggrin:
The Bianchi IX model in Loop Quantum Cosmology
Authors: Martin Bojowald, Ghanashyam Date, Golam Mortuza Hossain
Comments: 41 pages, 3 figures, revtex4
Report-no: IMSc/2004/04/16, AEI-2004-028
The Bianchi IX model has been used often to investigate the structure close to singularities of general relativity. Its classical chaos is expected to have, via the BKL scenario, implications even for the approach to general inhomogeneous singularities. Thus, it is a popular model to test consequences of modifications to general relativity suggested by quantum theories of gravity. This paper presents a detailed proof that modifications coming from loop quantum gravity lead to a non-chaotic effective behavior. The way this is realized, independently of quantization ambiguities, suggests a new look at initial and final singularities
http://arxiv.org/abs/gr-qc/0404039
The Bianchi IX model in Loop Quantum Cosmology
Authors: Martin Bojowald, Ghanashyam Date, Golam Mortuza Hossain
Comments: 41 pages, 3 figures, revtex4
Report-no: IMSc/2004/04/16, AEI-2004-028
The Bianchi IX model has been used often to investigate the structure close to singularities of general relativity. Its classical chaos is expected to have, via the BKL scenario, implications even for the approach to general inhomogeneous singularities. Thus, it is a popular model to test consequences of modifications to general relativity suggested by quantum theories of gravity. This paper presents a detailed proof that modifications coming from loop quantum gravity lead to a non-chaotic effective behavior. The way this is realized, independently of quantization ambiguities, suggests a new look at initial and final singularities
http://arxiv.org/abs/gr-qc/0404039
How does LQG get rid of cosmological singularities in homogeneous models for e.g. Bianchi V? Any good references to papers?
Is the Belinskii-Khalatnikov-Lifschitz scenario mostly used in analyzing quantum gravity solutions for cosmological singularities?
Thanks for any help (am just a beginner :redface:) .
How does LQG get rid of cosmological singularities in homogeneous models for e.g. Bianchi V? Any good references to papers?
.
Maddy, almost all the papers I know of by Bojowald etc have dealt with the isotropic case.
I will find some URL for ones dealing with anisotropic, but it is a comparatively recent effort AFAIK.
I havent seen anything about Bianchi V. I only recall stuff about Bianchi IX.
People sometimes come here (stingray, nonunitary) who might be able to respond more usefully.
As for papers about the homogenous (but not isotropic) case here are some references:
the most recent review of progress in the field is
"Loop Quantum Cosmology: Recent Progress"
http://arxiv.org/gr-qc/0402053
this contains only two refs that are explicitly to the homogeneous case and they are from last year
"Homogeneous loop quantum cosmology" gr-qc/0303073
"Homogeneous loop quantum cosmology: the role of the spin connection" gr-qc/0311004
maybe you can find more if you look in the references of these
there was also this, which I have not looked at so cannot say if it mentions Bianchi V,
"Quantum suppression of the generic chaotic behavior close to cosmological singularities" gr-qc/0311003
it looks to me as if the homog. case is just barely being scratched at
and that the papers mostly go back only to November of last year
but I am not knowledgeable about homog. case and you might find out things
are different if you take a closer look
most LQC papers involve homogenenous and isotropic simplification and
in effect use a quantized form of the Friedmann equation (which is what most cosmology depends on anyway)---again this is just my limited view and we could hopefully get some expert comment responding to your question
http://arxiv.org/abs/gr-qc/0404055
Ln(3) and Black Hole Entropy
Authors: Olaf Dreyer
Comments: Contribution to the Proceedings of the 3rd International Symposium on Quantum Theory and Symmetries, Cincinnati, September 2003
"We review an idea that uses details of the quasinormal mode spectrum of a black hole to obtain the Bekenstein-Hawking entropy of A/4 in Loop Quantum Gravity. We further comment on a recent proposal concerning the quasinormal mode spectrum of rotating black holes. We conclude by remarking on a recent proposal to include supersymmetry. "
This paper try to fix the Immirzi parameter
Give a look to page 2. It contains the formula for the entropy of a Black hole according to Loop Quantum Gravity
this thread is serving as a surrogate sticky "reference library". Thanks to all who have contributed so far!
-------Loop Gravity texts--------
Rovelli posted the 30 December 2003 draft of his book "Quantum Gravity", to be published this year by Cambridge University Press.
The PDF file is at his homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
The book is around 350 pages long and takes a few (like ten?) minutes to download and convert.
To download the 30 December 2003 draft of the book directly:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf
Here are Thiemann's Lecture Notes (they have been published in Berlin by Springer Verlag)
"Lectures on Loop Quantum Gravity".
A draft is online at
http://arxiv.org/gr-qc/0210094
-----a recent review article----
http://arxiv.org./abs/gr-qc/0404018
Ashtekar and Lewandowski
"Background Independent Quantum Gravity: a Status Report"
125 pages
many references
---------a newsletter: "Matters of Gravity"----
Jorge Pullin's newsletter "Matters of Gravity"
http://arxiv.org./abs/gr-qc/0403051
this is the Spring 2004 issue
-------Quantum Gravity Phenomenology and DSR---------
some recent phenomenology and DSR papers:
Ted Jacobson, Stefano Liberati, David Mattingly
"Quantum Gravity Phenomenology and Lorentz Violation"
http://arxiv.org./abs/gr-qc/0404067
15 April 2004
Giovanni Amelino-Camelia
"A perspective on quantum gravity phenomenology"
http://www.arxiv.org/abs/gr-qc/0402009
dated 2 February 2004
Giovanni Amelino-Camelia, Jerzy Kowalski-Glikman, Gianlucca Mandanici, and Andrea Procaccini
"Phenomenology of Doubly Special Relativity"
http://arxiv.org/gr-qc/0312124
dated 30 December 2003
Jerzy Kowalski-Glikman
"Doubly Special Relativity and quantum gravity phenomenology"
http://arxiv.org/hep-th/0312140
dated 12 December 2003
Jerzy Lukierski
"Relation between quantum κ-Poincare framework and doubly special relativity"
http://arxiv.org./hep-th/0402117
dated 18 February 2004
other less recent ones:
Jerzy Kowalski-Glikman and Sebastian Nowak
"Doubly Special Relativity and de Sitter space"
http://arxiv.org/hep-th/0304101
dated 11 October 2003
M. Daszkiewicz, K. Imilkowska, J. Kowalski-Glikman
"Velocity of particles in Doubly Special Relativity"
http://arxiv.org/hep-th/0304027
dated 3 April 2003
---------Loop Quantum Cosmology-------
Martin Bojowald
"Loop Quantum Cosmology: Recent Progress"
http://arxiv.org/gr-qc/0402053
One of the invited plenary talks at the January 2004 ICGC
conference (see list of recent conferences)
The Bianchi IX model in Loop Quantum Cosmology
Martin Bojowald, Ghanashyam Date, Golam Mortuza Hossain
41 pages
http://arxiv.org/abs/gr-qc/0404039
"Inflationary Cosmology and Quantization Ambiguities in Semi-Classical Loop Quantum Gravity"
Martin Bojowald, James E. Lidsey, David J. Mulryne, Parampreet Singh, Reza Tavakol
15 pages, 8 figures
http://arxiv.org./abs/gr-qc/0403106
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology and Boundary Proposals"
http://arxiv.org/gr-qc/0312103
dated 23 December 2003
Martin Bojowald
"Quantum Gravity and the Big Bang"
http://arxiv.org./astro-ph/0309478
dated 17 September 2003, briefly summarizes how
LQG can serve to cure the big bang singularity and
motivate inflationary expansion. Short and less technical
than the other two papers.
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology, Boundary Proposals, and Inflation"
http://arxiv.org/gr-qc/0303072
dated 19 March 2003
Shinji Tsujikawa, Parampreet Singh, Roy Maartens
"Loop quantum gravity effects on inflation and the CMB"
http://arxiv.org/astro-ph/0311015
from the Tsujikawa/Singh/Maartens abstract:
"In loop quantum cosmology, the universe avoids a big bang singularity and undergoes an early kinetic-dominated super-inflation phase, with a quantum-corrected Friedmann equation. As a result, an inflaton field is driven up its potential hill, thus setting the initial conditions for standard inflation. We show that this effect can raise the inflaton high enough to achieve sufficient e-foldings in the standard inflation era. We analyze the cosmological perturbations and show that loop quantum effects can leave a signature on the largest scales in the CMB, with some loss of power and running of the spectral index."
Viqar Husain and Oliver Winkler "On singularity resolution in quantum gravity"
http://arxiv.org/gr-qc/0312094
this is especially interesting because they duplicate LQC results (for example by Bojowald) using the older version of quantum gravity, ADM variables, quantized metric. Shows that the removal of the big bang singularity is "robust"---doesnt depend on using a particular formalism.
as a background reference for classical (non-quantum) cosmology:
Charles Lineweaver
"Inflation and the Cosmic Microwave Background"
http://arxiv.org/astro-ph/0305179
dated 12 May 2003
-------recent conferences------
Strings meet Loops (Albert Einstein Institute, MPI-Potsdam) October 2003
http://www.aei-potsdam.mpg.de/events/stringloop.html
Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm
International Conference on Gravity and Cosmology (India) January 2004
http://www.cusat.ac.in/icgc04/
Quantum Gravity Phenomenology, (40th annual Polish Winterschool in Theoretical Physics) February 2004
http://www.ws2004.ift.uni.wroc.pl/html.html
--------upcoming conferences--------
Loop/SpinFoam Conference (Marseille) May 2004
http://w3.lpm.univ-montp2.fr/~philippe/quantumgravitywebsite/
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?03Aug.1
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?04Feb.2
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?10
General Relativity Conference (Dublin) July 2004
more annoucements at
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference
------observational means for testing quantum gravity------
Floyd Stecker
"Cosmic Physics: the High Energy Frontier
http://arxiv.org/astro-ph/0309027
dated September 2003
Stecker discusses the various earth-based and orbital instruments, currently operating, or under construction, or planned, or proposed, and the kind of data becoming available. Among many other things he discusses GLAST, planned to start operating 2007, which, if there are tiny energy-dependent differences in speed among gamma-ray-burst photons, may be able to detect same. Also discusses neutrino observation.
------links to an unselective assortment of current work------
Carlo Rovelli and Winston Fairbairn
"Separable Hilbert space in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0403047
John Baez
"Quantum Quandaries: A Category-Theoretic Perspective"
http://arxiv.org/quant-ph/0404040
Livine's thesis
http://arxiv.org/gr-qc/0309028
Girelli and Livine
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032
Oriti's thesis
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"
Karim Noui and Philippe Roche
"Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
The cosmological constant occurs in a number of recent quantum gravity papers, for instance the one by Girelli/Livine.
Velhinho "On the structure of the space of generalized connections"
http://arxiv.org/math-ph/0402060
Noui and Perez "Three dimensional loop quantum gravity: physical scalar product and spin foam models"
http://arxiv.org/gr-qc/0402110
Noui and Perez "Three dimensional loop quantum gravity: coupling to point particles"
http://arxiv.org/gr-qc/0402111
Noui and Perez "Dynamics of Loop Quantum Gravity and Spin Foam Models in Three Dimensions"
http://arxiv.org/gr-qc/0402112
Noui and Perez "Observability and Geometry in Three Dimensional Quantum Gravity"
http://arxiv.org/gr-qc/0402113
Freidel and Louapre "Ponzano-Regge model revisited, I."
http://arxiv.org/hep-th/0401076
Gambini and Pullin "Canonical Quantum Gravity..."
http://arxiv.org/gr-qc/0402062
Buffenoir, Henneaux, Noui, Roche
Hamiltonian Analysis of Plebanski Theory
http://arxiv.org./gr-qc/0404041
(spin foam, BF)
----------fundamental constants, planck units, time-keeping-------
Historical source for Planck units, the 1899 paper (thanks arivero!)
http://www.bbaw.de/bibliothek/digital/struktur/10-sitz/1899-1/jpg-0600/00000494.htm
In December 2003, the National Institute of Standards and Technology (NIST) posted new CODATA recommended values for the basic planck units
http://physics.nist.gov/cuu/Constants/
choose "universal" from the menu to find (among other things) the recommended values of
planck mass
planck length
planck time
planck temperature
A 1997 article on timekeeping, discussing GR effects allowed-for in the GPS
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
-------science journalism----
"The Duel: Strings versus loops"
http://arxiv.org/abs/physics/0403112
A translation of Rudy Vaas' article in the German
science magazine "Bild der Wissenschaft" roughly
comparable to the "Scientific American"
========
simply to have this link on LaTex handy:
http://www.physicsforums.com/misc/howtolatex.pdf
Woit's blog:
http://www.math.columbia.edu/~woit/blog/
the responses are getting interesting too
------------------
Recent paper by Olaf Dreyer
http://arxiv.org./gr-qc/0404055
-------------------
this is spillover from the main page of links
which is full
-------------------
a new QG Phenomenology paper
http://arxiv.org./gr-qc/0404113
"On alternative approaches to Lorentz violation invariance in loop quantum gravity inspired models
Jorge Alfaro, Marat Reyes, Hugo A. Morales-Tecotl and L.F. Urrutia
------------
there is also a new Quantum Gravity Phenomenology
paper by Ted Jacobson et al
dealing with QG predictions of Lorentz violation and
their testability
http://arxiv.org/gr-qc/0404067
"Quantum Gravity Phenomenology and Lorentz Violation"
Ted Jacobson, Stefano Liberati, David Mattingly
looks like some category theory may be needed to do
quantum gravity (e.g. Velhinho, also several Baez papers)
http://www.folli.uva.nl/CD/1999/library/pdf/barrwells.pdf
Barr is at McGill and Wells is at U Virginia
its >100 pages of lecture notes
http://www.dcs.ed.ac.uk/home/dt/CT/categories.pdf
these notes are by Daniele Turi at U. Edinburgh
they are based on Saunders Mac Lane book
"Categories for the working mathematician"
Hendryk Pfeiffer has a new preprint on arxiv
called
"Quantum Gravity and the Classification of
Smooth Manifolds"
http://arxiv.org./gr-qc/0404088
"...d = 3 + 1, if it can indeed be constructed, will offer the same potential. This relationship is the main theme of the present article.
The special role of space-time dimension d = 3 + 1 in differential topology is summarized by the following result.
Theorem 1.1. Let M be a topological d-manifold, d = 1, 2, 3, 4,... If M admits an infinite number of pairwise inequivalent differentiable structures, then d = 4.
This is a corollary of several theorems by various authors. We explain in this article why this result is related to the search for a quantum theory of general relativity..."
here is something unique about the dimension 4
It is the only possible dimension for spacetime to be if you want to have
plenty of possible smooth-manifold structures.
In this sense, dim = 5 is not OK, and dim = 6 is not OK.
This is a surprising theorem. I didnt know that d = 3+1 was mathematically so special as that. It is so surprising that I think I must be failing to understand. but Pfeiffer is I think very good and there it is written in black and white as theorem 1.1, so will go back and try to understand.
-------had to do something else and just got back-----
Pfeiffer says, on page 16:
"In d ≥ 3 + 1, no analogous result is available. There exist countably infinite families of (compact) smooth 4-manifolds [13] which are pairwise non-diffeomorphic, but which have homeomorphic underlying topological manifolds. There is therefore a considerable discrepancy between C∞- and C0-QFTs in d = 3 + 1 space-time dimensions.
The most striking result even concerns the standard space R4 [14, 15].
Theorem 4.1. Consider the topological manifold Rd, d ε N.
• If d < 4, then there exists a differentiable structure for Rd which is unique up to diffeomorphism.
• If d = 4, then there exists an uncountable family of pairwise non diffeomorphic differentiable structure for Rd.
Non-uniqueness of differentiable structures persists in higher dimensions, for example, there are 28 inequivalent differentiable structures on the sphere S7, or 992 inequivalent differentiable structures on S11, [16], but in dimension d ≥ 4 + 1 (d ≥ 5 + 1 if the manifold has a non-empty boundary), there never exists more than a finite number of non-diffeomorphic differentiable structures on the same underlying topological manifold.
The space-time dimension d = 3 + 1 is distinguished by the feature that there can exist an infinite number of homeomorphic, but non-diffeomorphic smooth manifolds."
------then on page 19 Pfeiffer says------
"Scenario for quantum gravity.
We have reached a first goal: the diffeomorphism gauge symmetry of general relativity on a closed space-time manifold has been translated into a purely combinatorial problem involving triangulations that consist of only a finite number of simplices, and their manipulation by finite sequences of Pachner moves.
If not only the partition function, but also the full path integral of general relativity in d ≤ 5+1 is given by a PL-QFT, we know that all observables are invariant under Pachner moves.
The partition function of quantum general relativity is an invariant of PL-manifolds, too, and can be computed by purely combinatorial methods for any given combinatorial manifold.
A generic expression of such a partition function is the state sum,
Z =
\sum_{ colourings } \prod_{ simplices }
(amplitudes),
where the sum is over all labelings of the simplices with elements of some set of colours, and the integrand is a number that can be computed for each such labeling. In Section 5 below, we give examples and illustrate that the partition function of quantum general relativity in d = 2 + 1 is precisely of this form.
If quantum general relativity in d = 3+1 is indeed a PL-QFT, the following two statements which sound philosophically completely contrary,
• Nature is fundamentally smooth.
• Nature is fundamentally discrete.
are just two different points of view on the same underlying mathematical structure: equivalence classes of smooth manifolds up to diffeomorphism."
also on page 20, right after this, there is a picture which illustrates what are Pachner moves in 2 dimensions and 3 dimensions.
this business on pages 19 and 20 of Pfeiffer paper seems interesting. I never heard talk like this. I have bolded some exerpts for emphasis.
http://arxiv.org/abs/gr-qc/0404083
Spectrum of quantized black hole, correspondence principle, and holographic bound
Authors: I.B. Khriplovich
Comments: 9 pages
An equidistant spectrum of the horizon area of a quantized black hole does not follow from the correspondence principle or from general statistical arguments. On the other hand, such a spectrum obtained in loop quantum gravity (LQG) either does not comply with the holographic bound, or demands a special choice of the Barbero-Immirzi parameter for the horizon surface, distinct from its value for other quantized surfaces. The problem of distinguishability of edges in LQG is discussed, with the following conclusion. Only under the assumption of partial distinguishability of the edges, the microcanonical entropy of a black hole can be made both proportional to the horizon area and satisfying the holographic bound
selfAdjoint
Apr22-04, 09:38 AM
That's a new thought. All edges are equal but some edges are more equal.
I knew long time ago about these peculiarity of 4-dimensional manifolds. I thought it was a mainstream knowledge.
By some stupid reason i have no acces to arwiv for around a week so i couldn´t read the article. So while waitng i´ll ask about other, slighly related, things.
I have a few generic questions/refelxions aobut some of the themes LQG is addressing.
Let´s beging by the question of entropy. My deal is whether the concept of entropy makes sense in GR at all. A lest in the same sense that in ordinay statistichal mehcanics.
I know about two main results. The one, of wich i have a reasonable understanding , about the black hole area behaving like entropy. I also have notice about (but no understanding at all) results of Penrose relating the Weyl tensor to entropy, at least in cosmologicla scenarys
The question is that in the microcanonical device the entropy is reltaed to the number of micro-states compatible with an energy. But in GR there is no a good (and less local) definition of the energy of the gravitatory field.
I think these is commonly known. Anyway i would like to know how it has been addressed.
In order to get my own understanding i revised the whole idea (i never have had a deep basics in statistichal mecanics and it was a good exercice for me) of micro-states.
It works fine because it is used mainly for quantum mechanical systems with a discrete spectrum. But i wanted to understand how it could work incontinuous systems, other than the gravitatory field, an dalso in continous spectrum of quantum systems.
I begined revising the black-body radiation. There the thermal equiibrium is suposed to be achieved by influence of the walls of a cavity.
By general (non rigurous) arguments (kirchov laws and it´s subleties/consecuences) it is assumed that an arbitrary (as far as i know expermintally tested for simply connected) shaped body can be simulated by a box shaped resonant cavity.
Them the number of microstates of the EM field can be charazterized by thre numbers (k1.k2,k3). And by standard manipulations they can be related to the frecuency.
Ok, i will not reexplain all the remaining basic facts. My point is here. We can give a meaning to the number of microstates because we are dealing with a configuration in wich the EM field is in a compact region. ALso it is important to note that it seems that we are faced with a topological nature of the black body radiation. It is invariant under topological changes of the shap of the cavity.
So i try to think on the vibration numbers like something with a topologicla meaning. ¿woul the black body reaction could have another behaviour for non topologically trivial shapes of the caity?
Well, i know these hass addresed me far form the original question of the entropy of the gravitatory field. But my gues is that we are triying to aplly blindly a concept which works fine in non relativistic discret spectred hamiltonians. Maybe in some more general situations it needs some refinements.
For example, ¿whay about yang-mills fields?
Of course you will be saying ¿What about at finite temperature field theory?
I have not studied it in deep. But the whole idea seems to me so merely formal that doesn´t convince me as a good way to gain deep understanding of the problems. But of course probably it is my problem.
Well, i have more conceptual issues. But i´ll expose them in another moment.
this is interesting
QG has already challenged the Big Bang singularity
quantizing seems to remove it (Bojowald and others say)
maybe there is no black hole singularity, just a very deep well
maybe quantizing GR will remove this singularity also
also even without quantizing GR some alternative models avoid a singularity
http://arxiv.org/astro-ph/9908113
"Alternate Explosions: Collapse and Accretion Events with Red Holes instead of Black Holes"
html version:
http://arxiv.org./html/astro-ph/9908113
http://arxiv.org./astro-ph/9912322
"Red Hole Gamma-Ray Bursts: A New Gravitational Collapse Paradigm
Explains the Peak Energy Distribution And Solves the GRB Energy Crisis"
gammaray bursts are especially interesting and may even now not be
satisfactorily explained---awful lot of energy in them, more than supernovas
what has happened to these 1999 conjectures?
EDIT: by the way the author, Jim Graber, sometimes posts here at PF.
why doesnt someone ask him about this alternative picture of gravitational collapse
looks like some category theory may be needed to do
quantum gravity (e.g. Velhinho, also several Baez papers)
http://www.folli.uva.nl/CD/1999/library/pdf/barrwells.pdf
Barr is at McGill and Wells is at U Virginia
its >100 pages of lecture notes
http://www.dcs.ed.ac.uk/home/dt/CT/categories.pdf
these notes are by Daniele Turi at U. Edinburgh
they are based on Saunders Mac Lane book
"Categories for the working mathematician"
not being a whiz with categories or else they're just not very familiar, I'm
a bit bothered by their infiltrating into QG
there was just this paper by John Baez
"Quantum Quandaries: A Category-Theoretic Perspective"
http://arxiv.org/quant-ph/0404040
and this other recent paper by Hendryk Pfeiffer
is categorical in its approach
"Quantum Gravity and the Classification of
Smooth Manifolds"
http://arxiv.org./gr-qc/0404088
and back in February there was this paper by Velhinho
"On the structure of the space of generalized connections"
http://arxiv.org/math-ph/0402060
and now there is a woman mathematician who has published with Louis Crane and lives in New Zealand---her name is Marni Dee Sheppeard.
Her paper is so categorical that it seems the same to me whether I read it front to back or back to front. That is, I see a lot of diagrams with arrows and dont understand anything. but it purports to be about quantum gravity and I like the name Marni Dee so here's the link:
http://arxiv.org/gr-qc/0404121
On state sums, internalisation and unification
M. D. Sheppeard
35 pages
Abstract: "In this mostly expository article, elements of higher category theory essential to the construction of a class of four dimensional quantum geometric models are reviewed. These models improve current state sum models for Quantum Gravity, such as the Barrett-Crane model, in that they appear, for instance to remove degeneracies which swamp the partition function. Much work remains to be done before a complete construction is reached, but the crucial categorical notion of internalisation already illuminates the idea that a full unified model may result from few, albeit as yet poorly understood, additional principles. In particular, a spacetime and matter duality principle is employed through an understanding of the role of pseudomonoidal objects in categorified cohomology."
the good news: these models improve on the Barrett-Crane model which is pretty much the main spin-foam studied and which has some numerical crankiness (Baez published a paper in 2002 about this)
the good news: somehow putting spacetime and matter on the same footing?
the bad news: what the devil is pseudomonoidal objects in categorified cohomology?! it sounds like a new disease and I just hope it's not painful.
well fraid theres something else to learn about in QG
selfAdjoint
Apr30-04, 04:48 PM
The Sheppeard paper looks good! I am going to read it, and see if I can keep up with it.
I just love the sniffy prose style too, which I associate with Oxbridge and its colonies like The Economist :
"The first and simplest way of regarding a manifold M as a category is to give it, albeit rather trivially, a groupoid structure. That is, the points of the manifold are the objects of the category and each point has attached an identity arrow, which is of course invertible."
Of course!
The Sheppeard paper looks good! I am going to read it, and see if I can keep up with it.
I am much cheered by your positive reaction.
I think I might be able to get something out of pages 6 thru 10
as they are a basic expostion of category theory.
although I'm less than confident of getting much of the rest of the paper.
I didnt notice at first that she even had a prose style! when I first looked i was just very impressed: she seemed awesomely intelligent and mainly bewildering. Will try to attribute some of this to her English.
This is a nice efficient way to start talking about categories (maybe it is the standard definition):
"Whereas a set has elements, and a map between sets takes elements to elements, a category has both elements, called objects, and relationships between elements, called arrows. Every object A is equipped with at least an identity arrow 1A from A to A. Maps between categories, called functors, take objects to objects and arrows to arrows. Arrows may be composed f ◦g..."
she makes this point first off that I dont believe Ive heard put so clearly by anyone:
background independence is basic to QG, she says, and
if you want background independence
then you have to go categories.
rovelli never told me that, baez only hinted at it, she flatly asserts it.
there is a bold magesterial quality turn to her thinking. Doesnt mince words. Says right out front where she's going. E.g. here is the first of the introduction:
"1 Introduction
The philosophy behind the construction outlined here is that any reasonable
attempt to describe quantum gravity within a unified framework ought to
respect some quantum principle of general covariance. Recall that in coming
to accept general covariance in the first place [37], Einstein needed to rid
himself of the idea that spacetime points had physical meaning outside of
their use in the metric tensor field. Put another way: no gravitational field,
no spacetime.
It is argued here that categorical internalisation is an essential element
required of a successful mathematical description of such a principle. There is mounting support for this point of view from studies of, for instance,..."
selfAdjoint
Apr30-04, 09:55 PM
Yes. The only problem with doing the first pages is that she front loads the abstract stuff and expects her readers to have some problems with that. Of course with my weird background, that's the part I like!
http://arxiv.org/abs/hep-th/0405036
Remarks on the black hole entropy and Hawking spectrum in Loop Quantum Gravity
Authors: A. Alekseev, A.P. Polychronakos, M. Smedback
Report-no: CCNY-HEP-04/3, UUITP-13/04
In this note we reply to the criticism by Corichi concerning our proposal for an equidistant area spectrum in loop quantum gravity. We further comment on the emission properties of black holes and on the statistics of links.
Doesn't LQG propose disconnected quanta of space-time? Or are these quanta of space-time connected by infinitesimal strings between them? Does LQG really propose communication through something of no dimensional thickness?
Sorry for the off-topic.
As I told in the previous post I couldn´t download the last pdf´s because my acces to arxiv was temporarilly locked.
The nnouncement said "one week", but it was taking more time to be unlocked so I mailed arxiv and the lock was deletted.
But i have been warned about using prefetching software. ¿Wha´s that?, could it be the acrobat reader active-w wich executes from inside the explorer.
I ask because i wouldn´t want to be locked agian. I can´t follow properly these interesting threads without acces to the pdf´s, you know.
thanks in advance and sorry again for the off-topic.
Sauron I was also banned one week from arxiv, but it was because my acrobat reader was of an old version. They said that if i updated to the newer version of acrobat I wouldn't be banned again, so I did and so I haven't been banned anymore
Sauron and Meteor, I haven't yet experienced this difficulty with arxiv
but if it happens, knowing about the
possibility will make it less of a shock. thanks for the warning!
----------------
a September 2004 conference in quantum gravity has been announced
by Jonathan Halliwell of Imperial College London
"Current Themes in Quantum Gravity: A Two-day Conference
in Honour of the 60th Birthday of Chris Isham
Imperial College, September 6-7, 2004.
Inspired by the 60th birthday of Prof. Chris Isham, we are pleased to announce a conference whose aim is to take stock of the current status of quantum gravity, its mathematical foundations, conceptual problems and physical predictions for cosmology and black holes. Aspects of quantum theory related to gravity will also be considered. The main themes of the meeting will be as follows:
The canonical Hamiltonian approach to gravity and the loop variable approach to quantum gravity.
Alternatives to the canonical approach, such as causal sets and other discrete models.
The phenomenology of quantum gravity, in particular, quantum cosmology and quantum black holes.
Aspects of the foundations of quantum theory related to quantum gravity, in particular, the decoherent histories approach and topos-theoretic ideas.
Conceptual problems in quantum gravity, in particular, the problem of time.
The list of speakers will include,
Abhay Ashtekar (Penn State)
Stanley Deser (Brandeis)
Gary Gibbons (Cambridge)
Jim Hartle (Santa Barbara)
Stephen Hawking (Cambridge)
Karel Kuchar (Utah)
Renate Loll (Utrecht)
Roger Penrose (Oxford)
Carlo Rovelli (Marseille)
Rafael Sorkin (Syracuse)
"
the announcement says more detail will eventually be available on the
website
http://theory.ic.ac.uk.
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?04Apr.11
a recent paper co-written by Renate Loll
http://arxiv.org./hep-th/0404156
"Emergence of a 4D World from Causal Quantum Gravity"
my unqualified&inexpert opinion: Renate Loll kicks butt, anybody studying quantum gravity would be lucky to get a chance to study with her at Utrecht (netherlands)
they (Ambjorn, Jurkiewicz, Loll) did a monte carlo simulation (massive randomized computer experiment) and 4 dimensions emerged probabilistically from their quantum gravity setup---they probably helped and it isnt conclusive but it is a suggestive preliminary finding at the least
Renate Loll is one of the featured speakers (with Stephen Hawking, Carlo Rovelli, Roger Penrose,...) at that Quantum Gravity conference happening in London this september. what odds she talks about this monte carlo QG research
A few hours after I wrote this (about the AJL paper) John Baez posted the "Marseille Workshop" thread at PF calling attention to his "Week 206" which is mostly about the Ambjorn Jurkiewicz Loll paper. Here is a link to Baez Week 206:
http://math.ucr.edu/home/baez/week206.html
Judging from what Baez says, and from today's reaction by Thomas Larsson on SPR, it could be a landmark paper.
Larsson's comment on AJL in reply to Baez week 206 was:
"If the numerical evidence in this paper is true, and it seems quite strong, then we see a new field open up here...I would not be surprised if this is the next bandwagon and a lot of smart people will jump onto it."
Hi Marcus (ranyart here) you not be aware of this 'amazing' paper:http://uk.arxiv.org/abs/gr-qc/0405060
One question before the storm?..the Dynamical Volume Operator here proposed is 3-D, Volumes do not exist in 2-Dimensional Spacetimes.
Straight Lengths for 2-D spaces, Curved Lengths for 3-D!
we are on our way ;)
...not be aware of...:http://uk.arxiv.org/abs/gr-qc/0405060
another by Thomas Thiemann!
he is busy this year
hope all is well with you
thanks for the link
getting back what we were talking about earlier
here is a photograph of Renate Loll
http://www1.phys.uu.nl/wwwitf/fotopagina's/Medewerkers/Renate.htm
for some reason it does not form an active link and must be copy-pasted into the browser
the snapshot appears to have been taken at Utrecht institute for theoretical physics
with ears discretely concealed (in case of being a Vulcan)
a recent paper co-written by Renate Loll
http://arxiv.org./hep-th/0404156
"Emergence of a 4D World from Causal Quantum Gravity"
my unqualified&inexpert opinion: Renate Loll kicks butt, anybody studying quantum gravity would be lucky to get a chance to study with her at Utrecht (netherlands)
they (Ambjorn, Jurkiewicz, Loll) did a monte carlo simulation (massive randomized computer experiment) and 4 dimensions emerged probabilistically from their quantum gravity setup---they probably helped and it isnt conclusive but it is a suggestive preliminary finding at the least
Renate Loll is one of the featured speakers (with Stephen Hawking, Carlo Rovelli, Roger Penrose,...) at that Quantum Gravity conference happening in London this september. what odds she talks about this monte carlo QG research
the best introductory presentation of Loll's style of quantum gravity that I have found so far is
http://arxiv.org/hep-th/0212340
"A Discrete History of the Lorentzian Path Integral"
this is dated 13 January 2003 and describes results in d = 2 and d = 3.
These foreshadow the results just announced for d = 4.
On page 16 she says:
"... In d = 4, the first numerical simulations are currently being set up."
so this is a good pedagogical exposition of just before the recent major result
http://arxiv.org./hep-ph/0306/0306198
Measuring quantum states of neutrons in the Earth's gravitational field
this experiment was a first
(objects falling in the earth's grav field seem to descend in jumps from level to level because their grav potential energy is quantized)
it was a beautiful experiment
first announced in a 2-page note in Nature vol 415 (2002)
http://www.nature.com/nsu/020114/020114-8.html
but this 9-page (Phys Rev D) article is online and beautifully illustrated
and more complete than the earlier one in Nature
[this thread is serving as a "surrogate sticky" quantum gravity link-basket,
or a reference library of links to QG stuff, the quantization of energy levels of a falling neutron seems at least periferally related to quantum gravity]
Nesvizhevsky's team is at Grenoble France but now
according to ZapperZ in another thread
there may be related work at Mainz
selfAdjoint
May14-04, 10:00 AM
Umm, Marcus, that experiment has been interpreted as showing not that gravity is quantized but that the particle's ENERGY is quantized. So whatever the gravity potential is, they can only acquire energy from it in steps. But of course that's old news.
so does the link belong in General Physics?
and not in the Quantum Gravity link-basket?
I would like to keep tabs on it one place or the other
but dont care about the wording
if you suggest an editing change I will be glad to make it
[PS I think I see the objection so I went ahead and changed
the wording in the post to emphasize that the connection with QG may be
only periferal.]
pelastration
May14-04, 06:05 PM
Marcus,
This Loll et All approach reminds me somehow to Buckminster Fuller's work , but he didn't started from 'dust-type' particles.
Buckminster was a very gifted guy. He was very consequent in his idea's, also on politics. This is the offical website: http://www.bfi.org
In his giant standard work http://www.rwgrayprojects.com/synergetics/synergetics.html you will some find some amazing concepts. I believe you have to enter that link first (for copyright).
Then I find it helpful to go first to the index about images : http://www.rwgrayprojects.com/synergetics/findex/findex.html.
By these image numbers you can look for the real texts (paragraph numbers) in the Table of content http://www.rwgrayprojects.com/synergetics/toc/toc.html.
Buckminster: "Please now think of all the tensional forces of Universe as one single membrane containing all the radiational, explosive forces we have enumerated. Now think of the original compression sphere exploding into many ... individual, exploded-apart, spherical mass components, each of which is tightly embraced by the membrane - leaving only intervening perpendicular linear tubes."
I think you will appreciate his logic.
Dirk
A new paper by Amelino-Camelia
http://arxiv.org/gr-qc/0405084
A new one by Livine and Oriti
http://arxiv.org/gr-qc/0405085
any comments are welcome. I have to go out for part of the evening, so
may not get around to discussing these for a while. I would guess the
Livine/Oriti paper is interesting.
L/O:
----quote---
Does a quantum gravity theory with an invariant length and a discrete spectrum for geometric observables necessarily break Lorentz symmetry or necessarily require some sort of modification/deformation of it? The answer, as we will see, is simply “no”.
---end quote---
I believe they are Two Gems!
this thread is serving as a surrogate sticky "reference library" for useful LQG links. Thanks to all who have contributed so far!
the last time I updated the main list of references was post #163 about thirty posts back, so it's about time to update the list again
I will break it down into some categories, with textbooks and introductory survey lectures and such coming first
------- texts--------
Rovelli posted the 30 December 2003 draft of his book "Quantum Gravity", to be published this year by Cambridge University Press.
The PDF file is at his homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
The book is around 350 pages long and takes a few (like ten?) minutes to download and convert.
To download the 30 December 2003 draft of the book directly:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf
Here are Thiemann's Lecture Notes (they have been published in Berlin by Springer Verlag)
"Lectures on Loop Quantum Gravity".
A draft is online at
http://arxiv.org/gr-qc/0210094
-----a recent review article----
http://arxiv.org./abs/gr-qc/0404018
Ashtekar and Lewandowski
"Background Independent Quantum Gravity: a Status Report"
125 pages
many references
another recent survey:
Enrique Alvarez
http://arxiv.org/gr-qc/0405107
"Quantum Gravity"
( Lectures given at Karpacz. 40 pages)
this next is older and interesting partly for historical and broader perspective.
it is a Rovelli survey at a 1997 GR conference (plenary at GR15)
and you get not just LQG and string but some other approaches that
were tried in the 1990s:
http://arxiv.org/gr-qc/9803024
Carlo Rovelli
"Strings, loops and others: a critical survey of the present approaches to quantum gravity"
" I illustrate the main achievements and the main difficulties in: string theory, loop quantum gravity, discrete quantum gravity (Regge calculus, dynamical triangulations and simplicial models), Euclidean quantum gravity, perturbative quantum gravity, quantum field theory on curved spacetime, noncommutative geometry, null surfaces, topological quantum field theories and spin foam models...."
---------a newsletter: "Matters of Gravity"----
Jorge Pullin's newsletter "Matters of Gravity"
http://arxiv.org./abs/gr-qc/0403051
this is the Spring 2004 issue
-------Quantum Gravity Phenomenology and DSR---------
some recent phenomenology and DSR papers:
A new paper by Amelino-Camelia
http://arxiv.org/gr-qc/0405084
http://arxiv.org./gr-qc/0404113
"On alternative approaches to Lorentz violation invariance in loop quantum gravity inspired models
Jorge Alfaro, Marat Reyes, Hugo A. Morales-Tecotl and L.F. Urrutia
Ted Jacobson, Stefano Liberati, David Mattingly
"Quantum Gravity Phenomenology and Lorentz Violation"
http://arxiv.org./abs/gr-qc/0404067
15 April 2004
Giovanni Amelino-Camelia
"A perspective on quantum gravity phenomenology"
http://www.arxiv.org/abs/gr-qc/0402009
dated 2 February 2004
Giovanni Amelino-Camelia, Jerzy Kowalski-Glikman, Gianlucca Mandanici, and Andrea Procaccini
"Phenomenology of Doubly Special Relativity"
http://arxiv.org/gr-qc/0312124
dated 30 December 2003
Jerzy Kowalski-Glikman
"Doubly Special Relativity and quantum gravity phenomenology"
http://arxiv.org/hep-th/0312140
dated 12 December 2003
Jerzy Lukierski
"Relation between quantum ?-Poincare framework and doubly special relativity"
http://arxiv.org./hep-th/0402117
dated 18 February 2004
other less recent ones:
Jerzy Kowalski-Glikman and Sebastian Nowak
"Doubly Special Relativity and de Sitter space"
http://arxiv.org/hep-th/0304101
dated 11 October 2003
M. Daszkiewicz, K. Imilkowska, J. Kowalski-Glikman
"Velocity of particles in Doubly Special Relativity"
http://arxiv.org/hep-th/0304027
dated 3 April 2003
---------Loop Quantum Cosmology-------
Martin Bojowald
"Loop Quantum Cosmology: Recent Progress"
http://arxiv.org/gr-qc/0402053
One of the invited plenary talks at the January 2004 ICGC
conference (see list of recent conferences)
The Bianchi IX model in Loop Quantum Cosmology
Martin Bojowald, Ghanashyam Date, Golam Mortuza Hossain
41 pages
http://arxiv.org/abs/gr-qc/0404039
"Inflationary Cosmology and Quantization Ambiguities in Semi-Classical Loop Quantum Gravity"
Martin Bojowald, James E. Lidsey, David J. Mulryne, Parampreet Singh, Reza Tavakol
15 pages, 8 figures
http://arxiv.org./abs/gr-qc/0403106
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology and Boundary Proposals"
http://arxiv.org/gr-qc/0312103
dated 23 December 2003
Martin Bojowald
"Quantum Gravity and the Big Bang"
http://arxiv.org./astro-ph/0309478
dated 17 September 2003, briefly summarizes how
LQG can serve to cure the big bang singularity and
motivate inflationary expansion. Short and less technical
than the other two papers.
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology, Boundary Proposals, and Inflation"
http://arxiv.org/gr-qc/0303072
dated 19 March 2003
Shinji Tsujikawa, Parampreet Singh, Roy Maartens
"Loop quantum gravity effects on inflation and the CMB"
http://arxiv.org/astro-ph/0311015
from the Tsujikawa/Singh/Maartens abstract:
"In loop quantum cosmology, the universe avoids a big bang singularity and undergoes an early kinetic-dominated super-inflation phase, with a quantum-corrected Friedmann equation. As a result, an inflaton field is driven up its potential hill, thus setting the initial conditions for standard inflation. We show that this effect can raise the inflaton high enough to achieve sufficient e-foldings in the standard inflation era. We analyze the cosmological perturbations and show that loop quantum effects can leave a signature on the largest scales in the CMB, with some loss of power and running of the spectral index."
Viqar Husain and Oliver Winkler "On singularity resolution in quantum gravity"
http://arxiv.org/gr-qc/0312094
this is especially interesting because they duplicate LQC results (for example by Bojowald) using the older version of quantum gravity, ADM variables, quantized metric. Shows that the removal of the big bang singularity is "robust"---doesnt depend on using a particular formalism.
as a background reference for classical (non-quantum) cosmology:
Charles Lineweaver
"Inflation and the Cosmic Microwave Background"
http://arxiv.org/astro-ph/0305179
dated 12 May 2003
-----in case of category theory----
http://www.folli.uva.nl/CD/1999/library/pdf/barrwells.pdf
Barr is at McGill and Wells is at U Virginia
its >100 pages of lecture notes
http://www.dcs.ed.ac.uk/home/dt/CT/categories.pdf
these notes are by Daniele Turi at U. Edinburgh
they are based on Saunders Mac Lane book
"Categories for the working mathematician"[/QUOTE]
here are some recent conferences and other stuff (I still need to edit and bringh some of it up to date)
------recent conferences------
Strings meet Loops (Albert Einstein Institute, MPI-Potsdam) October 2003
http://www.aei-potsdam.mpg.de/events/stringloop.html
Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm
International Conference on Gravity and Cosmology (India) January 2004
http://www.cusat.ac.in/icgc04/
Quantum Gravity Phenomenology, (40th annual Polish Winterschool in Theoretical Physics) February 2004
http://www.ws2004.ift.uni.wroc.pl/html.html
Loop/SpinFoam Conference (Marseille) May 2004
http://w3.lpm.univ-montp2.fr/~philippe/quantumgravitywebsite/
Baez report on it
http://math.ucr.edu/home/baez/week206.html
--------upcoming conferences------
General Relativity Conference (GR17) at Dublin 3 July 2004
http://www.dcu.ie/~nolanb/gr17.htm
more annoucements at
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference
Chris Isham's 60th Birthday conference
Imperial College London, around September 6-7th
for info go to the Imperial College site and click on
"Isham 60 Conference"
http://theory.ic.ac.uk/
------links to an unselective assortment of current work------
Ambjorn Jurkeiwicz Loll
http://arxiv.org./hep-th/0404156
"Emergence of a 4D World from Causal Quantum Gravity"
Carlo Rovelli and Winston Fairbairn
"Separable Hilbert space in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0403047
John Baez
"Quantum Quandaries: A Category-Theoretic Perspective"
http://arxiv.org/quant-ph/0404040
Hendryk Pfeiffer has a new preprint on arxiv
called
"Quantum Gravity and the Classification of
Smooth Manifolds"
http://arxiv.org./gr-qc/0404088
Livine's thesis
http://arxiv.org/gr-qc/0309028
Girelli and Livine
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032
Oriti's thesis
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"
Karim Noui and Philippe Roche
"Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
The cosmological constant occurs in a number of recent quantum gravity papers, for instance the one by Girelli/Livine.
Velhinho "On the structure of the space of generalized connections"
http://arxiv.org/math-ph/0402060
Noui and Perez "Three dimensional loop quantum gravity: physical scalar product and spin foam models"
http://arxiv.org/gr-qc/0402110
Noui and Perez "Three dimensional loop quantum gravity: coupling to point particles"
http://arxiv.org/gr-qc/0402111
Noui and Perez "Dynamics of Loop Quantum Gravity and Spin Foam Models in Three Dimensions"
http://arxiv.org/gr-qc/0402112
Noui and Perez "Observability and Geometry in Three Dimensional Quantum Gravity"
http://arxiv.org/gr-qc/0402113
Freidel and Louapre "Ponzano-Regge model revisited, I."
http://arxiv.org/hep-th/0401076
Gambini and Pullin "Canonical Quantum Gravity..."
http://arxiv.org/gr-qc/0402062
Buffenoir, Henneaux, Noui, Roche
Hamiltonian Analysis of Plebanski Theory
http://arxiv.org./gr-qc/0404041
(spin foam, BF)
http://arxiv.org/abs/hep-th/0405036
Remarks on the black hole entropy and Hawking spectrum in Loop Quantum Gravity
Authors: A. Alekseev, A.P. Polychronakos, M. Smedback
Report-no: CCNY-HEP-04/3, UUITP-13/04
In this note we reply to the criticism by Corichi concerning our proposal for an equidistant area spectrum in loop quantum gravity. We further comment on the emission properties of black holes and on the statistics of links.
A new one by Livine and Oriti
http://arxiv.org/gr-qc/0405085
----------fundamental constants, planck units, time-keeping-------
Historical source for Planck units, the 1899 paper (thanks arivero!)
http://www.bbaw.de/bibliothek/digital/struktur/10-sitz/1899-1/jpg-0600/00000494.htm
In December 2003, the National Institute of Standards and Technology (NIST) posted new CODATA recommended values for the basic planck units
http://physics.nist.gov/cuu/Constants/
choose "universal" from the menu to find (among other things) the recommended values of
planck mass
planck length
planck time
planck temperature
A 1997 article on timekeeping, discussing GR effects allowed-for in the GPS
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
------prospects for testing quantum gravity observationally------
Floyd Stecker
"Cosmic Physics: the High Energy Frontier
http://arxiv.org/astro-ph/0309027
dated September 2003
Stecker discusses the various earth-based and orbital instruments, currently operating, or under construction, or planned, or proposed, and the kind of data becoming available. Among many other things he discusses GLAST, planned to start operating 2007, which, if there are tiny energy-dependent differences in speed among gamma-ray-burst photons, may be able to detect same. Also discusses neutrino observation.
-------science journalism----
"The Duel: Strings versus loops"
http://arxiv.org/abs/physics/0403112
A translation of Rudy Vaas' article in the German
science magazine "Bild der Wissenschaft" roughly
comparable to the "Scientific American"
========
simply to have this link on LaTex handy:
http://www.physicsforums.com/misc/howtolatex.pdf
quotes about physics:
http://www.angelo.edu/faculty/kboudrea/cheap/cheap2_physics.htm
Michael Flohr's great set of notes on group theory in physics:
http://www.itp.uni-hannover.de/~flohr/lectures
(scroll down to "Physical Applications of Group Theory")
http://arxiv.org/hep-th/0405160
Maulik K. Parikh
A Secret Tunnel Through The Horizon
(First prize in the Gravity Research Foundation Essay Competition)
7 pages
A poster on SPR named Chris Weed has noted Parikh's paper and recommended it, together with another that has recently appeared
http://arxiv.org/gr-qc/0405111
some more new ones:
http://arxiv.org/hep-th/0405183
"No black hole information puzzle in a relational universe"
Rodolfo Gambini, Rafael Porto, Jorge Pullin
4 pages
http://arxiv.org/gr-qc/0405119
"Automorphism covariant representations of the holonomy-flux *-algebra"
Andrzej Okolow, Jerzy Lewandowski
32 pages
Lewandowski/Okolow abstract: "We continue an analysis of representations of cylindrical functions and fluxes which are commonly used as elementary variables of Loop Quantum Gravity. We consider an arbitrary principal bundle of a compact connected structure group and following Sahlmann's ideas define a holonomy-flux *-algebra whose elements correspond to the elementary variables. There exists a natural action of automorphisms of the bundle on the algebra; the action generalizes the action of analytic diffeomorphisms and gauge transformations on the algebra considered in earlier works. We define the automorphism covariance of a *-representation of the algebra on a Hilbert space and prove that the only Hilbert space admitting such a representation is a direct sum of spaces L^2 given by a unique measure on the space of generalized connections. This result is a generalization of our previous work (Class. Quantum. Grav. 20 (2003) 3543-3567, gr-qc/0302059) where we assumed that the principal bundle is trivial, and its base manifold is R^d."
there is a handy utility at Spires that I just learned about today
namely a feature of their search engine that is especially
designed to find highly cited papers. It is at:
http://www.slac.stanford.edu/spires/hep/
to illustrate, suppose you want to find influential or highly cited papers by Jan Ambjorn.
Ambjorn is the "dynamical triangulations" researcher at Niels Bohr inst. and at Utrecht, who has published recently with Renate Loll. ("Emergence of a 4D World from Causal Quantum Gravity")
In the main search field if you type
find a ambjorn and topcite 50+
then it will list those of Jan Ambjorn's papers which have received 50 or more citations to date
It turns out he has authored 34 papers which topped 50 citations.
Or you can say "topcite 100+" to restrict the search still further,
and find several of Ambjorn's papers which have topped 100.
The search engine takes several different formats and for one of them instead of saying "find a thiemann" you have to say
"find author thiemann". but the default seems to be use the letter A to stand for author.
there is a handy utility at Spires that I just learned about today
namely a feature of their search engine that is especially
designed to find highly cited papers. It is at:
http://www.slac.stanford.edu/spires/hep/
In the main search field if you type
find author ambjorn and topcite 50+
then it will list Jan Ambjorn's papers which have received 50 or more
citations to date
or you can say "topcite 100+" to restrict the search still further.
Ambjorn is the "dynamical triangulations" researcher at Niels Bohr inst. and at Utrecht, who has published recently with Renate Loll. ("Emergence of a 4D world...")
It turns out Ambjorn has published several "100+" citations papers.
This is an lqg thread. Of course maybe your posting this because you now believe lqg is wrong and you've decided to change religions.
A new PF poster named setAI pointed out a good 6-page essay by Lee Smolin today, so I will add it to this collection of links:
Sample from page 5:
"The debate between proponents of background-dependent and background independent theories is in fact just the modern version of an ancient debate. Since the Greeks, the argument has raged between those who believed that space and time have an eternally fixed, absolute character and those who thought space and time are no more than relations between events that themselves evolve in time. Plato, Aristotle, and Newton were absolutists. Heraclites, Democritus, Leibniz, Mach, and Einstein were relationalists. When we demand that the quantum theory of gravity be background-independent, we are saying we believe that the triumph that general relativity represented for the relational point of view is final and will not be reversed.
Much of the argument between string and loop theorists is a continuation of this debate. Most string theorists were trained as elementary-particle physicists and worked their whole lives in a single fixed spacetime. Many of them have never even heard of the relational/absolute debate, which is the basic historical and philosophical context for Einstein's work. Most people who work in loop quantum gravity do so because at some point in their education they understood the relational, dynamical character of spacetime as described in general relativity, and they believe in it. They don't work on string theory because they cannot take seriously any candidate for a quantum theory of gravity that is background-dependent and hence loses (or at best hides) the relational, dynamical character of space and time."
http://www.edge.org/3rd_culture/smolin03/smolin03_p5.html
It is a good essay because it combines a clearsighted overview with a personal insider's take, and also tells the history of this approach to quantum gravity from a participant's perspective.
Sample from page 3:
"Loop quantum gravity started in the early 1980s with some discoveries about classical general relativity by Amitaba Sen, then a postdoc at the University of Maryland. These were made into a beautiful reformulation of Einstein's theory by Abhay Ashtekar, then at Syracuse University and now director of the Center for Gravitational Physics at Penn State—a reformulation that brought the mathematical and conceptual language we use to describe space and time closer to the language used in particle physics and quantum physics."
http://www.edge.org/3rd_culture/smolin03/smolin03_p3.html
Another quote from page 5:
"Another reason that string theory cannot be the final word is that in string theory one studies strings moving in a fixed classical spacetime. Thus, string theory is what we call a background-dependent approach. It means that one defines the strings as moving in a fixed space and time. This may be a useful approximation, but it cannot be the fundamental theory. One of the fundamental discoveries of Einstein is that there is no fixed background. The very geometry of space and time is a dynamical system that evolves in time. The experimental observations that energy leaks from binary pulsars in the form of gravitational waves—at the rate predicted by general relativity to the unprecedented accuracy of eleven decimal places—tells us that there is no more a fixed background of spacetime geometry than there are fixed crystal spheres holding the planets up. The fundamental theory must unify quantum theory with a completely dynamical description of space and time. It must be what we call a background-independent theory. Loop quantum gravity is such a one; string theory is not."
http://www.edge.org/3rd_culture/smolin03/smolin03_p5.html
From page 6:
"So while I disagree with the leading string theorists about methodology, this hasn't kept me from working on string theory. After all, they don't own it; its open problems are there for anyone to try to solve. So I decided a few years ago to ignore their advice and try to construct the background independent form of M theory. In the process of inventing loop quantum gravity, we gained a lot of knowledge about how to make quantum theories of space and time that are background-independent."
http://www.edge.org/3rd_culture/smolin03/smolin03_p6.html
the essay seems to have been written in latter half of 2003, so is fairly recent.
thanks to setAI for flagging this one
selfAdjoint
May29-04, 10:59 AM
Thinking about all this LQG vs. string business, I wonder if the "beauitiful reformulation" of Ashtekar isn't like the "beautiful reformulation of string theory" of Schwartz and Witten that has motivated so much string research. I am wondering whether in the final analysis, beauty is all it's cracked up to be as a search strategy.
The two great historical exemplars of beauty first were Einstein and Dirac. In each case their approach achieved a great success early but then led them into unproductive wastelands. And it is at least arguable that both string physics and LQG research in the Ashtekar tradition are right now spinning their wheels. Maybe it's time for a younger generation, playing Feynman and Dyson to the Witten - Ashtekar version of Einstein-Dirac to have their say. Which is why I am very interseted in the AJL paper, a possibly rough hewed (remember Feyman's early rep?) but undoubtedly novel approach to the problem of background independent quantum mechanics (and THAT, not just quantum gravity is the big kahuna).
This post is possibly not in line with your intent to have this as a colllection of documents, but I just couldn't resist, seeing the same old same old out of Smolin being posted once again.
Thinking about all this LQG vs. string business,...
...Which is why I am very interseted in the AJL paper, a possibly rough hewed (remember Feyman's early rep?) but undoubtedly novel approach to the problem of background independent quantum mechanics (and THAT, not just quantum gravity is the big kahuna)...
Amen to that.
Background independent quantum mechanics is the big kahuna.
this turns up the lights on something that was creeping around the edges of my mind also
I'm very interested in the AJL dynamical triangulations approach too. We could continue in the Marseille thread that Baez started (it is largely about AJL but Marseille was billed as a Loop+Foam conference---a lot of family resemblances: loop to foam and foam to simplicial QG---making too sharp distinctions could be a mistake.
Anyway, if we leave this thread as a catchbasket for LQG-and-related links we could followup on AJL etc at the Marseille thread if that suits you, or start a new one on the Big Kahuna!
http://arxiv.org/abs/hep-th/0405273
the title is "Introduction to DSR"
there was a Quantum Gravity symposium in poland
this February and Jerzy Kowalski-Glikman (the organizer) lectured on
DSR and its relation to QG
this paper was developed from his lectures at the Winterschool, and
submitted to Springer for publication in its "lecture notes in physics" series.
anything calling itself an Introduction could potentially
be useful.
the idea of DSR is that the usual minkowski space and lorentz group symmetries
of special relativity are what results from forcing c to be the same
for all observers
what if you force TWO physical quantities, not just the speed of light but also the planck length or the planck mass, to be the same for all observers.
----------------
more DSR news, this time from Alejandro's city of Zaragoza
just out:
http://arxiv.org/hep-th/0405285
Quantum Uncertainty in Doubly Special Relativity
Authors: Jose Luis Cortes, J. Gamboa
4 pages, no figures
The modification of the quantum mechanical commutators in a relativistic theory with an invariant length scale (DSR) is identified...
---------QG phenomenology-----
a new paper:
http://arxiv.org/quant-ph/0406007
"Could Energy Decoherence due to Quantum Gravity be observed?"
Christoph Simon, Dieter Jaksch
7 pages, no figures
Sample from abstract:
"It has recently been proposed that quantum gravity might lead to the decoherence of superpositions in energy, corresponding to a discretization of time at the Planck scale....
... We also show how local energy decoherence, which acts separately on system and phase reference, could be detected with remarkable sensitivity and over a wide range of length scales by long-distance Ramsey interferometry with metastable atomic states. The sensitivity of the experiments can be further enhanced using multi-atom entanglement."
the Spires database is an amazing resource for keeping track of activity in various research lines and seeing what topics are attracting interest
several links within that site have proven useful (for me) recently
Here is the 2003 edition of the topcites for all categories, gr-qc as well as hep-th and the rest
http://www.slac.stanford.edu/library/topcites/2003/eprints/index.shtml
Here is the general index for topcites for all the years 1992-2003:
http://www.slac.stanford.edu/library/topcites/
I posted earlier another special Spires feature which lets you find the most-cited papers by a particular author. To use it you need to know that in their code the letter A stands for author, so you say "a ambjorn" to find papers authored by ambjorn.
there is a handy utility at Spires that I just learned about today
namely a feature of their search engine that is especially
designed to find highly cited papers. It is at:
http://www.slac.stanford.edu/spires/hep/
to illustrate, suppose you want to find influential or highly cited papers by Jan Ambjorn.
Ambjorn is the "dynamical triangulations" researcher at Niels Bohr inst. and at Utrecht, who has published recently with Renate Loll. ("Emergence of a 4D World from Causal Quantum Gravity")
In the main search field if you type
find a ambjorn and topcite 50+
then it will list those of Jan Ambjorn's papers which have received 50 or more citations to date
It turns out he has authored 34 papers which topped 50 citations.
Or you can say "topcite 100+" to restrict the search still further,
and find several of Ambjorn's papers which have topped 100.
The search engine takes several different formats and for one of them instead of saying "find a thiemann" you have to say
"find author thiemann". but the default seems to be use the letter A to stand for author.
lot of good information to get out of Spires, bravo to Stanford and SLAC for hosting it, I feel I've just scratched the surface
PAM Dirac was (one of) the first to try
to construct a quantum version of
General Relativity------to quantize gravity.
So this thread being for LQG links we should have a Dirac link
and Pelastration just supplied one with two photos of Dirac and
a newspaper interview
http://faculty.rmwc.edu/tmichalik/dirac.htm
cant recommend but thought it interesting enough to keep tabs on
"Toward a Background Independent Quantum Theory of Gravity"
Authors: Vishnu Jejjala, Djordje Minic, Chia-Hsiung Tze
Comments: Awarded Honorable Mention, 2004 Gravity Research Foundation Essay Competition; 8 pages
http://arxiv.org/gr-qc/0406037
This just appeared on arXiv today.
It looks like a keeper:
http://arxiv.org/gr-qc/0406042
"Oscillatory Universes in Loop Quantum Cosmology and Initial Conditions for Inflation"
James E. Lidsey, David J. Mulryne, N. J. Nunes, Reza Tavakol
6 pages, 4 figures
Several of them wrote a paper with Martin Bojowald that posted a couple of months back. Otherwise I dont recall seeing their names before
Rovelli has redone his homepage.
The book is planned to hit the bookstores in October
there is a picture of the book and a link to the Cambridge U. P.
catalog entry for it
planned price is 45 pounds sterling
the draft is still available free at his site, by agreement with C.U.P.
He mentions another piece of writing---popular----
called "What is space? What is time?"
so far just in Italian. I would guess there will be an English version
If you want to look for it in Italian (which I understand some people can read) the title is
Che cos' e lo spazio? Che cos' e il tempo?
Rovelli homepage:
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
Rovelli has redone his homepage.
The book is planned to hit the bookstores in October
there is a picture of the book and a link to the Cambridge U. P.
catalog entry for it
planned price is 45 pounds sterling
the draft is still available free at his site, by agreement with C.U.P.
He mentions another piece of writing---popular----
called "What is space? What is time?"
so far just in Italian. I would guess there will be an English version
If you want to look for it in Italian (which I understand some people can read) the title is
Che cos' e lo spazio? Che cos' e il tempo?
Rovelli homepage:
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
Thanks Marcus, its a must buy!
I like the cover of the book, the use of Geometry is very evident.
The cover outline is framed with precise measurment at its edge, with the Backgound uniform colour.
As one moves inward the Author and Title are framed by a 'casimir-effect'..and a sea of points 'Quantum-Background' are pretty hazy?
Cool1 :smile: :approve:
http://assets.cambridge.org/0521837332/cover/0521837332.jpg
Quantum gravity is perhaps the most important open problem in fundamental physics. It is the problem of merging quantum mechanics and general relativity, the two great conceptual revolutions in the physics of the twentieth century. The loop and spinfoam approach, presented in this book, is one of the leading research programs in the field. The first part of the book discusses the reformulation of the basis of classical and quantum Hamiltonian physics required by general relativity. The second part covers the basic technical research directions. Appendices include a detailed history of the subject of quantum gravity, hard-to-find mathematical material, and a discussion of some philosophical issues raised by the subject. This fascinating text is ideal for graduate students entering the field, as well as researchers already working in quantum gravity. It will also appeal to philosophers and other scholars interested in the nature of space and time.
http://titles.cambridge.org/catalogue.asp?isbn=0521837332
It will be a must buy for myself as well.
But I wonder about the issue of quantum geometry. How will this be formulated into the LQG perspective, as it has in strings?
The Elegant Universe, by Brian Greene, pg 231 and Pg 232
"But now, almost a century after Einstein's tour-de-force, string theory gives us a quantum-mechanical discription of gravity that, by necessity, modifies general relativity when distances involved become as short as the Planck length. Since Reinmannian geometry is the mathetical core of genral relativity, this means that it too must be modified in order to reflect faithfully the new short distance physics of string theory. Whereas general relativity asserts that the curved properties of the universe are described by Reinmannian geometry, string theory asserts this is true only if we examine the fabric of the universe on large enough scales. On scales as small as planck length a new kind of geometry must emerge, one that aligns with the new physics of string theory. This new geometry is called, quantum geometry."
I am seeing similarities arising not only from this perspective but from the current link Marcus supplied on the cosmological association (LQC (http://www.physicsforums.com/showthread.php?t=30258) ).
Olias and Sol2, I too am glad to see "Quantum Gravity" well on its
way to being available, and I will certainly buy a copy although
45 pounds sterling is a fair-size chunk of cash.
It was thoughtful of Carlo to work out an agreement with Cambridge where
the publisher will allow him to keep a draft version available free online at his website. that way if someone can't afford the book they can at least get the draft and print it out at home, or just keep it on the computer---which is already pretty useful.
speculation is always risky but I am speculating that because it's a fast-moving field there will be several editions of this book
what is coming out this year (planned for October) will be the first edition
but----with ongoing developments in quantum cosmology and the simplicial or "dynamical triangulations" approach, and whatever else (so hard to predict)----there may be a second edition, and possibly others as years go along.
and then the draft on rovelli's website will be a kind of "zero-th edition".
my favorite page in the draft version of the book is page 7---the part about the whale. I also like some things around page 52
I also really like the philosophical essays at the end
and the historical accounts
although it has a lot for the general reader, the book is primarily for graduate students looking for PhD thesis work to do and for established researchers wanting to move into the field of QG.
That is to say, it has generally accessible portions (which are admirable and enlightening, in my opinion) but also (in case other people besides Olias and Sol2 are reading this thread I want to stress) plenty that is not so accessible.
...
But I wonder about the issue of quantum geometry. How will this be formulated into the LQG perspective, as it has in strings?
The Elegant Universe, by Brian Greene, pg 231 and Pg 232
"But now, almost a century after Einstein's tour-de-force, string theory gives us a quantum-mechanical discription of gravity that, by necessity, modifies general relativity when distances involved become as short as the Planck length. Since Reinmannian geometry is the mathetical core of genral relativity, this means that it too must be modified in order to reflect faithfully the new short distance physics of string theory. Whereas general relativity asserts that the curved properties of the universe are described by Reinmannian geometry, string theory asserts this is true only if we examine the fabric of the universe on large enough scales. On scales as small as planck length a new kind of geometry must emerge, one that aligns with the new physics of string theory. This new geometry is called, quantum geometry."
....
this quote is very interesting and raises an important issue. maybe we will eventually have a thread devoted to it. for starters
how about going to arXiv and putting "quantum geometry"
into the abstract box
and doing a search for articles that say "quantum geometry" in their
abstract summary
It would give an idea of what the experts mean by it, in a technical sense.
Also in the title box, for the arXiv search engine. To find whatever
books and articles have been written about quantum geometry have that in the title. (I know some, but most likely not all.)
I am not promising that a good thread would come of this, or a clear resolution of how the term is used, even. but it is something to think about
selfAdjoint
Jun11-04, 12:04 PM
Putting "quantum geometry" in the latest year search at hep-th brought up four papers, all of which used Quantum Geometry as a synonym for the Ashtekar program, aka LQG.
Putting "quantum geometry" in the latest year search at hep-th brought up four papers, all of which used Quantum Geometry as a synonym for the Ashtekar program, aka LQG.
go back to earlier papers
there is completely different stuff called quantum geometry
I seem to recall Majid using the term
and maybe Connes
most likely others
no clear connection with string tho
I think maybe Brian Greene was fantasizing a little
or looking ahead to a desirable future, but could be wrong
the development of a quantum geometry has to come but
may have no clear connection with string IMHO
this quote is very interesting and raises an important issue. maybe we will eventually have a thread devoted to it. for starters
how about going to arXiv and putting "quantum geometry"
into the abstract box
and doing a search for articles that say "quantum geometry" in their
abstract summary
It would give an idea of what the experts mean by it, in a technical sense.
Also in the title box, for the arXiv search engine. To find whatever
books and articles have been written about quantum geometry have that in the title. (I know some, but most likely not all.)
I am not promising that a good thread would come of this, or a clear resolution of how the term is used, even. but it is something to think about
The dimensional significance of this topic is really a difficult issue for myself as well, and the statistics really surpirsed me that you have offerred.
If such a geometry was to emerge what exactly are we describing? Jeff's comments in regard to supersymmetry are valid statements because of the complexity of the issue in regards to the metric. The complexity of points really seem to flow when you come to that level, yet it has encapsulated the ideas of dimension. So geometry has its work cut out for it no doubt.
What exactly is the hierarchy problem?The gist of it is that the universe seems to have two entirely different mass scales, and we don't understand why they are so different. There's what's called the Planck scale, which is associated with gravitational interactions. It's a huge mass scale, but because gravitational forces are proportional to one over the mass squared, that means gravity is a very weak interaction. In units of GeV [billions of electron volts], which is how we measure masses, the Planck scale is 10 to the 19th GeV. Then there's the electroweak scale, which sets the masses for the W and Z bosons. These are particles that are similar to the photons of electromagnetism and which we have observed and studied well. They have a mass of about 100 GeV. So the hierarchy problem, in its simplest manifestation, is how can you have these particles be so light when the other scale is so big. (http://www.physicsforums.com/showpost.php?p=231252&postcount=16)
I had mentioned in the topic of the new math thread (http://www.physicsforums.com/showpost.php?p=231852&postcount=17), that such attempts at a discription woud have to be formulated in much the same way Smolin did? Klein's Ordering of geometries is really quite interesting in terms of Quantum Evolution? :smile:
Because we understand this dynamical movement in plasmatic features as supersymmetical conisderation one would have to understand how gravity moves to supergravity. If we understand the gravity field can have its differences( dimensional relationship?) then how we look at the Q<--->Q measure becomes a interesting relation in terms of understanding the metric in a different way.
ds2 = (cdt)2 - dl2
On a cosmological level this directs my attention, yet I recognize the complexity of the movement in the quantum world. Why classically does this not fit at that quantum level and what do we have to reconsider here?
Do you understand how this subject might evolve in this context?
...But I wonder about the issue of quantum geometry. How will this be formulated into the LQG perspective, as it has in strings?
The Elegant Universe, by Brian Greene, pg 231 and Pg 232
"But now, almost a century after Einstein's tour-de-force, string theory gives us a quantum-mechanical discription of gravity that, by necessity, modifies general relativity when distances involved become as short as the Planck length. Since Reinmannian geometry is the mathetical core of genral relativity, this means that it too must be modified in order to reflect faithfully the new short distance physics of string theory. Whereas general relativity asserts that the curved properties of the universe are described by Reinmannian geometry, string theory asserts this is true only if we examine the fabric of the universe on large enough scales. On scales as small as planck length a new kind of geometry must emerge, one that aligns with the new physics of string theory. This new geometry is called, quantum geometry."
...
hi Sol2, you copied in this Brian Greene quote which could be the start of a new thread so I started one, and hope we can continue the discussion there (in a harmonious fashion! :smile: I might add.)
there is no collective name for the group of background independent QG approaches aimed at quantizing GR
Ashtekar says Quantum Geometry, but means Loop
Thiemann says Canonical Quantum General Relativity, but means Loop
Gambini says Canonical Quantum Gravity, meaning his type of Loop
the most widely used term is Loop---LQG for short
Ashtekar and Lewandowski recently used Background Independent Q.G.
in a review article, meaning Loop
then there are closely allied approaches called Spin Foam
and Simplicial Quantum Gravity (dynamical triangulations in particular)
and one of the Simplicial people has used the term Quantum Geometry
but it does not mean exactly the same as what Ashtekar means
these approaches got together at the May 2004 "loop/foam conference"
but there is no agreed on collective noun
So I have tried to construct a keyword search in arXiv that would turn up these things and this seems to work. Here are numbers of preprints
by year 1992-present.
Year 1992:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1992/0/1
Year 1993:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1993/0/1
Year 1994:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1994/0/1
Year 1995:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1995/0/1
Year 1996:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1996/0/1
Year 1997:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1997/0/1
Year 1998:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1998/0/1
Year 1999:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1999/0/1
Year 2000:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/2000/0/1
Year 2001:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/2001/0/1
Year 2002:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/2002/0/1
Year 2003:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/2003/0/1
Last twelve months (e.g. 14 June 2003 to 14 June 2004):
http://lanl.arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/past/0/1
this is designed to catch:
loop quantum gravity
loop quantum cosmology
canonical quantum gravity
simplicial quantum gravity
nonperturbative quantum gravity
spin foam
dynamical triangulation
[EDIT afterthought]
BTW Rovelli has a new paper out on arXiv, together with Oriti and Speziale.
predictably, the last link in the above search list turned it up
http://arxiv.org/gr-qc/0406063
"...The model sheds light also on several other features of spinfoam quantum gravity: the reality of the partition function; the geometrical interpretation of the Newton constant; and the fact that the partition function of general relativity is finite in spite of the divergence of the BF one."
Here is a more inclusive version of the above search:
2001:
http://lanl.arxiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/2001/0/1
2002:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/2002/0/1
2003:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/2003/0/1
Last Twelve Months:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/past/0/1
------------------
http://arxiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/past/0/1
another Loop quantum cosmology paper at arxiv today. the page of links needs to be brought up to date:
Qualitative Approach to Semi-Classical Loop Quantum Cosmology
G.V. Vereshchagin
http://arxiv.org/abs/gr-qc/0406108
this thread is serving as a surrogate sticky for useful loop-and-related quantum gravity links
I update it periodically.
to get a picture of QG developments in the first half of 2004 it is helpful to quote Baez post after the May 2004 marseille conference:
I just got back from the Marseille conference on loop quantum gravity and spin foams:
http://w3.lpm.univ-montp2.fr/~philippe/quantumgravitywebsite/
It was really great, so I devoted "week206" of my column This Week's Finds entirely to this conference:
http://math.ucr.edu/home/baez/week206.html
In particular, I spend a lot of time giving a very simple nontechnical introduction to the recent work of Ambjorn, Jurkiewicz and Loll in which they seem to get a 4d spacetime to emerge from a discrete quantum model - something that nobody had succeeded in doing before!
http://www.arXiv.org/abs/hep-th/0404156
I hope this lays to rest certain rumors here that I'd burnt out on quantum gravity. :devil:
a key paper mentioned here
Ambjorn Jurkiewicz Loll
"Emergence of a 4D World from Causal Quantum Gravity"
http://www.arXiv.org/abs/hep-th/0404156
other key papers, connecting quantum gravity with outgrowths of DSR namely "DDSR" or "TSR" (smolin's name: triply special relativity) and moffat's NGT an outgrowth of MOND
Kowalski-Glikman, Smolin
"Triply Special Relativity"
http://arxiv.org/abs/hep-th/0406276
Girelli Livine Oriti
"Deformed Special Relativity as an effective flat limit of quantum gravity"
http://arxiv.org/gr-qc/0406100
Moffat
Modified Gravitational Theory as an Alternative to Dark Energy and Dark Matter
http://arxiv.org/astro-ph/0403266
another interesting development, resolution of the "Black Hole Information Paradox" using relational time----a quantum mechanical clock rather than absolute ideal time
Gambini Porto Pullin
"Realistic clocks, universal decoherence and the black hole information paradox"
http://arxiv.org/abs/hep-th/0406260
also their earlier paper
“No black hole information puzzle in a relational universe,”
http://arxiv.org/hep-th/0405183.
it is interesting that this information-loss paradox has been worked on rather hard by some wellknown stringy people like Susskind and, it seems, Maldacena--but in stringy context it is still a challenging outstanding problem which people are working on. So it is a bit of a coup to resolve it as GPP do, to resolve it at all would be respectable and they do it, as well, with apparent ease and not a lot of mess.
Since this thread is a linkbasket for links to recent LQG stuff that might be useful I will put these recent things here:
Penrose book is about LQG to some degree. "The Road to Reality"
It came out in July.
Carlo Rovelli book "Quantum Gravity" is coming out in Fall 2004 from Cambridge Uni Press. this is the first comprehensive LQG graduate-level textbook.
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
Lee Smolin has posted "An Invitation to Loop Quantum Gravity" a 50-page survey and intro with FAQ for physicists in other fields who want to switch fields and do QG research. http://arxiv.org/hep-th/0408048
"Invitation" is intended for Reviews of Modern Physics.
John Baez gave an introduction and survey at the Dublin GR17 conference, which is available online at Baez site.
"Loop Quantum Gravity, Quantum Geometry and Spin Foams"
http://math.ucr.edu/home/baez/lectures.html#lqg
"Quantum Gravity Phenomenology" was the topic of the Winterschool-2004 (WS-2004) symposium this year, 4-14February, a 10-day conference on the initial and planned efforts to test QG by empirical observation.
http://ws2004.ift.uni.wroc.pl/html.html
click on lectures if you want slides from the various talks given at WS-2004
Observational tests of QG have already had a considerable impact as discussed by Smolin in the "Invitation" survey article starting page 27.
LQG is rapidly reaching a point where it can guide experiment---if one counts certain kinds of astronomical observation as experiment.
Cambridge Uni Press is also publishing "Universe or Multiverse" which will contain a chapter by Smolin called "Scientific Alternatives to the Anthropic Principle" where he offers an evolutionary Multiverse hypothesis that generates testable (numerical) predictions and therefore has meaning as a part of science.
http://arxiv.org/hep-th/0407213
Smolin's Multi is falsifiable, using today's tools.
------------------
In his recent paper Smolin cites "personal communication" from Martin Bojowald regarding work on eliminating the Black Hole singularity (by the appropriate quantization of gravity). So far Bojowald has only published his preliminary work on this, not the final result:
Martin Bojowald
Spherically Symmetric Quantum Geometry: States and Basic Operators
http://arxiv.org/abs/gr-qc/0407017
26 pages
"The kinematical setting of spherically symmetric quantum geometry, derived from the full theory of loop quantum gravity, is developed. This extends previous studies of homogeneous models to inhomogeneous ones where interesting field theory aspects arise. A comparison between a reduced quantization and a derivation of the model from the full theory is presented in detail, with an emphasis on the resulting quantum representation. Similar concepts for Einstein-Rosen waves are discussed briefly."
Martin Bojowald and Rafal Swiderski
The Volume Operator in Spherically Symmetric Quantum Geometry
http://arxiv.org/abs/gr-qc/0407018
25 pages
"The spherically symmetric volume operator is discussed and all its eigenstates and eigenvalues are computed. Even though the operator is more complicated than its homogeneous analog, the spectra are related in the sense that the larger spherically symmetric volume spectrum adds fine structure to the homogeneous spectrum. The formulas of this paper complete the derivation of an explicit calculus for spherically symmetric models which is needed for future physical investigations."
http://arxiv.org/hep-th/0407115
Loop Quantum Gravity and the Cyclic Universe
Martin Bojowald, Roy Maartens, Parampreet Singh
6 pages
"Loop quantum gravity introduces strong non-perturbative modifications to the dynamical equations in the semi-classical regime, which are responsible for various novel effects, including resolution of the classical singularity in a Friedman universe. Here we investigate the modifications for the case of a cyclic universe potential, assuming that we can apply the four-dimensional loop quantum formalism within the effective four-dimensional theory of the cyclic scenario. We find that loop quantum effects can dramatically alter the near-collision dynamics of the cyclic scenario. In the kinetic-dominated collapse era, the scalar field is effectively frozen by loop quantum friction, so that the branes approach collision and bounce back without actual collision."
--------------------
Representations of the Weyl Algebra in Quantum Geometry
Christian Fleischhack
63 pages
http://arxiv.org/abs/math-ph/0407006
substantial progress beyond where this was taken by Hanno Sahlmann, Thiemann, Lewandowski, Okolow. It may be that Fleishhack has reached to goals set by this earlier work
----------------------
In the next post, Meteor calls attention to the work of Date and Hossain
who showed that both Inflation and the Big Bounce were generic in isotropic LQC. I think Meteor mentioned two of their three recent papers and I will add the other one for completeness:
"Genericity of Big Bounce in isotropic loop quantum cosmology"
http://arxiv.org/gr-qc/0407074
http://arxiv.org/abs/gr-qc/0407073
Effective Hamiltonian for Isotropic Loop Quantum Cosmology
http://arxiv.org/abs/gr-qc/0407069
Genericity of inflation in isotropic loop quantum cosmology
http://arxiv.org/abs/hep-th/0407072
Corrections to the Planck's radiation law from loop quantum gravity
I've been looking quickly the third article, and apart of a proposed modification of Planck's law, there's also a proposed modification for Wien's displacement law
this thread is serving as a surrogate sticky for useful loop-and-related quantum gravity links
I update it periodically.
for a concise and up-to-date survey of LQG and allied approaches see
John Baez talk at Dublin, given Tuesday 20July2004:
Loop Quantum Gravity, Quantum Geometry and Spin Foams
It is online at his website
http://math.ucr.edu/home/baez/lectures.html#lqg
For more QG developments in the first half of 2004 here is Baez post after the May 2004 marseille conference:
I just got back from the Marseille conference on loop quantum gravity and spin foams:
http://w3.lpm.univ-montp2.fr/~philippe/quantumgravitywebsite/
It was really great, so I devoted "week206" of my column This Week's Finds entirely to this conference:
http://math.ucr.edu/home/baez/week206.html
In particular, I spend a lot of time giving a very simple nontechnical introduction to the recent work of Ambjorn, Jurkiewicz and Loll in which they seem to get a 4d spacetime to emerge from a discrete quantum model ---something that nobody had succeeded in doing before!
http://www.arXiv.org/abs/hep-th/0404156
I hope this lays to rest certain rumors here that I'd burnt out on quantum gravity. :devil:
a key paper mentioned here
Ambjorn Jurkiewicz Loll
"Emergence of a 4D World from Causal Quantum Gravity"
http://www.arXiv.org/abs/hep-th/0404156
-----------------
for me, one of the most enlightening things that has come online recently is the slides from a talk Lee Smolin gave in Poland in February at the Winterschool WS-2004. This was a 10-day symposium and the topic this year was Quantum Gravity Phenomenology
there were talks by a dozen or so experts and they are online at the WS-2004 site. Smolin gave 3 lectures and it is the third I found especially interesting.
http://ws2004.ift.uni.wroc.pl/html.html
click on lectures and scroll down to Smolin's three.
-------------------
Roger Penrose's new book "The Road to Reality" just appeared at the bookstores. It is 1000 pages. Key ideas were presented at his public lecture "Fashion, Faith, Fantasy in Modern Physics" at Dublin last week. Also last October Penrose gave 3 evening lectures on these three themes at Princeton, they are online to listen, with sketchy video.
http://www.princeton.edu/WebMedia/lectures/
scroll down to October 2003 and find the three lectures by Penrose
-----------------------------
A number of papers have appeared recently connecting quantum gravity with extensions of Special Relativity. Examples are outgrowths of DSR such as "DDSR" or "TSR" (smolin's name: triply special relativity). Another aspect involves moffat's NGT an outgrowth of MOND
Kowalski-Glikman, Smolin
"Triply Special Relativity"
http://arxiv.org/abs/hep-th/0406276
Girelli Livine Oriti
"Deformed Special Relativity as an effective flat limit of quantum gravity"
http://arxiv.org/gr-qc/0406100
Moffat
Modified Gravitational Theory as an Alternative to Dark Energy and Dark Matter
http://arxiv.org/astro-ph/0403266
another interesting development, resolution of the "Black Hole Information Paradox" using relational time----a quantum mechanical clock rather than absolute ideal time
Gambini Porto Pullin
"Realistic clocks, universal decoherence and the black hole information paradox"
http://arxiv.org/abs/hep-th/0406260
also their earlier paper
“No black hole information puzzle in a relational universe,”
http://arxiv.org/hep-th/0405183.
this is a very incomplete listing of what has recently become available online, by way of Loop-and-related QG sources.
I really should mention Leonardo Modesto removing the Black Hole singularity before posting this.
http://arxiv.org/gr-qc/0407097
Disappearance of the Black Hole Singularity in Quantum Gravity
Also the popular series of 3 articles on LQG by Rudy Vaas (translated from German by Amitabha Sen and Martin Bojowald)
Beyond Space And Time
Ruediger Vaas
7 pages, English translation of "Jenseits von Raum und Zeit"
http://arxiv.org/physics/0401128
The Duel: Strings versus Loops
Ruediger Vaas
10 pages, English translation of "Das Duell: Strings gegen Schleifen"
http://arxiv.org/physics/0403112
The Inverted Big-Bang
Ruediger Vaas
8 pages, English translation of "Der umgestuelpte Urknall"
http://arxiv.org/physics/0407071
a more accurate translation of the title would be
"the turned-inside-out Big Bang" or "the everted Big Bang"
in Loop Quantum Cosmology the volume element gets
turned inside out at the moment of the quantum bounce
where there used to be a singularity
Should also include the recalculation of the Immirzi parameter by
Domagala, Lewandowski, and Meissner
"Black Hole Entropy from Quantum Geometry"
http://arxiv.org/gr-qc/0407051
"Black Hole Entropy in Loop Quantum Gravity"
http://arxiv.org/gr-qc/0407052
http://arxiv.org/abs/http://www.arxiv.org/abs/gr-qc/0408033
On the counting of black hole states in loop quantum gravity
Authors: Sergei Alexandrov
Comments: 4 pages
Report-no: SPIN-04/09, ITP-UU-04/15
We argue that counting black hole states in loop quantum gravity one should take into account only states with the minimal spin at the horizon
BTW, Alexandrov has posted a reply to Lubos Motl
http://www.physicsforums.com/showthread.php?t=39780&highlight=alexandrov
BTW, Alexandrov has posted a reply to Lubos Motl
http://www.physicsforums.com/showthread.php?t=39780&highlight=alexandrov
hee hee
Lubos, a Czech, has gotten himself embroiled with a Russian
and has already acheived the level of (East European equivalent to)
racial epithets. The Czechs have cause to remember the Soviet period
to which Lubos refers:
"It seems to me like a person who wants to get the result 1917 apples, but he gets, by an explicit counting, 1991. Well, the remaining 74 are anti-socialist macroapples and they should not be counted, should they? LM"
Meteor thanks so much for the link. Sergei is at Utrecht (where also that
Renate Loll is----simplicial model gravity) and I earlier got the impression of him that he is quite bright, if also somewhat of a maverick (or wild horse). the world is beautiful.
selfAdjoint
Aug21-04, 05:41 PM
It's one thing for a moderator or mentor to also be a discussant, as we all are here. It's another to use your technical moderator power to respond to somebody's post before they can respond to yours. It seems to me that LM misuses the moderator powers this way, at least in spirit.
It's one thing for a moderator or mentor to also be a discussant, as we all are here. It's another to use your technical moderator power to respond to somebody's post before they can respond to yours. It seems to me that LM misuses the moderator powers this way, at least in spirit.
and his spirit is mean. we know this. Lubos is a viper.
however he is extraordinarily charming and we must cherish him
as we do poisonous flowers
to me, that crack about the 74 anti-socialist macro-apples that should not be counted (well almost) makes up for all the times he has unfairly insulted John Baez (tho JB might not think so)
oh, I see better now. You mean [i]as a moderator[/b]
all right to be nettlesome in private, but must be the unbiased moderator
you are right
it does demean the office of moderator to be so biased and
to take such advantage.
but after all it is only SPS.
it would drive me crazy if I had to cope with something like that at PF
but the PF moderators (yrslf incl.) are pretty OK thank goodness
Forget lubos's personality. His posts are full of valuable insights that are difficult to come by for most people here. Anyway, he never commits the crime of pretending to be something he's not, which is more than I can say for some of the people here.
I guess we can make allowances for some personalites, hey Jeff? :smile: We make them for you? :rofl:
Self Adjoint made a interesting point about comments at the same time as the posting. It would appear to me that as a moderator the content is being look at, which I guess is the moderators job, but why not after its posted.
As to our opinons of Lubos's Knowkedge, I do not think anyone is questioning that.
Smolin and those involved in Loop were well aware of what was happening in strings?. It seems that they go hand in hand, and for Smolin, critical summations of where he had been might not of answered, where he would like to go.
I am not putting words in his mouth just pointing out the excellents papers that have come and summation of those other areas of research of theoretical positions.
Susskind and Smolin rebuttal towards each other, is very interesting to me. This kind of particpation under the Edge banner by John Brock request for letters, helps people decide what is going on.
New paper to consideration:
http://arxiv.org/abs/gr-qc/0408094
Time dependence in Quantum Gravity
Authors: Martin Bojowald, Parampreet Singh, Aureliano Skirzewski
Comments: 33 pages, 17 figures
Report-no: AEI-2004-065, IGPG-04/8-2
The intuitive classical space-time picture breaks down in quantum gravity, which makes a comparison and the development of semiclassical techniques quite complicated. By a variation of the group averaging method to solve constraints one can nevertheless introduce a classical coordinate time into the quantum theory, and use it to investigate the way a semiclassical continuous description emerges from discrete quantum evolution. Applying this technique to test effective classical equations of loop cosmology and their implications for inflation and bounces, we show that the effective semiclassical theory is in good agreement with the quantum description even at short scales.
Plus the follow-up of Alexandrov after some days of reflection
http://www.physicsforums.com/showthread.php?p=300078#post300078
Sean Carroll was asked a good series of questions about the testing of GR and possible alternatives to the theory, and posted his answers:
http://preposterousuniverse.blogspot.com/2004/08/testing-general-relativity.html
The American Physical Society is having a meeting of
the division of Particles and Fields at riverside
(this gives a picture of what High Energy Physics, or Particle Physics,
is doing these days)
http://dpf2004.ucr.edu/program.html
a fair number of the talks were on astronomy, cosmology, astrophysics topics, you can see by running down the list of plenary talks
Sean Carroll's overview of cosmology (theory) was today Tuesday 31Aug,
it is online and takes about 4 minutes to download
lots of graphs and other visual data
Finally, do you think that GR will ultimately prove to be wrong (or incomplete) at some level?
Yes. Everybody (in their right mind) does. GR is a classical theory, fundamentally inconsistent with the quantum world in which we live. At the very least we will have to find a quantum version of GR; more likely, we will have to find some more profound theory that is intrinsically quantum-mechanical and reduces to GR in the appropriate circumstances. If experiments reveal deviations from GR at even the classical level, so much the better.
http://preposterousuniverse.blogspot.com/2004/08/testing-general-relativity.html
DA..... you think? :laughing:
http://arxiv.org/abs/gr-qc/0409006
"Semiclassical Quantum Gravity: Statistics of Combinatorial Riemannian Geometries"
Or "How to use statistical geometry to quantify uncertainties"
http://arxiv.org/abs/gr-qc/0409020
Causal Loop Quantum Gravity and Cosmological Solutions
Authors: Ali Shojai, Fatimah Shojai
Comments: 16 pages, 4 figures
"We shall present here the causal interpretation of canonical quantum gravity in terms of new variables. Then we shall apply it to the minisuperspace of cosmology. A vacuum solution of quantum cosmology is obtained, and the Bohmian trajectory is investigated. At the end a coherent state with matter is considered in the cosmological model. "
Seems like a new interpretation of LQG in terms of Bohmian mechanics.
Hidden variables in LQG?
Florian Conrady
Free vacuum for loop quantum gravity
http://arxiv.org/abs/gr-qc/0409036
Olaf Dreyer, Fotini Markopoulou, Lee Smolin
Symmetry and entropy of black hole horizons
http://arxiv.org/abs/hep-th/0409056
Fotini Markopoulou, Lee Smolin
Gauge fixing in Causal Dynamical Triangulations
http://arxiv.org/abs/hep-th/0409057
Olaf Dreyer
Background Independent Quantum Field Theory and the Cosmological Constant Problem
http://arxiv.org/hep-th/0409048
http://arxiv.org/abs/gr-qc/0409045
I've read a bit of the paper, and in a nutshell: in gr-qc/9401028, there were a pair of guys trying to unify LQG with Yang-Mills theory, but there were difficulties. In this new paper, the authors say that they achieve this unification in the Euclidean signature
Daniele Colosi, Carlo Rovelli
Global particles, local particles
http://arxiv.org/abs/gr-qc/0409054
brief sample from conclusions section:
---quote---
...the distinction between global and local states can therefore be safely neglected in concrete utilizations of QFT. However, the distinction is conceptually important because it bears on three related issues: (i) whether particles are local or global objects in conventional QFT; (ii) the extent to which the quantum field theoretical notion of particle can be extended to general contexts where gravity cannot be neglected; and furthermore, more in general, (iii) whether particles can be viewed as the fundamental reality (the “ontology”) described by QFT. Let us discuss these three issues separately. ...
...Can we base the ontology of QFT on local particles? Yes, but local particle states are very different from global particle states. Global particle states such as the Fock particle states are defined once and for all in the theory, while each finite size detector defines its own bunch of local particle states. Since in general the energy operators of different detectors do not commute ([HR, HR'] nonzero) there is no unique “local particle basis” in the state space of the theory, as there is a unique Fock basis. Therefore, we cannot interpret QFT by giving a single list of objects represented by a unique list of states. In other words, we are in a genuine quantum mechanical situation in which distinct particle numbers are complementary observables. Different bases that diagonalize different HR operators have equal footing. Whether a particle exists or not depends on what I decide to measure. In such a context, there is no reason to select an observable as “more real” than the others.
The world is far more subtle than a bunch of particles that interact.
---end quote---
selfAdjoint
Sep15-04, 11:14 AM
Apropos of this Colosi-Rovelli attempt to generalize particles, see today's post on Peter Woit's Not Even Wrong about Grothendiek and his toposes. He generalized the Nullstellenstatz view, which you have explicated so clearly, Marcus, in which the points of a continuum are represented as the prime ideals of the algebra of continuous functions on it. G. represents points of a space X as sheafs over X; a sheaf is a kind of category, and this leads to G.'s definition of topos, which we have had some discussion about in connection with Chris Isham's papers. G. was looking to define the "group of a point", and he actually reached a good definition.
Much of the perceived beauty of string theory is actually the beauty of G. and his generation's work in topology and algebraic geometry, which people like Witten have scarfed up and instantiated in physical models, orbifolds, for example. See the survey of this work by Jacques Cartier which Woit gives a link to.
I am glad to see the web of connections to this Rovelli-Colosi paper
extended in such a distinguished way.
On another matter, it has always been an impediment and a concern that there is no introductory textbook for LQG.
Alejandro Perez posted this, just today:
Introduction to Loop Quantum Gravity and Spin Foams
http://arxiv.org/abs/gr-qc/0409061
selfAdjoint
Sep16-04, 08:24 AM
Yes I saw Perez's paper. I have only just glanced at it but I am glad to see that he doesn't stint on discussing the quantization problems.
meteor noticed this short paper by Gambini, Olson, Pullin
http://arxiv.org/abs/gr-qc/0409045
It was posted 10September but didn't get noted at the time.
Unified model of loop quantum gravity and matter
4 pages, dedicated to Michael P. Ryan on the occasion of his sixtieth birthday
"We reconsider the unified model of gravitation and Yang--Mills interactions proposed by Chakraborty and Peldán, in the light of recent formal developments in loop quantum gravity. In particular, we show that one can promote the Hamiltonian constraint of the unified model to a well defined anomaly-free quantum operator using the techniques introduced by Thiemann, at least for the Euclidean theory. The Lorentzian version of the model can be consistently constructed, but at the moment appears to yield a correct weak field theory only under restrictive assumptions, and its quantization appears problematic."
wolram found this
http://jdc.math.uwo.ca/spin-foams/
it is a great QG resource
Franz Hinterleitner
Canonical DSR
http://arxiv.org/gr-qc/0409087
"For a certain example of a "doubly special relativity theory" the modified space-time Lorentz transformations are obtained from momentum space transformations by using canonical methods. In the sequel an energy-momentum dependent space-time metric is constructed, which is essentially invariant under the modified Lorentz transformations. By associating such a metric to every Planck volume in space and the energy-momentum contained in it, a solution of the problem of macroscopic bodies in doubly special relativity is suggested."
may have a solution to the "Soccer Ball" problem mentioned in several recent papers on multi-special relativity (links in this thread to papers on DSR by Smolin, Kowalski-Glikman, Livine, Girelli, Oriti etc. mentioning this problem of the momentum of macroscopic bodies sometimes called soccer ball problem) I have to go but will get back to this and start a separate thread about this paper if it looks to me like Hinterleitner has made some headway with this.
nonunitary
Oct8-04, 06:44 PM
Hi,
There are two upcoming activities related to loop quantum gravity:
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/index.php
and
http://www.nuclecu.unam.mx/~gravit/EscuelaVI/english.html
Hi,
There are two upcoming activities related to loop quantum gravity:
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/index.php
and
http://www.nuclecu.unam.mx/~gravit/EscuelaVI/english.html
Thanks nonunitary! I see there's an interesting lineup of talks at the "Quantum Gravity in the Americas" conference, reflecting people's current research:
---quote from the program---
Workshop on Quantum Gravity in the Americas: Status and future directions
October 29 - 31, 2004
Brunnemann: Volume Operator and Recoupling Theory
Chen: Quantum Liouville Theory and Black Hole
Conrady: Vacuum State for LQG
Dittrich: Status of the Master Constraint Programme
Henson: Consequences of space-time discreteness on wave propagation
Manrique: On the macroscopic limit of vacuum compact QED
Meusburger: Phase space quantization of 2+1 gravity in Chern-Simons formulation
Perrini: Asymptotic safety for quantum gravity
Reyes: Higgs propagation in loop quantum geometry
Singh: Phenomenological aspects of LQC
Terno: Entropy and Entanglement for LQG Black Holes
Willis: Some Obstructions to Spin Networks for Non-Compact Gauge
*
Alfaro: Loop Quantum Gravity and High Energy Cosmic Rays
Bombelli: Coherent and semiclassical states for systems
Corichi: About Semi-Classical Quantum Gravity
Dowker: The `Problem of Time’ in a Sum-Over-Histories framework?
Freidel: Particles in 3d quantum gravity
Husain: Black Hole geometrodynamics revisited
Lewandowski: Black hole entropy
Major: Quantum Geometry Phenomenology: A Discrete Machian Model
Markopoulou: Locality in Quantum Gravity
Morales-Tecotl: Possible phenomenological limits for semi-classical LQG
Oeckl: General boundaries and transition amplitudes in QG
Oriti: Feynman propagator in spin foam QG: causality without time
Perez: Dynamics and Spin Foams in non perturbative QG
Pullin: Semi-discrete solution to the dynamics of LQG
Reisenberger: Canonical GR on null hypersurfaces
Sahlmann: String Theory with LQG methods
Smolin: Physics from Loop Quantum Gravity
Sudarski: Space-time granularity and Lorentz Invariance Violation
Urrutia: Synchrotron radiation in Myers-Pospelov electrodynamics
Winkler: Particles, cosmology and spinfoams
Zapata: Coarse graining in loop quantization
Ashtekar: Quantum geometry and black holes
Baez: Spin foams, 2-vector spaces and categorification
Sorkin: Is a past finite order the inner basis of space-time?
LIST OF POSTERS
Cambiaso: Lorentz symmetry violations: Constraints from ultra-high energy astronomy
Carrion: Wilson loop dynamics without regularization
Cortez: On nonequivalence of representations in QFT
Garcia-Islas
Giesel
Montesinos: Covariant Hamiltonian dynamics
Noui: About the Plebanski action
Vandersloot: A model of loop quantum cosmology and its physical Hilbert space
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/participants.php
Stanford SLAC Library collaborates with the German organization DESY
on the Spires HEP database which has a lot of physics papers available for keyword search. They have sharpened the focus on Quantum Gravity at Spires. You can see this by comparing how the search engine works for 2003 and 2004.
the main URL is
http://www.slac.stanford.edu/spires/hep/
Quantum Gravity is one of the Spires keyword options, and as of now if you try it for 2004 you get 132 papers. It is not that this is so many, quantity-wise, what impresses me is the quality of the pick (especially as compared with past years)
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+k+quantum+gravity+and+date+2004&SKIP=0
this gets all the papers on Quantum Gravity that appeared this year that they have cataloged as QG so far. It looks to me to be remarkably complete. the representation of LQG and allied approaches to QG is near total and a real improvement over the same search for 2002 or 2003
The above URL is what you get when you type this into the search engine box:
FIND K QUANTUM GRAVITY AND DATE 2004
K means "keyword", it is fairly self-explanatory and the "help" section is helpful. Spires has other modes of search as well, like by author, and you have the option of filtering the search for highly cited papers
For example this will get any Quantum Gravity paper which appeared in 2003 and which has been cited by 50 or more other papers:
FIND K QUANTUM GRAVITY AND TOPCITE 50+ AND DATE 2003
The result is:
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+K+QUANTUM+GRAVITY+AND+TOPCITE+50%2 B+AND+DATE+2003
Another useful feature Spires has is a "top 100" papers list for each year, that ranks papers according to number of citations received.
http://www.slac.stanford.edu/library/topcites/
BTW has anyone besides me listened to Michael Peskin's 8 October talk at Kavli. He was on the panel that came after talks by Steven Weinberg and Frank Wilczek
http://online.itp.ucsb.edu/online/kitp25/
His perspective was in sharp contrast to Weinberg's, as Peskin himself pointed out. Peskin does the annual Spires HEP review.
It is always hard to find an article about Quantum Gravity written for general audience, that is online.
Smolin's January 2004 Scientific American article was clear and accessible (really good i thought) but not online---required a trip to the library, or for you to have a subscription.
this one by Rovelli is not only good and written for general audience, but free for downloading. It prints out to 5 pages and even mention's Renate Loll's work (computer simulations of evolving geometry, with graphic output). I guess Abhay Ashtekar liked it so he scanned it and put it at his website :smile: for educational use.
http://cgpg.gravity.psu.edu/people/Ashtekar/articles/rovelli03.pdf
This thread is serving as a surrogate sticky (which I update periodically) for links to useful sources about Loop-and-related quantum gravity research.
the last update was back on page 15 around post #217 and another pass is overdue.
Other people's collections of links:
wolram found this
http://jdc.math.uwo.ca/spin-foams/
Introductions, surveys, books:
Abhay Ashtekar
Gravity and the Quantum
http://arxiv.org/gr-qc/0410054
Carlo Rovelli's book
Quantum Gravity
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
contains introduction, historical and philosophical perspective as well as technical stuff.
Best general audience article available online is probably
Rovelli's November 2003 "Physics World" article which Astekar has
at his website. The title was Loop Quantum Gravity
http://cgpg.gravity.psu.edu/people/Ashtekar/articles/rovelli03.pdf
Another good general audience article (but not available online) is
Lee Smolin's Atoms of Space and Time in the January 2004 issue
of the "Scientific American".
Probably the best introduction for physics students and physicists who are not specialists in QG is by Lee Smolin
An Invitation to Loop Quantum Gravity
http://arxiv.org/hep-th/0408048
50-page survey and intro with FAQ for physicists in other fields who want to switch fields and do QG research. Intended for the annual Reviews of Modern Physics. Contains list of unsolved problems to work on. List of main results so far.
Alejandro Perez
Introduction to Loop Quantum Gravity and Spin Foams
http://arxiv.org/abs/gr-qc/0409061
John Baez gave an introduction and survey at the Dublin GR17 conference, which is available online at Baez site.
"Loop Quantum Gravity, Quantum Geometry and Spin Foams"
http://math.ucr.edu/home/baez/lectures.html#lqg
--------
Ways to find out what's currently happening:
Conferences:
the 29,30,31 October conferences at Perimeter
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/participants.php
Search engines:
SPIRES search engine
http://www.slac.stanford.edu/spires/hep/
http://www.slac.stanford.edu/library/topcites/
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+k+quantum+gravity+and+date+2004&SKIP=0
The above URL is what you get when you type this into the search engine box:
FIND K QUANTUM GRAVITY AND DATE 2004
K means "keyword", the "help" section is helpful.
ARXIV search engine
2001:
http://lanl.arxiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/2001/0/1
2002:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/2002/0/1
2003:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/2003/0/1
Last Twelve Months:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/past/0/1
John Baez TWF, eg. his TWF #206 reports on the Marseille conference
http://math.ucr.edu/home/baez/week206.html
"In particular, I spend a lot of time giving a very simple nontechnical introduction to the recent work of Ambjorn, Jurkiewicz and Loll in which they seem to get a 4d spacetime to emerge from a discrete quantum model - something that nobody had succeeded in doing before!"
the paper mentioned here is
Ambjorn Jurkiewicz Loll
Emergence of a 4D World from Causal Quantum Gravity
http://www.arXiv.org/abs/hep-th/0404156
Wave's_Hand_Particle just called attention to the fact that Smolin has
co-authored a followup to this paper
Fotini Markopoulou, Lee Smolin
Gauge fixing in Causal Dynamical Triangulations
http://arxiv.org/hep-th/0409057
PAGE TWO (look back one post for the beginning)
NEWSLETTER: the APS Gravity newsletter
Jorge Pullin's Matters of Gravity
http://arxiv.org./abs/gr-qc/0403051
this is the Spring 2004 issue
the Fall 2004 issue is
http://arxiv.org/abs/gr-qc/0409046
==========
TEXTS:
Besides Rovelli's book there is a recent monograph by Ashtekar and Lewandowski
Background Independent Quantum Gravity: A Status Report
http://arxiv.org/gr-qc/0404018
and Thiemann's Lecture Notes
(they have been published in by Springer Verlag, Berlin)
Lectures on Loop Quantum Gravity
available online
http://arxiv.org/gr-qc/0210094
Rovelli's LivingReviews article
http://arxiv.org/abs/gr-qc/9710008
Rovelli and Upadhya "Primer"
http://arxiv.org/abs/gr-qc/9806079
Rovelli and Gaul lecture notes from WS-1999
http://arxiv.org/abs/gr-qc/9910079
========
SURVEY ARTICLES:
A survey of the whole field of approaches to QG
which is interesting partly for historical and broader perspective is
Carlo Rovelli's
Strings, loops and others: a critical survey of the present approaches to quantum gravity
http://arxiv.org/gr-qc/9803024
" I illustrate the main achievements and the main difficulties in: string theory, loop quantum gravity, discrete quantum gravity (Regge calculus, dynamical triangulations and simplicial models), Euclidean quantum gravity, perturbative quantum gravity, quantum field theory on curved spacetime, noncommutative geometry, null surfaces, topological quantum field theories and spin foam models...."
another broad survey
Enrique Alvarez
http://arxiv.org/gr-qc/0405107
Quantum Gravity
( Lectures given at Karpacz. 40 pages)
==============
QG PHENOMENOLOGY:
Efforts and proposals to test QG are of increasing importance.
QG testability (Phenomenology) was the topic of the Winterschool-2004 (WS-2004) symposium this year, 4-14February, at Karpacz.
http://ws2004.ift.uni.wroc.pl/html.html
click on lectures if you want slides from the various talks.
Many of the talks are now written up as journal articles and available
on arxiv----more complete and faster download.
Cambridge Uni Press is publishing "Universe or Multiverse" which will contain a chapter by Smolin called "Scientific Alternatives to the Anthropic Principle" where he offers an evolutionary Multiverse hypothesis that generates testable (numerical) predictions.
http://arxiv.org/hep-th/0407213
Giovanni Amelino-Camelia
Planck-scale Lorentz-symmetry test theories
http://arxiv.org/abs/astro-ph/0410076
====================
EXTENSIONS OF SPECIAL RELATIVITY
Much QG phenomenology focuses on modifications of Lorentz symmentry---connecting quantum gravity with DSR namely "DDSR" or "TSR" ( triply special relativity)
Kowalski-Glikman, Smolin
"Triply Special Relativity"
http://arxiv.org/abs/hep-th/0406276
Girelli Livine Oriti
"Deformed Special Relativity as an effective flat limit of quantum gravity"
http://arxiv.org/gr-qc/0406100
======================
LOOP QUANTUM COSMOLOGY (only a few of the many papers)
meteor recently flagged this one
Martin Bojowald, Parampreet Singh, Aureliano Skirzewski
Time dependence in Quantum Gravity
http://arxiv.org/abs/gr-qc/0408094
This recent paper has an extensive bibliography with many arxiv links, so
I will refer to that instead of posting them.
Here is the abstract:
"The intuitive classical space-time picture breaks down in quantum gravity, which makes a comparison and the development of semiclassical techniques quite complicated. By a variation of the group averaging method to solve constraints one can nevertheless introduce a classical coordinate time into the quantum theory, and use it to investigate the way a semiclassical continuous description emerges from discrete quantum evolution. Applying this technique to test effective classical equations of loop cosmology and their implications for inflation and bounces, we show that the effective semiclassical theory is in good agreement with the quantum description even at short scales."
Martin Bojowald
Loop Quantum Cosmology: Recent Progress
http://arxiv.org/gr-qc/0402053
Martin Bojowald
Quantum Gravity and the Big Bang
http://arxiv.org./astro-ph/0309478
Shinji Tsujikawa, Parampreet Singh, Roy Maartens
Loop quantum gravity effects on inflation and the CMB
http://arxiv.org/astro-ph/0311015
from their abstract:
"In loop quantum cosmology, the universe avoids a big bang singularity and undergoes an early kinetic-dominated super-inflation phase, with a quantum-corrected Friedmann equation. As a result, an inflaton field is driven up its potential hill, thus setting the initial conditions for standard inflation. We show that this effect can raise the inflaton high enough to achieve sufficient e-foldings in the standard inflation era. We analyze the cosmological perturbations and show that loop quantum effects can leave a signature on the largest scales in the CMB, with some loss of power and running of the spectral index."
Viqar Husain and Oliver Winkler
On singularity resolution in quantum gravity
http://arxiv.org/gr-qc/0312094
this is especially interesting because they duplicate LQC results (for example by Bojowald) using an older version of quantum gravity, ADM variables, quantized metric. Shows that the removal of the big bang singularity doesnt depend on using a particular formalism.
PAGE THREE OF THE LINKS
Stingray told us about this talk by Ashtekar
http://www.phys.psu.edu/events/index.html?event_id=934;event_type_ids=7;span=2004-08-20.2004-12-25
The talk was given 20 September at Penn State and is called
Black Hole Evaporation and Information Loss: Recent Advances
As you listen to the audio you have to step from one slide to the next
in synch with the talk. In this talk you get a foretaste of two papers by Ashtekar and Bojowald which are not on arxiv yet.
Ashtekar refers to the papers in
http://arxiv.org/abs/gr-qc/0410054
They are:
Ashtekar A and Bojowald M 2004 Non-Singular Quantum Geometry of the Schwarzschild Black Hole Interior Preprint
Ashtekar A and Bojowald M 2004 Black hole evaporation: A paradigm Preprint
nonunitary provided a link to an article giving the definitions of dynamical horizon and isolated horizon. Ashtekar uses these concepts in his talk
Abhay Ashtekar, Badri Krishnan
Isolated and dynamical horizons and their applications
http://arxiv.org/gr-qc/0407042
other papers of interest
Gambini Porto Pullin
Realistic clocks, universal decoherence and the black hole information paradox
http://arxiv.org/abs/hep-th/0406260
also their earlier
No black hole information puzzle in a relational universe
http://arxiv.org/hep-th/0405183
==========================
Wave's_Hand_Particle
Oct21-04, 01:46 AM
Marcus, it may be of interest if you could collate papers that are pertaining to each other, for intstance these appeared a couple of days ago:http://uk.arxiv.org/abs/hep-th/0409057
http://uk.arxiv.org/abs/hep-th/0409056
This is a reply from Smolin-Dreyer-Markopoulou, to the Loll-Ambjorn paper?
Marcus, it may be of interest if you could collate papers that are pertaining to each other, for intstance these appeared a couple of days ago:http://uk.arxiv.org/abs/hep-th/0409057
http://uk.arxiv.org/abs/hep-th/0409056
This is a reply from Smolin-Dreyer-Markopoulou, to the Loll-Ambjorn paper?
Thanks for pointing this out WHP. After I read your post I went back and edited it into post #245, that is 3 or 4 back from here.
The Smolin-Markopoulou hep-th/0409057 is definitely a response to the Ambjorn-Jurkiewicz-Loll paper. it supports the idea that the AJL technique does not single out any particular time coordinate (which would be a weakness in their approach). So it is a friendly corroboration that AJL looks like it is on the right track. However more needs to be done in that direction, as they note. If you see more dynamical triangulations papers please flag them. I'm looking forward to seeing some appear in the next few months.
selfAdjoint
Oct21-04, 10:58 AM
From hep-th/0409057:
It appears that, while other features of sum-over-histories quantum gravity, such as locality or fundamental discreteness, appear desirable, they are not shared by Causal Dynamical Triangulations. As it is the latter that possesses the good low-energy behavior, this implies either that these desirable features are misguided, or that our previous expectations for the geometrical interpretation of the path-integral histories as the microscopic description of spacetime were naive.
I am coming to believe that all the present day theories, as viewed from say 2040, will be seen to be infected with galloping naivite. Mathematical sophistication without the deep insight that is going to be discovered one of these days.
From hep-th/0409057:
I am coming to believe that all the present day theories, as viewed from say 2040, will be seen to be infected with galloping naivite. Mathematical sophistication without the deep insight that is going to be discovered one of these days.
Could this more fundamental physical principle be a deeper insight into entropy? Could entropy and information have a more direct connection to forms of logic? Certainly, inductive logic considers the probabilities of events, and so does quantum mechanics. So could there be rules of entropy applied to inductive logic that might also give rise to QFT? I wonder. If so, then perhaps the laws of physics and be derived from logic.
selfAdjoint
Oct21-04, 05:53 PM
No I don't think it's entropy. I don't think it's anything that's obvious, because hundreds of physicists are searching, and everything obvious has been tried or soon will be. I think there's a missing piece of mathematics. When I was in grad school, there wasn't any such thing as K-theory; then it was developed, and within a few years Witten was using it to refine string theory. Unlike the situation when I was growing up, the advance of physics is closely tied to the advance of mathematics.
Dan Christensen, Louis Crane
Causal sites as quantum geometry
20 pages, 3 figures
http://arxiv.org/abs/gr-qc/0410104
A seminar talk at Penn State by Martin Bojowald
http://www.phys.psu.edu/events/index.html?event_id=516;event_type_ids=0;span=2002-12-26.2003-05-31
Quantum Cosmology: An Overview
27 January 2003
audio with slides.
more informative in some ways than journal articles
e.g. more computer-graphic images.
The first slide is here:
http://www.phys.psu.edu/events/display.html?event_id=516&file=0
No I don't think it's entropy. I don't think it's anything that's obvious, because hundreds of physicists are searching, and everything obvious has been tried or soon will be. I think there's a missing piece of mathematics. When I was in grad school, there wasn't any such thing as K-theory; then it was developed, and within a few years Witten was using it to refine string theory. Unlike the situation when I was growing up, the advance of physics is closely tied to the advance of mathematics.
I appreciate your input as always, but I don't know, guy. We are talking about the emergence of structure amoung alternative possibilities. I thing entropy is inherent in those considerations. To date I don't think that entropy has been applied to the structure of particle/strings or the structure of spacetime itself. But if we imagine the very first possible structure in the universe, you might think that there is entropy involved with any structure.
As I understant it, we don't know the reason that QM is as it is. We are still looking for fundamental principles that give rise to the formulism of QM. I have to wonder if that might not be some sort of conservation of entropy or at least a restriction of it rate. I consider for example whether the amplitude/phase of QM might derive from the necessity of alternatives when each alternative has some structure. The structure would be neg-entropy, and the increase of alternative would be entropy, thus QM. Or something like that.
I mean, the mere fact that we are talking about various kinds of structures in the universe and how they interact makes probabilities and entropy relevant. And since we are considering all the possibilities in our investigations, I think it may be inevitable that the final formalism must relate how all possibilities interact/interfere with each other to product the final result. We are now looking for why QM gives us the way that these possibilities interact/interfere.
Hi Mike, do you think you could start a thread on this so we can keep focused here on collecting links to source material for Loop and allied approaches to QG?
Dan Christensen, Louis Crane
Causal sites as quantum geometry
20 pages, 3 figures
http://arxiv.org/abs/gr-qc/0410104
...
ordinary (loop) QG is done on a set of points called a differentiable manifold-----a continuum---analog of ordinary 3D space but without
a precommittment to some particular geomety---a floppy continuum
that was the basis for classic 1915 GR too.
Now Christensen and Crane want to replace the diff-manif.
they want to get rid of the point set continuum and replace it with a new mathematical arena called a Site.
Grothendieck made up Sites. A site is a category with a "Grothendieck topolopy"
you consider your old pointset topological space and you notice that the subsets A of X form a partially ordered-by-inclusion structure and you abstract this notion. Now you have a bunch of "subsets" but they dont have points they are just abstract entities with an ordering relation (taken from the old "order-by-inclusion")
that's not all, these things (A, B,....) are also ordered by causality. One of them can precede another, sometimes.
Grotend. made up a topology to put on this kind of thing, and various
superstructure---presheaf, gerbe, bundle, gadgetry---which he and his friends always enjoyed doing.
Along come christensen crane and notice it would be a neat thing to do QG on instead of doing it on a manifold.
Einstein always said that the points of the manifold had no physical existence. So maybe christensen crane are purifying. and sometimes
when you purify it is like throwing overboard the balast and the ship or balloon can get off the ground.
so i want to call attention to this paper. it has the beginnings of a new approach. mostlikely one that will fail!!! of course. that is the game. one must try anyway. good luck to them.
[edit: it might succeed too, might be a really good idea---can't tell at this point]
you consider your old pointset topological space and you notice that the subsets A of X form a partially ordered-by-inclusion structure and you abstract this notion. Now you have a bunch of "subsets" but they dont have points they are just abstract entities with an ordering relation (taken from the old "order-by-inclusion")
Is it possible that the sets of a causal site can be shrunk down to points? Are causal site a generalization of point sets?
Is it possible that the sets of a causal site can be shrunk down to points? Are causal site a generalization of point sets?
I started a thread about the christensen crane paper, in case there is some interest in discussing it. QG-wise it's really new mathematics, could have potential. Anyway I think it rates a thread.
Let's discuss it there, if you want, and not load this link-basket thread with discussion
Here's a picture of Martin Bojowald
from the May 2004 conference at Marseille
http://perimeterinstitute.ca/images/marseille/marseille017.JPG
Martin is the guy in the gray T-shirt and black frames, not looking at the camera.
Turns out he gave two seminar talks at Penn State last year, both on Quantum Cosmology.
To get recorded seminar talks at Penn State you go here:
http://phys.psu.edu/events/
and select whatever semester.
In "spring 2003" there is a long list that includes
Bojowald's talks Quantum Cosmology: An Overview
and Quantum Cosmology: Formalism
the links for the slides and audio for these two are
http://phys.psu.edu/events/index.html?event_id=516;event_type_ids=0;span=2002-12-26.2003-05-31
http://phys.psu.edu/events/index.html?event_id=521;event_type_ids=0;span=2002-12-26.2003-05-31
Ashtekar's recent talk is part of the same collection
Black Hole Evaporation and Information Loss: Recent Advances
http://www.phys.psu.edu/events/index.html?event_id=934;event_type_ids=7;span=2004-08-20.2004-12-25
If I could, i would edit this into the DSR section of a page of links a few posts back, but those pages are closed to further editing, so i will just stick it on here:
Liberati, Sonego, Visser
Interpreting doubly special relativity as a modified theory of measurement
http://arxiv.org/gr-qc/0410113
Madhavan Varadarajan
The graviton vacuum as a distributional state in kinematic Loop Quantum Gravity
44 pages
http://arxiv.org/abs/gr-qc/0410120
---quote---
Abstract:
The quantum behaviour of weak gravitational fields admits an adequate, albeit approximate, description by those graviton states in which the expectation values and fluctuations of the linearised gravitational field are small. Such states must approximate corresponding states in full quantum gravity. We analyse the nature of this approximation for the graviton vacuum state in the context of kinematical Loop Quantum Gravity (LQG) wherein the constraints are ignored. We identify the graviton vacuum state with kinematically non-normalizable, distributional states in LQG by demanding that relations between linearised operator actions on the former are mirrored by those of their non-linear counterparts on the latter. We define a semi- norm on the space of kinematical distributions and show that the identification is approximate upto distributions which are small in this semi-norm. We argue that our candidate states are annihilated by the linearised constraints (expressed as operators in the full theory) to leading order in the parameter characterising the approximation. This suggests the possibility, in a scheme such as ours, of solving the full constraints order by order in this parameter. The main drawback of our considerations is that they depend on certain auxilliary constructions which, though mathematically well defined, do not arise from physical insight. Our work is an attempt to implement an earlier proposal of Iwasaki and Rovelli.
I. Introduction:
A Dirac constraint quantization of a Hamiltonian formulation of gravity is defined through the following steps. First, a “kinematical” representation of the Poisson bracket algebra of a large enough set of functions on the unconstrained phase space is constructed such that these functions act as linear operators on the representation space. Next, the constraints of the theory are represented as quantum operators in this representation and physical states are identified with their kernel. Finally, an inner product on the space of physical states is chosen which enforces hermiticity conditions on a complete set of operators corresponding to Dirac observables, thus converting the physical state space to a Hilbert space...
---end quote---
Varadarajan's 19 archived papers go back to 1993. He has co-authored with Abhay Ashtekar, and also at various times both with Rafael Sorkin, and with Fernando Barbero.
Here are the two Iwasaki-Rovelli papers he says were his point of departure
J. Iwasaki and C. Rovelli, Int.J.Mod.Phys.D1, 533 (1993);
J. Iwasaki and C. Rovelli, Class.Quant.Grav.11, 1653 (1994).
Since the graviton is supposed to be quantized gravity which is also supposed to be quantized geometry, would gravitons exist everywhere and constitute the construction of spacetime itself? This would be opposed to gravition moving through space. Instead, are graviton space itself, always motionless with respect to the observer?
... would gravitons exist everywhere and constitute the construction of spacetime itself?
I believe not, Mike. That is, this paper does not indicate that gravitons are real----rather that they are a mathematical fiction which plays a part in a possibly useful approximation. Analysis of the gravitational field using gravitons would be appropriate, as this paper suggests, in the weak field case.
We should start a thread about ontology. You seem to me to have an abiding interest in ontological questions---what is really there, what is reality made of. I would say, in answer to such questions, "the field is the field."
---here is a short essay that could serve to start an ontology thread---
To say it in a lot more words: the field is the field and it is not made of anything more basic---it is not made of gravitons or anything else, it is really there. It may or may not sometimes be useful (as an approximation) to describe it in some special case in terms of the mathematical device of postulated gravitons and then it may or may not be helpful to talk among ourselves about "gravitons". But that is an overlay of mathematics, it is not ontological. Ontologically speaking there is nothing more basic than the field.
The field is not something defined "in space and time" because space and time arise from the field. space and time are appearances or phenomena which emerge from the field. We must arrive at ways of describing the field which do not depend on imagining a prior space and time in which to define it.
The field is a quantum animal, living in a Hilbertspace of all possible fields. The central problem in physics today is to find a satisfactory formal way of to describe the gravitational field, and the Hilbertspace of its possibilities.
When and if that is found, everything else will be describable relative to, and on top of, the field.
----let's continue this in a separate thread, so as not to choke the linkbasket---
Viqar Husain and Oliver Winkler
Quantum resolution of black hole singularities
http://arxiv.org/abs/gr-qc/0410125
this can be seen as continuation of their work in 2003 where they resolved the big bang singularity (following Bojowald lead but by their own methods)
Viqar Husain and Oliver Winkler
On singularity resolution in quantum gravity
http://arxiv.org/gr-qc/0312094
(they duplicated Bojowald LQC results using an older version of quantum gravity, ADM variables, quantized metric, showing that the removal of the big bang singularity doesnt depend on using a particular formalism)
the history is like this: in 2001 Martin Bojowald used Loop QG to resolve the big bang singularity---his landmark paper is Absence of singularity in Loop Quantum Cosmology
in the intervening time people have repeatedly confirmed his result using variations of LQG, it has been shown not to depend on the details but to be a basic thing. when you quantize Einstein 1915 Gen Rel, then you get a bounce at the beginning.
and Hussain and Winkler used their QG methods to corroborate this.
Now everybody was wondering "What about black hole?" does the black hole singularity also go away when you quantize? Bojowald and Ashtekar have a paper about this, not yet posted on arxiv.
And already a Rovelli postdoc named Leonardo Modesto has published, getting rid of the BH singularity
And now Hussain and Winkler too!
Although Bojowald and Ashtekar have not yet posted their resolution of BH singularity, Ashtekar gave a talk relating to it last month at Penn State.
Ashtekar's 20 September talk:
Black Hole Evaporation and Information Loss: Recent Advances
http://www.phys.psu.edu/events/index.html?event_id=934;event_type_ids=7;span=2004-08-20.2004-12-25
You can see his slides and listen to audio. what they think is at the heart of BH, at the former singularity, is shown around slide #12 and #13
======
I mentioned this earlier, but will repeat incase anyone missed it: a picture of Martin Bojowald from the May 2004 conference at Marseille
http://perimeterinstitute.ca/images/marseille/marseille017.JPG
Martin is the one not looking at the camera.
Laurent Freidel, David Louapre
Ponzano-Regge model revisited II: Equivalence with Chern-Simons
http://arxiv.org/abs/gr-qc/0410141
Laurent Freidel is interested in putting matter into 3d gravity----the 2+1 toy model that has proven easier to develop than the full 3+1 theory and which people look to for hints of how to proceed in building 4d quantum gravity.
He is giving a talk Saturday 29 October, on Symmetry and particles in 3D quantum gravity.
This paper may have some bearing on the talk. Here is the abstract:
---quote---
We provide a mathematical definition of the gauge fixed Ponzano-Regge model showing that it gives a measure on the space of flat connections whose volume is well defined. We then show that the Ponzano-Regge model can be equivalently expressed as Reshetikhin-Turaev evaluation of a colored chain mail link based on D(SU(2)): a non compact quantum group being the Drinfeld double of SU(2) and a deformation of the Poincare algebra. This proves the equivalence between spin foam quantization and Chern-Simons quantization of three dimensional gravity without cosmological constant. We extend this correspondence to the computation of expectation value of physical observables and insertion of particles.
---end quote---
Also just out today:
Daniele Oriti
The Feynman propagator for spin foam quantum gravity
http://arxiv.org/abs/gr-qc/0410134
---quote---
We link the notion causality with the orientation of the 2-complex on which spin foam models are based. We show that all current spin foam models are orientation-independent, pointing out the mathematical structure behind this independence. Using the technology of evolution kernels for quantum fields/particles on Lie groups/homogeneous spaces, we construct a generalised version of spin foam models, introducing an extra proper time variable and prove that different ranges of integration for this variable lead to different classes of spin foam models: the usual ones, interpreted as the quantum gravity analogue of the Hadamard function of QFT or as a covariant definition of the inner product between quantum gravity states; and a new class of causal models, corresponding to the quantum gravity analogue of the Feynman propagator in QFT, non-trivial function of the orientation data, and implying a notion of ''timeless ordering''.
---end quote---
Freidel and Oriti are scheduled to give talks day after tomorrow at the PI conference
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/program2.php
Saturday 29 October, Oriti 2PM and Freidel 3PM
the titles of the talks shed a bit of extra light on the topics of these papers
Saturday at 14:00
Oriti: Feynman propagator in spin foam quantum gravity: causality without time
Saturday at 15:00
Freidel: Symmetry and Particles in 3d quantum gravity
(the phrase "causality without time" is suggestive, since time may be something that emerges from the theory rather than being one of the things on which it is built, it would be nice to have a way of representing the causal ordering of events without a notion of clock or time axis)
John Baez gives a talk tomorrow morning (Friday 10AM) at the PI conference
http://math.ucr.edu/home/baez/lectures.html#dynamics
click on PDF to get the slides----lecture notes, rather more complete than slides usually are.
The Problem of Dynamics in Quantum Gravity
delineates and highlights major problems in both loop and foam approaches. attempts to set attainable goals, to guide gradual progress,
without minimizing the difficulties.
thoughtful of Baez to post his lecture notes the day before he gives the talk, often you have to wait for the online stuff until days or weeks after
Wave's_Hand_Particle
Oct29-04, 04:34 AM
Has anyone had problems with the Perimiter Seminar/Lectures links?..I have tried every single link, and not a single lecture can be viewed?
Has anyone had problems with the Perimiter Seminar/Lectures links?..I have tried every single link, and not a single lecture can be viewed?
WHP, I have listened to seminar talks online at Penn State and several other places, but never so far from Perimeter. I dont know about video.
I can't recall having seen any Perimeter talks that have even the audio available! Maybe I just havent tried.
this seems a shame. If anyone comes across audio and slides for some PI talks, please tell me. Maybe they plan to make such files available but havent gotten around to it yet.
a new Bojowald paper today:
Spherically Symmetric Quantum Horizons
Martin Bojowald, Rafal Swiderski
http://arxiv.org/abs/gr-qc/0410147
4 pages
Marc-Thierry Jaekel, Serge Reynaud
Gravity tests in the solar system and the Pioneer anomaly
http://arxiv.org/abs/gr-qc/0410148
4 page
---exerpt from Bojowald, Swiderski, conclusions section---
The isolated horizon framework provides an unambiguous condition which is local at the horizon. This makes it possible to impose the condition without full knowledge of physical solutions, which to our knowledge results in the first implementation of horizon conditions fully at the quantum level. It is this isolated horizon condition which leads to strong simplifications in the quantum Hamiltonian constraint exploited here.
Our results verify some of the earlier expectations concerning fluctuating horizons and make them more detailed. Moreover, we can show that the horizon area is an approximate quantum observable in the sense that it commutes with the dominant contribution to the Hamiltonian constraint. These calculations test several aspects of the constraint operator, in particular those which did not play a role in homogeneous models [14, 16, 17]. As we have seen, going to the horizon simplifies the analysis of some aspects of quantum observables since a horizon is much easier to impose on quantum states than an asymptotic regime where one could test the ADM mass.
The framework introduced here allows, e.g., to answer questions related to black hole evaporation [1]. There are several new possibilities not yet studied when matter Hamiltonians are coupled: First, the horizon conditions need to be generalized to dynamical horizons ...
---end quote---
reference [1] for this paper is listed simply as
[1] A. Ashtekar and M. Bojowald, in preparation.
According to Hossain, LQG explains some features of the CMB which have been observed, and makes in-principle testable predictions which distinguish it from the standard inflationary scenario. Hossain was at Albert Einstein Institute for a while this year and I believe is now back at Chennai, in India.
Golam M. Hossain
Primordial Density Perturbation in Effective Loop Quantum Cosmology
http://arxiv.org/gr-qc/0411012
30 pages
Here are Hossain's papers
http://arxiv.org/find/gr-qc/1/au:+Hossain_G/0/1/0/all/0/1
It looks to me as if this is a snapshot of Hossain at the table with Bojowald at the 2004 Marseille conference. Resembles an older shot which I know is him.
http://perimeterinstitute.ca/images/marseille/marseille017.JPG
Hossain mentions something interesting on page 26. Inflation scenarios were devised largely as a way to cope with the "horizon problem" and the temp uniformity of the CMB has always been a big talkingpoint for inflation.
But Loop cosmology resolves the horizon problem by eliminating the singularity, as Hossain points out. It also provides for well-behaved inflation with less fine-tuning. Hossain refers back to a paper with Date about this, and gives some further discussion in this paper as well.
Here's a link to the Hossain/Date paper
“Genericity of inflation in isotropic loop quantum cosmology,”
http://arxiv.org/gr-qc/0407069
--quote from gr-qc/0411012 abstract--
It is widely believed that quantum field fluctuation in an inflating background creates the primeval seed perturbation which through subsequent evolution leads to the observed large scale structure of the universe. The standard inflationary scenario produces scale invariant power spectrum quite generically but it produces, unless fine tuned, too large amplitude for the primordial density perturbation than observed. Using similar techniques it is shown that loop quantum cosmology induced inflationary scenario can produce scale invariant power spectrum as well as small amplitude for the primordial density perturbation without fine tuning. Further its power spectrum has a qualitatively distinct feature which is in principle falsifiable by observation and can distinguish it from the standard inflationary scenario.
---end quote---
Some new papers:
Leonardo Modesto
The Kantowski-Sachs Space-Time in Loop Quantum Gravity
http://arxiv.org/abs/gr-qc/0411032
Thomas Thiemann
Reduced Phase Space Quantization and Dirac Observables
http://arxiv.org/abs/gr-qc/0411031
Bianca Dittrich
Partial and Complete Observables for Hamiltonian Constrained Systems
http://arxiv.org/abs/gr-qc/0411013
Modesto is comparatively new to LQG (was doing String research until around a year and a half ago). His first paper was about using QG to remove the black hole classical singularity.
Bianca Dittrich is a student of Thiemann's at the Albert Einstein Institute (Gölm). She has been giving seminar-talks on the master constraint program (this spring at penn state and last month at perimeter)
there are some more details but I have to go out for the evening, so will add to this tomorrow
This may be a stupid question but hopefully someone can clarify.
Can spin networks describe a unit volume that is "sphere-like"?
Lee Smolin's SciAm article "Atoms of Space and Time" seems to be describing unit volume and area in spin networks in terms of polygonal structures e.g. pyramids, cubes, etc.?
Can a sphere-like volume of space be considered a unit volume since it doesn't seem to allow discrete interfaces with neighbouring unit volumes?
Hi maddy,
I'm not sure I understand what you mean by a unit of volume having some assigned shape. I will make this bold to focus attention on it and try to understand what you mean.
...Can spin networks describe a unit volume that is "sphere-like"?
Lee Smolin's SciAm article "Atoms of Space and Time" seems to be describing unit volume and area in spin networks in terms of polygonal structures e.g. pyramids, cubes, etc.?
Can a sphere-like volume of space be considered a unit volume since it doesn't seem to allow discrete interfaces with neighbouring unit volumes?
Maddy, as far as I know a spin network is not a lattice.
A common type of lattice has a UNIT CELL which is repeated over and over to form the lattice. Like a cubical lattice has a repeating cubical cell.
AFAIK there is no analogous concept in spin networks.
In the context of spin networks the only unit of volume AFAIK is the Planck volume unit and it has no assigned shape.
Like a pint or a quart or a gallon----they are unit volumes and they have no special shape.
I dont remember Lee Smolin saying anything about unit volumes having some definite polyhedral shape. If you have the SciAm article you could type in the paragraph that suggests this to you.
It is a pity that the Smolin SciAm article is not available online.
Rovelli has a popular, introductory article that is available free online and is thus easier to discuss---in case you'd like to start an entry-level discussion that anyone could follow.
Here is Ashtekar's list of popular articles on the web
http://cgpg.gravity.psu.edu/people/Ashtekar/articles.html
and among them I think the best is Rovelli's
from Physics World (November 2003)
http://cgpg.gravity.psu.edu/people/Ashtekar/articles/rovelli03.pdf
ah! it is dawning on me what your question might be about.
it might be about a certain specialized idea of duality in simplicial complexes.
(replace each point by an n-simplex, each line segment by an n-1 simplex...)
If so, then I think that is too sophisticated for an elementary discussion.
If Smolin brought it up in the SciAm article then he probably made things more difficult than necessary.
The answer to your question is no. You dont get spheres in that context.
In spin foam and spin networks this idea of duality does come up, though, and it is a pretty construction----for example in 3D you replace every point by a tetrahedron, every line by a triangle, every triangle by a line...., and you get a second pattern that is dual to the first-----in this sense it makes sense to associate volume with points and area with lines.
That may seem far-fetched. I am stretching to try to grasp what you have in mind.
Anyway the direct answer is no, sorry, no spheres anywhere in this picture.
Just to save making a new post for this, here's a new link for this thread
(in its capacity as LQG-and-allied linkbasket)
http://arxiv.org/abs/hep-th/0411154
Quantum kappa-Poincare Algebra from de Sitter Space of Momenta
J. Kowalski-Glikman, S. Nowak
10 pages
Abstract:"There is a growing number of physical models, like point particle(s) in 2+1 gravity or Doubly Special Relativity, in which the space of momenta is curved, de Sitter space. We show that for such models the algebra of space-time symmetries possesses a natural Hopf algebra structure. It turns out that this algebra is just the quantum kappa-Poincare algebra."
A new Martin Bojowald paper appeared today
http://arxiv.org/gr-qc/0411101
On Loop Quantum Gravity Phenomenology and the Issue of Lorentz Invariance
Martin Bojowald, Hugo A. Morales-Tecotl, Hanno Sahlmann
16 pages,
"A simple model is constructed which allows to compute modified dispersion relations with effects from loop quantum gravity. Different quantization choices can be realized and their effects on the order of corrections studied explicitly. A comparison with more involved semiclassical techniques shows that there is agreement even at a quantitative level..."
A couple more samples of Bojowald's output this year
http://arxiv.org/abs/gr-qc/0408094
Time dependence in Quantum Gravity
Martin Bojowald, Parampreet Singh, Aureliano Skirzewski
33 pages, 17 figures
"The intuitive classical space-time picture breaks down in quantum gravity, which makes a comparison and the development of semiclassical techniques quite complicated. By a variation of the group averaging method to solve constraints one can nevertheless introduce a classical coordinate time into the quantum theory, and use it to investigate the way a semiclassical continuous description emerges from discrete quantum evolution. Applying this technique to test effective classical equations of loop cosmology and their implications for inflation and bounces, we show that the effective semiclassical theory is in good agreement with the quantum description even at short scales."
http://arxiv.org/abs/gr-qc/0402053
Loop Quantum Cosmology: Recent Progress
Martin Bojowald
17 pages, 2 figures, Plenary talk at ICGC 2004
"Aspects of the full theory of loop quantum gravity can be studied in a simpler context by reducing to symmetric models like cosmological ones. This leads to several applications where loop effects play a significant role when one is sensitive to the quantum regime. As a consequence, the structure of and the approach to classical singularities are very different from general relativity: The quantum theory is free of singularities, and there are new phenomenological scenarios for the evolution of the very early universe including inflation. We give an overview of the main effects, focussing on recent results obtained by several different groups."
======
this thread serves to collect useful links to LQG-and-allied articles, and it needs periodic updating.
There are a lot of good links and at the moment I dont have time for a complete update right now. So I will just assemble a few specially good ones here:
Ashtekar's recent seminar talk at Penn State:
Black Hole Evaporation and Information Loss: Recent Advances
http://www.phys.psu.edu/events/index.html?event_id=934;event_type_ids=7;span=2004-08-20.2004-12-25
Ashtekar's list of links to online popular Loop Gravity articles
http://cgpg.gravity.psu.edu/people/Ashtekar/articles.html
Ashtekar's recent survey article is excellent, it presents the whole QG
picture in understandable concise terms:
http://arxiv.org/abs/gr-qc/0410054
Gravity and the Quantum
"A general review of quantum gravity addresed non-experts. To appear in the special issue "Space-time a Hundred Years Later" of the New Journal of Physics; J. Pullin and R. Price (editors)."
Thiemann and Dittrich may have found a handle on LQG dynamics
(successfully modified the Hamiltonian)
Thomas Thiemann
Reduced Phase Space Quantization and Dirac Observables
http://arxiv.org/abs/gr-qc/0411031
Bianca Dittrich
Partial and Complete Observables for Hamiltonian Constrained Systems
http://arxiv.org/abs/gr-qc/0411013
Gambini Pullin may also have a handle on the dynamics, by a
discretization that replaces the Hamiltonian constraint by a stepwise unitary evolution operator:
http://arxiv.org/abs/gr-qc/0409057
Consistent discretization and loop quantum geometry
Just keeping tabs on Ganashyam Date and Golam Hossain (one of several papers)
Genericity of inflation in isotropic loop quantum cosmology
http://arxiv.org/gr-qc/0407069
Parampreet Singh has 3 seminar talks on LQG Phenomenology
two of which are online (Fall 2004 semester at phys.psu.edu):
Phenomenological Issues in Loop Quantum Cosmology I, II
http://phys.psu.edu/events/index.html?event_id=935&event_type_ids=0&span=2004-08-20.2004-12-25
http://phys.psu.edu/events/index.html?event_id=936&event_type_ids=0&span=2004-08-20.2004-12-25
Jerzy Lewandowski has a recent seminar talk on BH entropy in LQG,
clearest thing on that I have seen so far:
Black Hole Entropy
http://phys.psu.edu/events/index.html?event_id=938&event_type_ids=0&span=2004-08-20.2004-12-25
Survey by Lee Smolin
http://arxiv.org/abs/hep-th/0408048
An invitation to loop quantum gravity
Lee Smolin
50 pages
"We describe the basic assumptions and key results of loop quantum gravity, which is a background independent approach to quantum gravity. The emphasis is on the basic physical principles and how one deduces predictions from them, at a level suitable for physicsts in other areas such as string theory, cosmology, particle physics, astrophysics and condensed matter physics. No details are given, but references are provided to guide the interested reader to the literature. The present state of knowledge is summarized in a list of 35 key results on topics including the hamiltonian and path integral quantizations, coupling to matter, extensions to supergravity and higher dimensional theories, as well as applications to black holes, cosmology and Plank scale phenomenology. We describe the near term prospects for observational tests of quantum theories of gravity and the expectations that loop quantum gravity may provide predictions for their outcomes. Finally, we provide answers to frequently asked questions and a list of key open problems."
the Debate between Lee Smolin and string-theorist Lenny Susskind
http://www.edge.org/3rd_culture/smolin_susskind04/smolin_susskind.html
that took place this summer (2004) under auspices of the online magazine Edge
Simulating the evolution of the geometry of the universe by Monte Carlo computer runs----AJL (Ambjorn, Jurkiewicz, Loll)
http://arxiv.org/abs/hep-th/0404156
Emergence of a 4D World from Causal Quantum Gravity]
and the follow-up paper
http://arxiv.org/abs/hep-th/0411152
Semiclassical Universe from First Principles
============
To an increasing extent the seminar talks at Penn State are turning out to be helpful. in some sense more up-to-date than preprint postings on ArXiv.
So here is how you go there:
http://phys.psu.edu/events/
and select whatever semester.
Mostly I have been referring to "this semester" (Fall 2004) but some earlier ones are good too, like:
in "spring 2003" there is a long list that includes
Bojowald's talks Quantum Cosmology: An Overview
and Quantum Cosmology: Formalism
The links for the slides and audio for these two are
http://phys.psu.edu/events/index.html?event_id=516;event_type_ids=0;span=2002-12-26.2003-05-31
http://phys.psu.edu/events/index.html?event_id=521;event_type_ids=0;span=2002-12-26.2003-05-31
We should also keep track of some parallel developments, as noted in this thread
http://physicsforums.com/showthread.php?t=47209
Running Newton Constant (no dark matter)
It is basically about papers of Martin Reuter to whom Ashtekar drew attention in his recent LQG survey Gravity and the Quantum:
M. Reuter, H. Weyer
Running Newton Constant, Improved Gravitational Actions, and Galaxy Rotation Curves
http://arxiv.org/abs/hep-th/0410117
"A renormalization group (RG) improvement of the Einstein-Hilbert action is performed which promotes Newton's constant and the cosmological constant to scalar functions on spacetime. ... It is found that a power law running of Newton's constant with a small exponent of the order 10^-6 would account for their non-Keplerian behavior without having to postulate the presence of any dark matter in the galactic halo."
M. Reuter, H. Weyer
Quantum Gravity at Astrophysical Distances?
http://arxiv.org/abs/hep-th/0410119
"Assuming that Quantum Einstein Gravity (QEG) is the correct theory of gravity on all length scales we use analytical results from nonperturbative renormalization group (RG) equations as well as experimental input in order to characterize the special RG trajectory of QEG which is realized in Nature and to determine its parameters. ...could provide a solution to the astrophysical missing mass problem which does not require any dark matter. We show that an extremely weak power law running of Newton's constant leads to flat galaxy rotation curves similar to those observed in Nature. Furthermore, a possible resolution of the cosmological constant problem is proposed by noting that all RG trajectories admitting a long classical regime automatically give rise to a small cosmological constant."
Also there's the Time in Quantum Gravity thread
http://physicsforums.com/showthread.php?t=48492
where Edgar1813 was discussing with us at some length about
Gambini and Pullin stuff.
I guess one can say there is developing a kind of non-string "Quantum Gravity Scene" consisting of several approaches to directly quantizing General Relativity----not what is done in string-type theories, indeed particle theorists have argued that GR is impossible to quantize---un-renormalizable.
So there is a group of non-string approaches to QG which includes Loop, and also Reuter's QEG ("quantum einstein gravity") and also Hawking's "Euclidean Path Integral" approach which was worked on in 1980s and I thought died in the 1990s, but which seems to be revitalized by the
Causal Dynamical Triangulations approach of AJL (ambjorn jurkiewicz loll).
we have AJL papers (which hark back to Hawking) making progress,
and we have Reuter and his co-workers
and we have Loop research lines also making progress on various fronts
and Gambini-Pullin's work one either thinks of as a close parallel or part of Loop, also making progress.
to me it is beginning to look like a small stampede.
Potentially these parallel efforts can be expected to support each other or even merge.
for instance, if Renate Loll's simplex gravity works or if Martin Reuter can really dispense with both dark energy and dark matter, then they will probably share these features which could also be assimilated by Loop as well. They are all trying to quantize Gen Rel and so are all, in that way, similar enterprises with some analogous mathematics.
This thread that selfAdjoint started is also a good one to keep in the picture
http://physicsforums.com/showthread.php?t=44414
String Gravitons yield GR. NOT
The thread discusses the recent paper of Thanu Padmanabhan
http://www.arxiv.org/abs/gr-qc/0409089
From Gravitons to Gravity: Myths and Reality
This paper (http://www.arxiv.org/abs/gr-qc/0409089) does a lot of testing of different kinds, and concludes that the string theorists assertion that the graviton reproduces the physics of GR in flat spacetime is a myth.
At least from my viewpoint it looks questionable whether stringy approaches have actually caught gravity---the fish may have slipped through their fingers. While on the other hand some non-string approaches are showing signs of overcoming the legendary intractability of Gen Rel, its famous resistence to being quantized. So there is a possible shift of balance under way.
mitchbicpu
Nov28-04, 07:14 PM
Check out two US PTO documents at www.epimedia.com/gravitypush/simple1.htm :surprised
the above post about pushgravity or some new theory of the cosmos seems out of place (i.e. is not relevant to this LQG thread)
here are three new LQG papers that came out today
a common theme seems to be the existence of a good semiclassical limit
in various cases of interest:
http://arxiv.org/abs/hep-th/0411245
Existence of a Semiclassical Approximation in Loop Quantum Gravity
Marco Frasca
5 pages
"We consider a spherical symmetric black hole in the Schwarzschild metric and apply Bohr-Sommerfeld quantization to determine the energy levels. The canonical partition function is then computed and we show that the entropy coincides with the Bekenstein-Hawking formula when the maximal number of states for the black hole is the same as computed in loop quantum gravity, proving in this case the existence of a semiclassical limit and obtaining an independent derivation of the Barbero-Immirzi parameter."
========
http://arxiv.org/gr-qc/0411124
Early Universe Dynamics in Semi-Classical Loop Quantum Cosmology
James E. Lidsey
14 pages
"Within the framework of loop quantum cosmology, there exists a semi-classical regime where spacetime may be approximated in terms of a continuous manifold, but where the standard Friedmann equations of classical Einstein gravity receive non-perturbative quantum corrections. An approximate, analytical approach to studying cosmic dynamics in this regime is developed for both spatially flat and positively-curved isotropic universes sourced by a self-interacting scalar field. In the former case, a direct correspondence between the classical and semi-classical field equations can be established together with a scale factor duality that directly relates different expanding and contracting universes. Some examples of non-singular, bouncing cosmologies are presented together with a scaling, power-law solution."
============
http://arxiv.org/gr-qc/0411125
Inflationary Cosmology and Oscillating Universes in Loop Quantum Cosmology
D. J. Mulryne, N.J. Nunes, R. Tavakol, J.E. Lidsey
7 pages
"We study oscillatory universes within the context of Loop Quantum Cosmology. We make a comparative study of flat and positively curved universes sourced by scalar fields with either positive or negative potentials. We investigate how oscillating universes can set the initial conditions for successful slow-roll inflation, while ensuring that the semi-classical bounds are satisfied. We observe rich oscillatory dynamics with negative potentials, although it is difficult to respect the semi-classical bounds in models of this type."
this is something I must use nearly every day.
it is the loop related papers that come up from an arxiv search
either for a given year or for the last twelve month period (LTM)
I will put the links here and afterwards tabulate how many papers the search comes up with for various years.
I think this bunch of papers about the semiclassical limit signals a bunch of new researchers, so one could expect a growing research output. Also the availability of standard textbooks facilitates entry. Interesting to watch the field grow.
----the Last Twelve Month count is as of 28 November 2004----
Year 1994:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/1994/0/1
Year 1995:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/1995/0/1
Year 1996:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/1996/0/1
Year 1997:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/1997/0/1
Year 1998:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/1998/0/1
Year 1999:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/1999/0/1
Year 2000:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/2000/0/1
Year 2001:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/2001/0/1
Year 2002:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/2002/0/1
Year 2003:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/2003/0/1
Last twelve months (e.g. 28 November 2003 to 28 November 2004):
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/past/0/1
1994 61
1995 83
1996 72
1997 70
1998 67
1999 76
2000 89
2001 98
2002 121
2003 140
LTM 180
---------
some news: Albert Einstein Institute, at Golm, is celebrating the first 10 years of its existence (and the Centennial of Einstein's 1905 relativity papers) this spring by holding a conference
http://www.aei.mpg.de/events/conference/
Abhay Ashtekar will do the invited lecture on Loop Gravity and related stuff.
The ever popular Brian Greene will also be giving one of the invited lectures.
Dates are April 5-8, 2005
the webpage says that the talks will be broadcast on the web from the AEI site.
the Mexico Loop and String conference (November 21-27) should be just finished now!
does anyone have any news to report?
http://www.nuclecu.unam.mx/~gravit/EscuelaVI/english.html
Loop quantum cosmology is experiencing rapid growth and getting a lot of results these days about the early universe, so we should probably collect a few links on the standard view of how matter was generated. Here is a survey article from the Summer 2004 institute at SLAC
http://arxiv.org/abs/hep-ph/0411301
Baryogenesis and Leptogenesis Mark Trodden
22 pages, extended version of lecture delivered at the SLAC 2004 Summer Science Institute
"The energy budget of the universe contains two components, dark matter and dark energy, about which we have much to learn. One should not forget, however, that the baryonic component presents its own questions for particle cosmology. In the context of cosmology, baryons would have annihilated with their antiparticles in the early universe, leaving a negligible abundance of baryons, in disagreement with that observed. In this general lecture, delivered at the SLAC 2004 Summer Science Institute, I provide an overview of the central issue and the general principles behind candidate models. I also briefly discuss some popular examples of models that are firmly rooted in particle physics."
http://arxiv.org/gr-qc/0412004
Physics of Deformed Special Relativity: Relativity Principle revisited
Florian Girelli, Etera R. Livine
24 pages
"In many different ways, Deformed Special Relativity (DSR) has been argued to provide an effective limit of quantum gravity in almost-flat regime. Some experiments will soon be able to test some low energy effects of quantum gravity, and DSR is a very promising candidate to describe these latter. Unfortunately DSR is up to now plagued by many conceptual problems (in particular how it describes macroscopic objects) which forbids a definitive physical interpretation and clear predictions. Here we propose a consistent framework to interpret DSR. We extend the principle of relativity: the same way that Special Relativity showed us that the definition of a reference frame requires to specify its speed, we show that DSR implies that we must also take into account its mass. We further advocate a 5-dimensional point of view on DSR physics and the extension of the kinematical symmetry from the Poincare group to the Poincare-de Sitter group (ISO(4,1)). This leads us to introduce the concept of a pentamomentum and to take into account the renormalization of the DSR deformation parameter kappa. This allows the resolution of the "soccer ball problem" (definition of many-particle-states) and provides a physical interpretation of the non-commutativity and non-associativity of the addition the relativistic quadrimomentum. In particular, the coproduct of the kappa-Poincare algebra is interpreted as defining the law of change of reference frames and not the law of scattering. This point of view places DSR as a theory, half-way between Special Relativity and General Relativity, effectively implementing the Schwarzschild mass bound in a flat relativistic context."
John Baez has alphabetized photos of many of the LQG people
at his website
http://math.ucr.edu/home/baez/marseille/
the last update of this thread was back on page 17, posts 245, 246, and 247.
I should probably update the main list of sources again.
BTW I see Baez is working on a paper with Urs Schreiber
(Urs has been doing category hocuspocus lately at the "coffee table")
wonders never cease in case you hadnt noticed :smile:
Baez Marseille pictures include a lot more than just the people, but
the snapshots of the people are labeled reliably with who they are
so finally, in case you were wondering what Alejandro Corichi looks like...
If you have Windows (with Media Player 10) then you have access to the videos----or else the slides/audio----of several dozen talks given at the October 2004 conference at Perimeter.
I have not seen them but another PF poster has checked several of them out. Here is the URL of a 4 page catalog of video talks:
http://streamer.perimeterinstitute.ca:81/MediasiteLive30/LiveViewer/FrontEnd/Front.aspx?cid=f8fb0405-e71a-4d04-a219-5080e1a8d535
Here is the main page for the conference:
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/
here's a one page list of the talks and who gave them:
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/participants.php
There were over 30 titles, several looked interesting to me and I wish it were convenient to watch the videos. Just to mention a few (not a complete list):
Ashtekar
Physical Ramifications of Quantum Geometry
Conrady
Vacuum State for LQG
Dittrich
Status of the Master Constraint Programme
Reyes
Higgs propagation in loop quantum geometry
Pullin
Semi-discrete solution to the dynamics of LQG
Sahlmann
String Theory with LQG methods
Smolin
Physics from Loop Quantum Gravity
Two interesting QG papers appeared on arxiv today
http://arxiv.org/abs/hep-th/0501114
Loop quantum gravity: an outside view
Hermann Nicolai, Kasper Peeters, Marija Zamaklar
50 pages, 11 figures
Report-no: AEI-2004-129
"We present a pedagogical review of loop quantum gravity, with the aim of enabling a precise but critical assessment of its achievements so far. Special attention is paid to the appearance of a large number of ambiguities in the theory, in particular in the formulation of the Hamiltonian constraint. We emphasise that the off-shell ('strong') closure of the constraint algebra is a crucial test of the consistency of the theory, and should be used as the main tool to select one (if any) of the proposed Hamiltonians. Developing suitable approximation methods to establish a connection with classical gravity on the one hand, and with the physics of elementary particles on the other, remains a major challenge."
Hermann Nicolai directs the relevant part of the Albert Einstein Institute (MPI Potsdam) where Thomas Thiemann and Martin Bojowald and several other Loop gravitists are (like Bianca Dittrich recently). Nicolai has done much of his research in String, but (unlike many US string theorists) he does not favor his own specialty exclusively and he supports research in Loop as well. Nicolai can provide a valuable outside understanding of LQG and what problems need to be worked on. I believe that Nicolai's insight into what he sees are the important features and unresolved questions about LQG should be very helpful to read. This is one i expect I shall print out.
This next one is in an interesting form. A three-expert Dialog, or since there are three, a "Trialog". Ted Jacobson's specialty is TESTING Quantum Gravity theories by astronomical observation---finding vulnerable points where observation may refute certain theories, or versions. Don Marolf has done reserarch in string theory, but is also familiar with LQG. Carlo Rovelli is one of the founders of LQG and contributed an early paper on Black Hole entropy. So this conversation should show contrasts between different expert viewpoints
http://arxiv.org/abs/hep-th/0501103
Black hole entropy: inside or out?
Ted Jacobson, Donald Marolf, Carlo Rovelli
42 pages, contribution to proceedings of Peyresq 9
"A trialogue. Ted, Don, and Carlo consider the nature of black hole entropy. Ted and Carlo support the idea that this entropy measures in some sense "the number of black hole microstates that can communicate with the outside world.'' Don is critical of this approach, and discussion ensues, focusing on the question of whether the first law of black hole thermodynamics can be understood from a statistical mechanics point of view."
I should also include a paper posted by Lee Smolin last week, this argues an important point that (rather in contrast to string theorizing) Smolin's LQG does make predictions that are clear and firm enough to test (with upcoming experiments) and could refute the theory. This is a "coming of age" thing. A theory is "grown up" when it is clear enough to be proven false. So this is potentially a contentious issue. Some people may not wish to acknowledge that LQG is mature enough as a theory to actually make predictions and be tested---to bet its life on future experimental outcomes.
http://arxiv.org/hep-th/0501091
Falsifiable predictions from semiclassical quantum gravity
Lee Smolin
9 pages
"Predictions are derived for the upcoming AUGER and GLAST experiments from a semiclassical approximation to quantum gravity. It is argued that to first order in the Planck length the effect of quantum gravity is to make the low energy effective spacetime metric energy dependent. The diffeomorphism invariance of the semiclassical theory forbids the appearance of a preferred frame of reference, consequently the local symmetry of this energy-dependent effective metric is a non-linear realization of the Lorentz transformations, which renders the Planck energy observer independent. This gives a form of deformed or doubly special relativity (DSR), previously explored with Magueijo, called the rainbow metric. The argument is general, and applies in all dimensions with and without supersymmetry, and is, at least to leading order, universal for all matter couplings. The argument is illustrated in detail in a specific example in loop quantum gravity.
A consequence of DSR realized with an energy dependent effective metric is a helicity independent energy dependence in the speed of light to first order in the Planck length. However, thresholds for Tev photons and GZK protons are unchanged from special relativistic predictions. These predictions of quantum gravity are falsifiable by the upcoming AUGER and GLAST experiments."
selfAdjoint
Jan16-05, 10:52 PM
In my opinion the Nicolai, Peeters, and Zamaklar paper, hep-th/0501114, is a wonderful discussion. I especially appreciated their explication of the constraint algebra closure issue which blew up in public around Thiemann's string quantization paper, but which, as NPZ show, was all the time simmering under the LQG surface. While all the other big names in LQG discretely stayed away from this uncomfortable issue, Thiemann had the guts to attack it head on (and is still attacking it with his new replacement for group averaging, a feature too new to get into LPZ). RTWT.
In this paper there are some references to recent work by Thiemann and Dittrich
http://arxiv.org/abs/hep-th/0501114
Loop quantum gravity: an outside view
Hermann Nicolai, Kasper Peeters, Marija Zamaklar
On page 22, references [79] and [80] are to the papers by Thiemann and Dittrich that came out in November 2004. (about 6 lines from the bottom of the page)
On page 41 reference [110] is to Thiemann's Loop-String paper (three lines from the bottom, where it refers to an "intense debate")
On page 34, where there is a discussion of group averaging and the way the diffeomorphism constraint is implemented, you will find a reference to the paper by Fairbairn and Rovelli
see reference [102] about 9 lines from the bottom of the page.
This is the Separable Hilbert Space in Loop Quantum Gravity which we discussed some at PF last year.
I read this paper as flagging major problems that (Nicolai thinks) would be worthwhile for his postdocs and visiting researchers at the Institute to tackle. See especially the conclusions paragraph on page 45.
As is usually the case with overviews and pedagogicial introductions (as this paper is in part) the paper only briefly touches on recent (2004) work
and gives a picture that is more "as-of-some-point-in-the-past". But even the brief mention of several of 2004 papers is helpful, or so I found, because it shows the authors' perspective on them.
[added in edit: I just saw your post. What does RTWT stand for? Ah! I bet TWT is Time Will Tell. still dont read the R]
selfAdjoint
Jan17-05, 09:12 AM
RTWT is blogger for Read the whole thing. And yes I did notice the discussion of nonseparable and separable Hilbert spaces. They are a little sly here, offering nonspeparability as a potential show stopper and then showing that it ha been handled. Or maybe this is just the result of having most of the paper written when the Fairbairn and Rovelli paper came out.
I am ashamed to say that after I promised to forge ahead on Thiemann's series of papers on his new technique, I sluffed off. With this NPZ insight on the constraint algebra problem I mean to tackle the applications of the technique this week.
no need to rush! what you have picked is a prime and hard objective.
Other than Thiemann, only Gambini (consistent discretization QG) seems to have an alternative way past the hamiltonian constraint difficulty----with him time is discretized and there is an evolution operator that advances things step by step, so there is no hamiltonian constraint
I am looking forward to any insights you have about Thiemann's approach.
On a different topic, here are two new papers Jorge Alfaro just posted on arxiv, which I want to keep tabs on.
http://arxiv.org/abs/hep-th/0501116
Loop Quantum Gravity Effects on the High Energy Cosmic Ray Spectrum
Jorge Alfaro, Gonzalo A. Palma
17 pages. Talk at The XVIIIth International Workshop High Energy Physics and Quantum Field Theory Saint-Petersburg, June 17-23, 2004
"Recent observations on ultra high energy cosmic rays (those cosmic rays with energies greater than about 4 x 10^{18} eV) suggest an abundant flux of incoming particles with energies above 1 x 10^{20} eV. These observations violate the Greisen-Zatsepin-Kuzmin cutoff. To explain this anomaly we argue that quantum-gravitational effects may be playing a decisive role in the propagation of ultra high energy cosmic rays. We consider the loop quantum gravity approach and provide useful techniques to establish and analyze constraints on the loop quantum gravity parameters arising from observational data. In particular, we study the effects on the predicted spectrum for ultra high energy cosmic rays and conclude that is possible to reconcile observations."
http://arxiv.org/abs/hep-th/0501129
LIV Dimensional Regularization and Quantum Gravity effects in the Standard Model
Jorge Alfaro
2 pages
"Recently, we have remarked that the main effect of Quantum Gravity(QG) will be to modify the measure of integration of loop integrals in a renormalizable Quantum Field Theory. In the Standard Model this approach leads to definite predictions, depending on only one arbitrary parameter. In particular, we found that the maximal attainable velocity for particles is not the speed of light, but depends on the specific couplings of the particles within the Standard Model. Also birrefringence occurs for charged leptons, but not for gauge bosons. Our predictions could be tested in the next generation of neutrino detectors such as NUBE. In this paper, we elaborate more on this proposal. In particular, we extend the dimensional regularization prescription to include Lorentz invariance violations(LIV) of the measure, preserving gauge invariance. Then we comment on the consistency of our proposal."
these are about testing for QG effects. I just want to keep track because it seems to be an expanding area of research. Alfaro is at a university in Chile and his co-author is at Cambridge.
Two new papers today of possible interest
http://arxiv.org/abs/quant-ph/0501135
The Computational Universe: Quantum gravity from quantum computation
Seth Lloyd
31 pages; 4 figures (gif); submitted to Science
"A theory of quantum gravity based on quantum computation is proposed. In this theory, fundamental processes are described in terms of quantum information processing: the geometry of space-time is a construct, derived from the underlying quantum computation. Explicit mechanisms are provided for the back-reaction of the metric to computational `matter,' black-hole evaporation, holography, and quantum cosmology."
Seth Lloyd is at MIT. the main body of the paper is 13 pages, then come references, appendices, and figures. some of the figures are in GIF format with the postscript version, so in PDF you only get placeholders.
Seth Lloyd has published 83 papers, a lot seem to be Physical Review A
and Physical Review Letters
but also two in Science, one in 2004 and another in 1996.
(preprint here---- http://arxiv.org/abs/quant-ph/9604015)
His specialty seems to be Quantum Computing. It is interesting that he thinks quantum theory of spacetime can be derived from theory of quantum computation
THE OTHER PAPER is one that I already flagged with its own thread, and one that John Baez highlighted in his report from the October 2004 Perimeter conference
http://arxiv.org/abs/hep-th/0501191
Quantum gravity in terms of topological observables
Laurent Freidel, Artem Starodubtsev
"We recast the action principle of four dimensional General Relativity so that it becomes amenable for perturbation theory which doesn't break general covariance. The coupling constant becomes dimensionless (G_{Newton} \Lambda) and extremely small 10^{-120}. We give an expression for the generating functional of perturbation theory. We show that the partition function of quantum General Relativity can be expressed as an expectation value of a certain topologically invariant observable. This sets up a framework in which quantum gravity can be studied perturbatively using the techniques of topological quantum field theory."
http://arxiv.org/abs/quant-ph/0501135
The Computational Universe: Quantum gravity from quantum computation
Seth Lloyd
I had a quick look at this. It doesn't look very enlightening. Where is SetAI when we need him? The idea of building QG with qubits is now well entrenched in certain circles.
Cheers
Kea :smile:
this thread has become a surrogate sticky for Loop (and allied) Quantum Gravity links, so we regularly post useful QG reference links here: new articles, websites and the like, and then every now and then I update the main list. It is getting to be time to do that again.
Today there were a couple of new papers by Etera Livine, solo and with Florian Girelli
http://arxiv.org/gr-qc/0501075
Reconstructing Quantum Geometry from Quantum Information: Spin Networks as Harmonic Oscillators
Florian Girelli, Etera R. Livine
16 pages, 3 figures
"Loop Quantum Gravity defines the quantum states of space geometry as spin networks and describes their evolution in time. We reformulate spin networks in terms of harmonic oscillators and show how the holographic degrees of freedom of the theory are described as matrix models. This allow us to make a link with non-commutative geometry and to look at the issue of the semi-classical limit of LQG from a new angle. This work is thought as part of a bigger project of describing quantum geometry in quantum information terms."
*
http://arxiv.org/gr-qc/0501076
Some Remarks on the Semi-Classical Limit of Quantum Gravity
Etera R. Livine
5 pages, Proceedings of the Second International Workshop DICE2004 (Castello di Piombino, Tuscany) "From Decoherence and Emergent Classicality to Emergent Quantum Mechanics"
"One of the most important issues in quantum gravity is to identify its semi-classical regime. First the issue is to define for we mean by a semi-classical theory of quantum gravity, then we would like to use it to extract physical predictions. Writing an effective theory on a flat background is a way to address this problem and I explain how the non-commutative spacetime of deformed special relativity is the natural arena for such considerations. On the other hand, I discuss how the definition of the semi-classical regime can be formulated in a background independent fashion in terms of quantum information and renormalisation of geometry."
this is one I dont want to lose track of because it may have some pedagogical usefulness
http://arxiv.org/abs/gr-qc/0501082
DISCRETE GRAVITY AS A LOCAL THEORY OF THE POINCARÉ GROUP IN THE FIRST ORDER FORMALISM
It deals in simple terms with the relation of Regge calculus to continuum General Relativity. By a Jesuit named Gabriele Gionti, who belongs to the Vatican Obsersatory Research Group at Seward in Arizona. He has a longstanding interest in quantum gravity and did his thesis in Dynamical Triangulations (the Ambjorn Loll approach but before they made the move to Lorentian (Causal) DT and were still getting pathologies.
G.Gionti, Discrete Approaches Toward the Definition of a Quantum Theory of Gravity, Ph.D. thesis SISSA (1998).
http://arxiv.org/gr-qc/9812080
this is one that Wolram spotted and flagged with its own thread
http://physicsforums.com/showthread.php?p=444123#post444123
http://lanl.arxiv.org/abs/gr-qc/0501053
The Search for Quantum Gravity Signals
G. Amelino-Camelia, C. Lämmerzahl, A. Macias, H. Müller
47 pages, submitted to AIP Conference Proceedings of the 2nd Mexican Meeting on Mathematical and Experimental Physics
"We give an overview of ongoing searches for effects motivated by the study of the quantum-gravity problem. We describe in greater detail approaches which have not been covered in recent "Quantum Gravity Phenomenology'' reviews. In particular, we outline a new framework for describing Lorentz invariance violation in the Maxwell sector. We also discuss the general strategy on the experimental side as well as on the theoretical side for a search for quantum gravity effects. The role of test theories, kinematical and dymamical, in this general context is emphasized. The present status of controlled laboratory experiments is described, and we also summarize some key results obtained on the basis of astrophysical observations.
Lee Smolin replied to Nicolai et al surve