Loop-and-allied QG bibliography

[marcus]

...

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 general 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]

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.

 

[marcus]

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

 

[sol2]

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.


I had mentioned in the topic of the new math thread, 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?

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?

 

[marcus]

...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 general 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! I might add.)

 

[marcus]

links for loop/foam/simplicial QG preprint numbers

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...am+AND+dynamical+triangulation/0/1/0/1992/0/1

Year 1993:
http://arXiv.org/find/nucl-ex,astro...am+AND+dynamical+triangulation/0/1/0/1993/0/1

Year 1994:
http://arXiv.org/find/nucl-ex,astro...am+AND+dynamical+triangulation/0/1/0/1994/0/1

Year 1995:
http://arXiv.org/find/nucl-ex,astro...am+AND+dynamical+triangulation/0/1/0/1995/0/1

Year 1996:
http://arXiv.org/find/nucl-ex,astro...am+AND+dynamical+triangulation/0/1/0/1996/0/1

Year 1997:
http://arXiv.org/find/nucl-ex,astro...am+AND+dynamical+triangulation/0/1/0/1997/0/1

Year 1998:
http://arXiv.org/find/nucl-ex,astro...am+AND+dynamical+triangulation/0/1/0/1998/0/1

Year 1999:
http://arXiv.org/find/nucl-ex,astro...am+AND+dynamical+triangulation/0/1/0/1999/0/1

Year 2000:
http://arXiv.org/find/nucl-ex,astro...am+AND+dynamical+triangulation/0/1/0/2000/0/1

Year 2001:
http://arXiv.org/find/nucl-ex,astro...am+AND+dynamical+triangulation/0/1/0/2001/0/1

Year 2002:
http://arXiv.org/find/nucl-ex,astro...am+AND+dynamical+triangulation/0/1/0/2002/0/1

Year 2003:
http://arXiv.org/find/nucl-ex,astro...am+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,...am+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,...D+spin+foam+AND+doubly+special/0/1/0/2001/0/1

2002:
http://arXiv.org/find/nucl-ex,astro...D+spin+foam+AND+doubly+special/0/1/0/2002/0/1

2003:
http://arXiv.org/find/nucl-ex,astro...D+spin+foam+AND+doubly+special/0/1/0/2003/0/1


Last Twelve Months:
http://arXiv.org/find/nucl-ex,astro...D+spin+foam+AND+doubly+special/0/1/0/past/0/1
------------------

http://arxiv.org/find/nucl-ex,astro...am+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

 

[marcus]

refreshing the link-basket

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.

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.

 

[marcus]

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

 

[meteor]

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

 

[marcus]

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.

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

 

[meteor]

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
https://www.physicsforums.com/showthread.php?t=39780&highlight=alexandrov

 

[marcus]

BTW, Alexandrov has posted a reply to Lubos Motl
https://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]

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.

 

[marcus]

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)

 

[marcus]

oh, I see better now. You mean 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

 

[jeff]

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.

 

[sol2]

I guess we can make allowances for some personalites, hey Jeff? We make them for you?

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.

 

[meteor]

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
https://www.physicsforums.com/showthread.php?p=300078#post300078

 

[marcus]

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

 

[sol2]

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:

 

[meteor]

http://arxiv.org/abs/gr-qc/0409006
"Semiclassical Quantum Gravity: Statistics of Combinatorial Riemannian Geometries"
Or "How to use statistical geometry to quantify uncertainties"

 

[meteor]

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?

 

[marcus]

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

 

[meteor]

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

 

[marcus]

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 ([H_R, H_R'] 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 H_R 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]

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.

 

[marcus]

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]

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.

 

[marcus]

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."

 

[marcus]

wolram found this
http://jdc.math.uwo.ca/spin-foams/

it is a great QG resource

 

[marcus]

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]

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

 

[marcus]

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

 

[marcus]

Quantum Gravity cataloging at Stanford SLAC/DESY Library

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%2B+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.

 

[marcus]

Best online article about Loop Gravity I've seen so far

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 for educational use.

http://cgpg.gravity.psu.edu/people/Ashtekar/articles/rovelli03.pdf

 

[marcus]

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,...D+spin+foam+AND+doubly+special/0/1/0/2001/0/1
2002:
http://arXiv.org/find/nucl-ex,astro...D+spin+foam+AND+doubly+special/0/1/0/2002/0/1
2003:
http://arXiv.org/find/nucl-ex,astro...D+spin+foam+AND+doubly+special/0/1/0/2003/0/1
Last Twelve Months:
http://arXiv.org/find/nucl-ex,astro...D+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

 

[marcus]

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 doesn't depend on using a particular formalism.

 

[marcus]

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]

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]

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]

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.

 

[Mike2]

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]

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.

 

[marcus]

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

 

[Mike2]

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.

 

[marcus]

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?

 

[marcus]

flagging the Causal Sites paper

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 don't 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]

 

[Mike2]

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 don't 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?

 

[marcus]

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

 

[marcus]

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