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marcus

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a sample---picking names more or less randomly to see what that person is going to talk about:

Speaker: Prof. John Baez

Title: New Progress in Spin Foam Models

Abstract: While the challenge of finding spin foam models with a well-behaved large-scale limit remains open, there has recently been interesting progress in a number of other directions. These include: relating spin foam models to the Feynman diagram formulation of ordinary quantum field theory on flat spacetime, finding a natural home for point particles in the spin foam formulation of 3d quantum gravity, and exploring a new background-free perturbative formulation of 4d gravity based on the BF formulation of the MacDowell-Mansouri theory. We hope to show that the ideas involved are beautiful and simple.

Speaker: Prof. Robbert Dijkgraaf

Title: Quantum geometry and topological strings

[my comment: "quantum geometry" is Ashtekar's name for LQG, he might be relating LQG and topol. string. Dijkgraaf already has a paper that hints at some connection to LQG---one thing is a "topological sector" of another thing. or he might mean something else by "quantum geometry", like a Loll kind of thing, no way to tell in advance]

Speaker: Dr. Laurent Freidel

Title: Effective Field theory from quantum gravity

Abstract: The Coupling of matter fields to spin foam models of quantum gravity will be discussed. We will show in the case of three dimensional gravity how the integration of quantum gravity degrees of freedom coupled to matter can be explicitely described in terms of an effective field theory. This theory is a new non commutative field theory obeying the principle of doubly special relativity. We will conclude on the extension of this approach to the four dimensional case.

Speaker: Prof. Martin Reuter

Title: Asymptotically safe quantum gravity: from the UV to the IR

Abstract: The construction of a microscopic theory of quantum gravity in the asymptotic safety scenario is reviewed, and the nonperturbative renormalization group flow of Quantum Einstein Gravity from the UV fixed point towards macroscopic scales is described. Furthermore, the emergence of a**fractal spacetime structure** will be discussed.

Speaker: Renate L:!)ll

Title: Emergence of Quantum Spacetime from Causal Dynamical Triangulations

Abstract: In this talk, I will summarize and discuss some recent results on the emergent macro- and microstructure of quantum spacetime obtained in the Causal Dynamical Triangulations approach to nonperturbative quantum gravity. Particularly compelling is the finding of a dynamically generated**scale-dependent dimension** of this spacetime, interpolating between the classical value four on large scales and two at short distances.

Speaker: Prof. John Baez

Title: New Progress in Spin Foam Models

Abstract: While the challenge of finding spin foam models with a well-behaved large-scale limit remains open, there has recently been interesting progress in a number of other directions. These include: relating spin foam models to the Feynman diagram formulation of ordinary quantum field theory on flat spacetime, finding a natural home for point particles in the spin foam formulation of 3d quantum gravity, and exploring a new background-free perturbative formulation of 4d gravity based on the BF formulation of the MacDowell-Mansouri theory. We hope to show that the ideas involved are beautiful and simple.

Speaker: Prof. Robbert Dijkgraaf

Title: Quantum geometry and topological strings

[my comment: "quantum geometry" is Ashtekar's name for LQG, he might be relating LQG and topol. string. Dijkgraaf already has a paper that hints at some connection to LQG---one thing is a "topological sector" of another thing. or he might mean something else by "quantum geometry", like a Loll kind of thing, no way to tell in advance]

Speaker: Dr. Laurent Freidel

Title: Effective Field theory from quantum gravity

Abstract: The Coupling of matter fields to spin foam models of quantum gravity will be discussed. We will show in the case of three dimensional gravity how the integration of quantum gravity degrees of freedom coupled to matter can be explicitely described in terms of an effective field theory. This theory is a new non commutative field theory obeying the principle of doubly special relativity. We will conclude on the extension of this approach to the four dimensional case.

Speaker: Prof. Martin Reuter

Title: Asymptotically safe quantum gravity: from the UV to the IR

Abstract: The construction of a microscopic theory of quantum gravity in the asymptotic safety scenario is reviewed, and the nonperturbative renormalization group flow of Quantum Einstein Gravity from the UV fixed point towards macroscopic scales is described. Furthermore, the emergence of a

Speaker: Renate L:!)ll

Title: Emergence of Quantum Spacetime from Causal Dynamical Triangulations

Abstract: In this talk, I will summarize and discuss some recent results on the emergent macro- and microstructure of quantum spacetime obtained in the Causal Dynamical Triangulations approach to nonperturbative quantum gravity. Particularly compelling is the finding of a dynamically generated

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marcus

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more sampling (I bolded something in rovelli's, for emphasis)

Speaker: Carlo Rovelli

Title: GENERAL RELATIVISTIC QUANTUM PHYSICS: Background independent scattering amplitudes, boundary formalism, local particles and partial observables

Abstract: (i) I present some preliminary results on background independent calculations of**particle scattering amplitudes**. In particular, I discuss the derivation of the graviton propagator, from loop quantum gravity and the spinfoam formalism. (ii) I illustrate the boundary formulation of quantum field theory, its role in a background independent context, and how "particles" emerge in this language. (iii) More in general, I discuss how systematic physical predictions can be extracted from a general relativistic quantum field theory: I illustrate the notion of "partial observable", and discuss the issue of the physical significance of the spectrum of these observables, which controls the interpretation of the area and volume discreteness.

Speaker: Prof. Lee Smolin

Title: Persistent puzzles in background independent approaches to quantum gravity

Abstract: The talk is intended to be a provacative review of what we have NOT accomplished so far in LQG, CDT and other background independent approches to quantum gravity. I will describe then work with collaborators motivated by new hypotheses about some of these persistent problems.

[COMMENT: I think by new hypotheses he means relatismo as laid out in "The Case for...", there is no denying that it does address some of the most persistent problems]

Speaker: Prof. Abhay Ashtekar

Title: Quantum Geometry and Space-time Singularities

Abstract: General relativity provides a subtle and powerful interplay between gravity and geometry, thereby opening numerous possibilities for novel phenomena. However, this interplay also implies that the space-time itself ends when the gravitational field becomes singular. In loop quantum gravity, the interplay is elevated to the quantum regime through quantum geometry. I will present examples which strongly suggest that the* physical* space-time **does not end at singularities**. Quantum geometry can serve as a bridge between vast space-time regions which are classically unrelated. Thus, contrary to one's initial intuition, ramifications of quantum geometry can reach far beyond the Planck regime.

[COMMENT: he could be talking about black hole gravitational collapse extending on into a kind of bounce----serving as a "bridge" to some other tract of spacetime. or he could have something else in mind. likes surprise]

Speaker: Prof. Roy Maartens

Title: COSMOLOGY AND QUANTUM GRAVITY

Abstract: I will review the basic features of the standard, classical model of cosmology, which is based on General Relativity, and how this model accounts for observed properties of the universe. Modifications to General Relativity that are inspired by quantum gravity need to be tested against cosmological observations. This is one of the key tests for any candidate quantum gravity theory. I will discuss in general terms some of the difficulties involved in this aim, and what is needed from theorists in order to achieve this aim. In particular, I will**compare some of the features of stringy cosmology and loop quantum cosmology**.

[COMMENT: selfAdjoint corrected a stupid mistake I made here.]

Speaker: Dr. Hanno Sahlmann

Title: On the measure theory underlying LQG

Abstract: We review recent results on**measures on spaces of connections** in connection with LQG and discuss new questions and work in progress in this area, interesting both for LQG and from the viewpoint of standard quantum field theory.

[COMMENT: measure theory is the bedrock of analysis. measures on spaces of functions and other geometrical objects. hardhat area]

Speaker: Bianca Dittrich

Title: Partial and Complete Observables for Canonical General Relativity

Abstract: We will apply the concepts of partial and complete observables to canonical general relativity. We will show, that by using these concepts, one can calculate Dirac observables for general relativity by dealing with just one constraint. Furthermore we will introduce Abelian Hamiltonian constraints, invariant under spatial diffeomorphisms.

[this is her good paper of the same title]

Speaker: Dr. Etera Livine

Title: Black Hole Entropy, Entanglement, and Area Renormalisation in LQG

Abstract: One can model a LQG black hole at the kinematical level as a space region bounded by a closed surface -the horizon- whose geometry an external observer can acquire information about only through measurements on the surface. The black hole state is then given by completely coarse-graining the geometry of the bounded region, which defines a density matrix on the space of intertwiners i.e. the quantum state of the boundary horizon. I'll explain how to derive the black hole entropy law in that context and how to explain the logarithmic correction in terms of entanglement/correlation between parts of the horizon. This will lead to introduce a concept of surface renormalisationin LQG.

[Livine and Terno, recent paper]

Speaker: Prof. Dan Christensen

Title: Finiteness and Positivity for the Lorentzian partition function

Abstract: This talk gives an overview of recent results on the Lorentzian Barrett-Crane model, a spin foam model of quantum gravity. I will begin with a concise proof of the finiteness of the Lorentzian 10j symbol, which generalizes to other integral expressions of the same form, such as causal models. I then describe work of my student Wade Cherrington which uses the generalized finiteness result above to show that the Lorentzian partition function for a fixed triangulation is finite in the Perez-Rovelli normalization. Finally I will outline joint work with Cherrington which shows that the Lorentzian partition function is non-negative. In fact, we show that the Lorentzian Barrett-Crane model can be reformulated as a model with non-negative amplitudes, and this leads to the possibility of efficient computations of expectation values using statistical methods.

[John Baez should be happy about this. there was worry about the Barrett-Crane model starting around 2002, which perhaps is being cleared up. Christensen does computer studies of spinfoam models so when he says "efficient computations" it is serious-----"statistical methods" could mean Monte Carlo. I think they have a good computer setup at Western Ontario where he is, or they use some other Canadian supercomputer installation---so this could be progress towards doing some of the same stuff Loll does, but with Foams instead of CDT. Cherrington recently posted a paper. quite a bit of this may be already available online]

Speaker: Dr. Josh Willis

Title: Computational Building Blocks for Lorentzian Spin Foams

Abstract: I discuss recent work on efficiently computing the basic functions of Lorenztian spin foam models, from which the amplitudes themselves are calculated. The focus is on computing the 6-J and 10-J symbols, and I show how starting from the expression for these functions as integrals over copies of the group SL(2,C), one can develop an efficient algorithm for the 6J symbol, which can then be applied in calculating the 10J symbol.

[Willis is also at Western Ontario, working with Christensen. Having the three researchers Christensen, Willis, Cherrington suggests that UWO is strong in the computational end of spinfoam research. The Loll example with CDT points up the critical importance of computer spacetime simulations and machine computation in quantum gravity.]

Speaker: Rafael Porto

Title: Fundamental gravitational limitations to quantum computing

Abstract: Lloyd has recently considered the ultimate limitations the fundamental laws of physics place on Quantum Computers (QC). Based on ordinary Quantum Mechanics, he concludes in particular that for an 'ultimate laptop' (a computer of one liter of volume and one kilogram of mass) the maximum number of operations per second is bounded by 10^51. Here we consider additional limits that are placed by quantum gravity ideas, namely the use of a relational notion of time and fundamental uncertainty relations that exists on time measurements. We then particularize fot the case of an ultimate laptop and show that the maximum number of operations is further constrained to 10^47 per second. In addition it is shown that black holes QC face similar limitations.

[Rafael Porto sometimes comes to PF and has posted here on some of our threads! Great going Rafael. I read this paper, that you are delivering! If you have time tell us something about the conference.]

Speaker: Artem Starodubtsev

Title: Scattering problem in background independent quantum gravity

Abstract: Four dimensional topological field theory allows to define matter particles as charges of the gauge group breaking the gauge symmetry at a point. This is analogous to Deser Jackew and t'Hooft particles in 3d gravity. The degrees of freedom of General Relativity can also be introduced by symmetry breaking of the same topological field theory. This provides a framework in which gravity and matter can be considered from a unified point of view. In this framework we study scattering of matter particles due to quantum gravity effects.

[OH MY! This sounds very interesting.]

Well that is a sample. I have to stop sampling the talks. Many interesting ones left out. Other people if you look at the program and find something that piques your interest please post it.

Speaker: Carlo Rovelli

Title: GENERAL RELATIVISTIC QUANTUM PHYSICS: Background independent scattering amplitudes, boundary formalism, local particles and partial observables

Abstract: (i) I present some preliminary results on background independent calculations of

Speaker: Prof. Lee Smolin

Title: Persistent puzzles in background independent approaches to quantum gravity

Abstract: The talk is intended to be a provacative review of what we have NOT accomplished so far in LQG, CDT and other background independent approches to quantum gravity. I will describe then work with collaborators motivated by new hypotheses about some of these persistent problems.

[COMMENT: I think by new hypotheses he means relatismo as laid out in "The Case for...", there is no denying that it does address some of the most persistent problems]

Speaker: Prof. Abhay Ashtekar

Title: Quantum Geometry and Space-time Singularities

Abstract: General relativity provides a subtle and powerful interplay between gravity and geometry, thereby opening numerous possibilities for novel phenomena. However, this interplay also implies that the space-time itself ends when the gravitational field becomes singular. In loop quantum gravity, the interplay is elevated to the quantum regime through quantum geometry. I will present examples which strongly suggest that the

[COMMENT: he could be talking about black hole gravitational collapse extending on into a kind of bounce----serving as a "bridge" to some other tract of spacetime. or he could have something else in mind. likes surprise]

Speaker: Prof. Roy Maartens

Title: COSMOLOGY AND QUANTUM GRAVITY

Abstract: I will review the basic features of the standard, classical model of cosmology, which is based on General Relativity, and how this model accounts for observed properties of the universe. Modifications to General Relativity that are inspired by quantum gravity need to be tested against cosmological observations. This is one of the key tests for any candidate quantum gravity theory. I will discuss in general terms some of the difficulties involved in this aim, and what is needed from theorists in order to achieve this aim. In particular, I will

[COMMENT: selfAdjoint corrected a stupid mistake I made here.]

Speaker: Dr. Hanno Sahlmann

Title: On the measure theory underlying LQG

Abstract: We review recent results on

[COMMENT: measure theory is the bedrock of analysis. measures on spaces of functions and other geometrical objects. hardhat area]

Speaker: Bianca Dittrich

Title: Partial and Complete Observables for Canonical General Relativity

Abstract: We will apply the concepts of partial and complete observables to canonical general relativity. We will show, that by using these concepts, one can calculate Dirac observables for general relativity by dealing with just one constraint. Furthermore we will introduce Abelian Hamiltonian constraints, invariant under spatial diffeomorphisms.

[this is her good paper of the same title]

Speaker: Dr. Etera Livine

Title: Black Hole Entropy, Entanglement, and Area Renormalisation in LQG

Abstract: One can model a LQG black hole at the kinematical level as a space region bounded by a closed surface -the horizon- whose geometry an external observer can acquire information about only through measurements on the surface. The black hole state is then given by completely coarse-graining the geometry of the bounded region, which defines a density matrix on the space of intertwiners i.e. the quantum state of the boundary horizon. I'll explain how to derive the black hole entropy law in that context and how to explain the logarithmic correction in terms of entanglement/correlation between parts of the horizon. This will lead to introduce a concept of surface renormalisationin LQG.

[Livine and Terno, recent paper]

Speaker: Prof. Dan Christensen

Title: Finiteness and Positivity for the Lorentzian partition function

Abstract: This talk gives an overview of recent results on the Lorentzian Barrett-Crane model, a spin foam model of quantum gravity. I will begin with a concise proof of the finiteness of the Lorentzian 10j symbol, which generalizes to other integral expressions of the same form, such as causal models. I then describe work of my student Wade Cherrington which uses the generalized finiteness result above to show that the Lorentzian partition function for a fixed triangulation is finite in the Perez-Rovelli normalization. Finally I will outline joint work with Cherrington which shows that the Lorentzian partition function is non-negative. In fact, we show that the Lorentzian Barrett-Crane model can be reformulated as a model with non-negative amplitudes, and this leads to the possibility of efficient computations of expectation values using statistical methods.

[John Baez should be happy about this. there was worry about the Barrett-Crane model starting around 2002, which perhaps is being cleared up. Christensen does computer studies of spinfoam models so when he says "efficient computations" it is serious-----"statistical methods" could mean Monte Carlo. I think they have a good computer setup at Western Ontario where he is, or they use some other Canadian supercomputer installation---so this could be progress towards doing some of the same stuff Loll does, but with Foams instead of CDT. Cherrington recently posted a paper. quite a bit of this may be already available online]

Speaker: Dr. Josh Willis

Title: Computational Building Blocks for Lorentzian Spin Foams

Abstract: I discuss recent work on efficiently computing the basic functions of Lorenztian spin foam models, from which the amplitudes themselves are calculated. The focus is on computing the 6-J and 10-J symbols, and I show how starting from the expression for these functions as integrals over copies of the group SL(2,C), one can develop an efficient algorithm for the 6J symbol, which can then be applied in calculating the 10J symbol.

[Willis is also at Western Ontario, working with Christensen. Having the three researchers Christensen, Willis, Cherrington suggests that UWO is strong in the computational end of spinfoam research. The Loll example with CDT points up the critical importance of computer spacetime simulations and machine computation in quantum gravity.]

Speaker: Rafael Porto

Title: Fundamental gravitational limitations to quantum computing

Abstract: Lloyd has recently considered the ultimate limitations the fundamental laws of physics place on Quantum Computers (QC). Based on ordinary Quantum Mechanics, he concludes in particular that for an 'ultimate laptop' (a computer of one liter of volume and one kilogram of mass) the maximum number of operations per second is bounded by 10^51. Here we consider additional limits that are placed by quantum gravity ideas, namely the use of a relational notion of time and fundamental uncertainty relations that exists on time measurements. We then particularize fot the case of an ultimate laptop and show that the maximum number of operations is further constrained to 10^47 per second. In addition it is shown that black holes QC face similar limitations.

[Rafael Porto sometimes comes to PF and has posted here on some of our threads! Great going Rafael. I read this paper, that you are delivering! If you have time tell us something about the conference.]

Speaker: Artem Starodubtsev

Title: Scattering problem in background independent quantum gravity

Abstract: Four dimensional topological field theory allows to define matter particles as charges of the gauge group breaking the gauge symmetry at a point. This is analogous to Deser Jackew and t'Hooft particles in 3d gravity. The degrees of freedom of General Relativity can also be introduced by symmetry breaking of the same topological field theory. This provides a framework in which gravity and matter can be considered from a unified point of view. In this framework we study scattering of matter particles due to quantum gravity effects.

[OH MY! This sounds very interesting.]

Well that is a sample. I have to stop sampling the talks. Many interesting ones left out. Other people if you look at the program and find something that piques your interest please post it.

Last edited:

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selfAdjoint

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Marcus said:[MY COMMENT: this is "Julia sets" Julia. I didn't know to expect him to show up at a Loops conference

Maybe we mean something different by Julia Sets? I mean the well-known classical fractal sets of that name; these were devised by Prof. Raoul Julia who fought in WWI and I don't suppose is around anymore.

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marcus

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to me it comes across as a galant gesture of hospitality that the main LQG researchers at Albert Einstein Institute (one could call them "senior" except that both are young) are not scheduling their own talks!

Thomas Thiemann and Martin Bojowald have each originated major lines of LQG investigation and lead considerable amounts of research in their respective areas---but have chosen to play the host

and NOT to put themselves on the programme WHICH THEY ORGANIZED AT THEIR AEI HOMEBASE.

maybe that will change, if some scheduled speaker can't make it and they have to fill an empty spot

but right now it looks very impressive

one thing it means is that nowadays you can put on a Loops conference and voluntarily exclude a couple of the major players and you still have a good show---there is a depth and breadth of resources

have to congratulate Drs. Bojowald and Thiemann for setting up such a program (it really looks very good, very exciting) and also thank Hermann Nicolai for deciding to have the conference at AEI!

Where will Loops 06 take place?

Thomas Thiemann and Martin Bojowald have each originated major lines of LQG investigation and lead considerable amounts of research in their respective areas---but have chosen to play the host

and NOT to put themselves on the programme WHICH THEY ORGANIZED AT THEIR AEI HOMEBASE.

maybe that will change, if some scheduled speaker can't make it and they have to fill an empty spot

but right now it looks very impressive

one thing it means is that nowadays you can put on a Loops conference and voluntarily exclude a couple of the major players and you still have a good show---there is a depth and breadth of resources

have to congratulate Drs. Bojowald and Thiemann for setting up such a program (it really looks very good, very exciting) and also thank Hermann Nicolai for deciding to have the conference at AEI!

Where will Loops 06 take place?

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marcus

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selfAdjoint said:Maybe we mean something different by Julia Sets? I mean the well-known classical fractal sets of that name; these were devised by Prof. Raoul Julia who fought in WWI and I don't suppose is around anymore.

thanks, mistake corrected

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This might sound like a silly question, but...

Is the picture on their homepage a rendering of CDT?

Is the picture on their homepage a rendering of CDT?

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marcus

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Berislav said:This might sound like a silly question, but...

Is the picture on their homepage a rendering of CDT?

it definitely is not a silly question

the "icon" chosen for a line of research can be a helpful handle on it

I think that the picture is actually a unification of two or more theories where in the FOREGROUND near the viewer I see the triangles of CDT

and FURTHER AWAY more in the background, I see loops emerging, or perhaps they are nonperturbative strings and branes, as some largerscale structures, already kind of foggy or blurred, emerge out of the primitive small scale triangles.

at least one of the pictures has this suggestive foreground-blending-into-background thing

http://loops05.aei.mpg.de/

If I were to meet hermann nicolai, I think I would ask him did he choose the artist or the pictures for this AEI website?

I think he is a clever far-seeing man and although his specialty is string, I think he WANTS THE BEST for all approaches to quantum gravity and he believes that they can work in a synergistic way to test each other and so on. a man like Nicolai could have decided on that picture.

(if anyone doesn't know: nicolai directs the Unified theories and Quantum gravity department of AEI-Potsdam where they are hosting the conference)

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marcus said:it definitely is not a silly question

the "icon" chosen for a line of research can be a helpful handle on it

I think that the picture is actually a unification of two or more theories where in the FOREGROUND near the viewer I see the triangles of CDT

and FURTHER AWAY more in the background, I see loops emerging, or perhaps they are nonperturbative strings and branes, as some largerscale structures, already kind of foggy or blurred, emerge out of the primitive small scale triangles.

at least one of the pictures has this suggestive foreground-blending-into-background thing

http://loops05.aei.mpg.de/

If I were to meet hermann nicolai, I think I would ask him did he choose the artist or the pictures for this AEI website?

I think he is a clever far-seeing man and although his specialty is string, I think he WANTS THE BEST for all approaches to quantum gravity and he believes that they can work in a synergistic way to test each other and so on. a man like Nicolai could have decided on that picture.

(if anyone doesn't know: nicolai directs the Unified theories and Quantum gravity department of AEI-Potsdam where they are hosting the conference)

One has to observe the interchanging three "logos" at the top left hand corner sidebar?..the one which emerges with a

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marcus

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whatever anyone sees or hears that in some way modifies the picture you get of current state of QG that is spelled out in the programme with its abstracts.

Another thing we can do is connect the dots so that someone who is not going to the conference can READ recent articles posted on arxiv that are likely to correspond somewhat to the talks, in some cases.

I can give some arxiv links, maybe other people can supply a few as well.

so we can have a Threepenny Opera version of the conference. an ersatz substitute for stay-at-homes, which with a little imagination might not be so bad.

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marcus

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the most illuminating thing I have seen since Friday when the program came out (bearing on the current QG picture) is this comment on Woit blog by JB

http://www.math.columbia.edu/~woit/wordpress/?p=251#comments

this post is #53 in the comments.

I can't easily copy the links so I put them as footnote at the end

---quote---

John Baez Says:

September 10th, 2005 at 10:19 pm

dan writes:

>Since Loll’s causal dynamic triangulation appears to have a

>well-behaved semi-classical limit, with non-trivial predictions

>on the Planck scale, shouldn’t that excite you to doing research

>in QG?

It does excite me; I think it’s one of the most exciting things to come along in quantum gravity during the last few years! Everyone should read this for a less technical description of what Ambjorn, Jurkiewicz and Loll have done - or these for more detail. I talked about this stuff in the issue of This Week’s Finds covering the 2004 Marseille conference on loops and spin foams, so you can also read that.

Unfortunately the most important work being done by these authors isn’t the sort of thing I’m good at. It involves lots of computer calculations. I’ve tried to get some computer whizzes interested, but so far nothing has come of it. So, I expect I’ll watch from the sidelines for a while.

There’s one place I can *imagine* helping out. Their theory makes crucial use of a time coordinate. This should wash out when they take the continuum limit, but it might not - in which case they would be studying not quantum gravity, but some other theory in a different “universality class”.

One can investigate this issue numerically. But it would be nice to find a variant of their model which did not make use of a chosen time coordinate, to simply sidestep this issue.

That’s the sort of thing I can *imagine* being able to do… but I haven’t actually been able yet. Since I’m making so much more progress on various kinds of math, I’ve been doing more of that.

---end quote---

"this" is The Universe from Scratch http://arxiv.org/abs/hep-th/0509010

"these" is Loll's stuff on the arxiv http://arxiv.org/find/gr-qc/1/au:+loll/0/1/0/all/0/1?skip=0&query_id=e9777db1cb585b3b

"that" is JB http://math.ucr.edu/home/baez/week206.html

---What this says to me---

first of all the picture for me is focused on 4 main areas----(apologies to Fay Dowker, Alain Connes, Raphael Sorkin, Steven Hawking, Gerard 't Hooft...and everybody else who has their own very interesting QG ideas)---it's focused on LQG, SpinFoam, CDT, and Reuter's QEG.

For me, THE OVERWHELMING FEATURE of the last two (CDT QEG) is that they are based on taking a limit. it is like the derivative is not defined in Calculus, you never actually get your hands on it, it only exists as a limit of ratios of differences. you never see it, all you see is an infinite SEQUENCE of approximations leading up to it.

Another thing about CDT QEG , that may or may not be related to this, is that they NOW SEEM NOT AMENABLE TO CATEGORIES and other high algebra.

I do not see anyone doing Category or Group Field with Loll triangles. So from a certain perspective it they are HARD TO MAKE SIMPLE AND BEAUTIFUL.

I hear this. And this for me is very very interesting, because the possibility of new mathematics is above all when people come across some ideas that are hard to make simple and beautiful. And then after a hard uncomfortable experience they see a creative way to do it, and that is new---it doesn't just use already existing tools and ideas.

So I like this situation and like to watch it develop. Hopefully it is not out of place for me to say this and I am not speaking out of turn. But in any case that is what I see as an onlooker and it is very interesting indeed right now.

I am waiting for SOMETHING LIKE A CATEGORY or a group field or some other beautiful elegant gadget to be applied to Loll Triangle gravity and I am almost certain that it will be something nobody has seen before.

There's no question Loll CDT is NOT MERE COMPUTER SIMULATION. I don't think anyone suggested that, so that is not exactly the issue. In fact the 2003 topology change papers with Westra for example are not computerish IIRC and have some rather neat combinatorics (what is it, Catalan numbers? and that's not all). It seems to me that the obstacle is not simply that computer simulations are involved------I would expect that in SpinFoam there will be more and more computer simulations as time goes on too. It seems to me that there is something deeper by way of an obstacle and it is that the model, as it stands, is not amenable to categories and algebra as they now are.

But the first two QG I mentioned, LQG and especially SpinFoam possibly ARE amenable to making simple and beautiful by Category and Algebra methods which exist or are within sight on the horizon. I don't know about this but JB says something about a hope of this kind in the abstract of his Loops 05 talk. And he indicates a friendly disagreement with Carlo Rovelli over whether "Group Field" method or Category method might work best so simplify and beautify SpinFoams.

http://www.math.columbia.edu/~woit/wordpress/?p=251#comments

this post is #53 in the comments.

I can't easily copy the links so I put them as footnote at the end

---quote---

John Baez Says:

September 10th, 2005 at 10:19 pm

dan writes:

>Since Loll’s causal dynamic triangulation appears to have a

>well-behaved semi-classical limit, with non-trivial predictions

>on the Planck scale, shouldn’t that excite you to doing research

>in QG?

It does excite me; I think it’s one of the most exciting things to come along in quantum gravity during the last few years! Everyone should read this for a less technical description of what Ambjorn, Jurkiewicz and Loll have done - or these for more detail. I talked about this stuff in the issue of This Week’s Finds covering the 2004 Marseille conference on loops and spin foams, so you can also read that.

Unfortunately the most important work being done by these authors isn’t the sort of thing I’m good at. It involves lots of computer calculations. I’ve tried to get some computer whizzes interested, but so far nothing has come of it. So, I expect I’ll watch from the sidelines for a while.

There’s one place I can *imagine* helping out. Their theory makes crucial use of a time coordinate. This should wash out when they take the continuum limit, but it might not - in which case they would be studying not quantum gravity, but some other theory in a different “universality class”.

One can investigate this issue numerically. But it would be nice to find a variant of their model which did not make use of a chosen time coordinate, to simply sidestep this issue.

That’s the sort of thing I can *imagine* being able to do… but I haven’t actually been able yet. Since I’m making so much more progress on various kinds of math, I’ve been doing more of that.

---end quote---

"this" is The Universe from Scratch http://arxiv.org/abs/hep-th/0509010

"these" is Loll's stuff on the arxiv http://arxiv.org/find/gr-qc/1/au:+loll/0/1/0/all/0/1?skip=0&query_id=e9777db1cb585b3b

"that" is JB http://math.ucr.edu/home/baez/week206.html

---What this says to me---

first of all the picture for me is focused on 4 main areas----(apologies to Fay Dowker, Alain Connes, Raphael Sorkin, Steven Hawking, Gerard 't Hooft...and everybody else who has their own very interesting QG ideas)---it's focused on LQG, SpinFoam, CDT, and Reuter's QEG.

For me, THE OVERWHELMING FEATURE of the last two (CDT QEG) is that they are based on taking a limit. it is like the derivative is not defined in Calculus, you never actually get your hands on it, it only exists as a limit of ratios of differences. you never see it, all you see is an infinite SEQUENCE of approximations leading up to it.

Another thing about CDT QEG , that may or may not be related to this, is that they NOW SEEM NOT AMENABLE TO CATEGORIES and other high algebra.

I do not see anyone doing Category or Group Field with Loll triangles. So from a certain perspective it they are HARD TO MAKE SIMPLE AND BEAUTIFUL.

I hear this. And this for me is very very interesting, because the possibility of new mathematics is above all when people come across some ideas that are hard to make simple and beautiful. And then after a hard uncomfortable experience they see a creative way to do it, and that is new---it doesn't just use already existing tools and ideas.

So I like this situation and like to watch it develop. Hopefully it is not out of place for me to say this and I am not speaking out of turn. But in any case that is what I see as an onlooker and it is very interesting indeed right now.

I am waiting for SOMETHING LIKE A CATEGORY or a group field or some other beautiful elegant gadget to be applied to Loll Triangle gravity and I am almost certain that it will be something nobody has seen before.

There's no question Loll CDT is NOT MERE COMPUTER SIMULATION. I don't think anyone suggested that, so that is not exactly the issue. In fact the 2003 topology change papers with Westra for example are not computerish IIRC and have some rather neat combinatorics (what is it, Catalan numbers? and that's not all). It seems to me that the obstacle is not simply that computer simulations are involved------I would expect that in SpinFoam there will be more and more computer simulations as time goes on too. It seems to me that there is something deeper by way of an obstacle and it is that the model, as it stands, is not amenable to categories and algebra as they now are.

But the first two QG I mentioned, LQG and especially SpinFoam possibly ARE amenable to making simple and beautiful by Category and Algebra methods which exist or are within sight on the horizon. I don't know about this but JB says something about a hope of this kind in the abstract of his Loops 05 talk. And he indicates a friendly disagreement with Carlo Rovelli over whether "Group Field" method or Category method might work best so simplify and beautify SpinFoams.

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- #12

marcus

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marcus said:the most illuminating thing I have seen since Friday when the program came out (bearing on the current QG picture) is this comment on Woit blog by JB

http://www.math.columbia.edu/~woit/wordpress/?p=251#comments

this post is #53 in the comments...

there is another thing that changes how I see the Loops05 programme, or shed some light on it for me and that is a passage from an earlier JB post on the same thread. it is hard to understand but points to something

---quote JB #29 in same thread---

John Baez Says:

September 5th, 2005 at 11:53 pm

... I have two grad students working on spin foam models: Derek Wise and Jeffrey Morton...

Among other things, we’re working on

particles as objects

spin networks going between particles as morphisms

spin foams going between spin networks as 2-morphisms

and this fact, when worked out in detail, gives a

But, to answer your question, what could get me working harder on quantum gravity is some evidence that we can find a mathematically elegant background-free quantum theory that can reduce to general relativity in a suitable limit.

This is precisely why I mentioned Carlo Rovelli’s new paper...

---end quote---

as a QG watcher I get several things:

1. look out for Jeffrey Morton and Derek Wise in future, I recall that Wise is scheduled to give a late-afternoon talk at Loops05

2. look out for Freidel and others incorporating matter into spinfoam spacetime, I guess by breaking the topology up a little, "loose ends".

Freidel talks about this in the abstract of his Loops05 talk and indicates he will discuss EXTENDING IT FROM 3D TO 4D, he has a lot of recent papers dealing with the 3D case which you could say is for practice.

3. look out for JB and others ideas of how to make this elegant---simpler and more beautiful---using category or other methods.

4. "such a thing might be found" if one does not require elegance might contain an acknowledgment of promise in the direction of more socall-grubby analytical methods of CDT and QEG, but it also may refer to Rovelli work that is then discussed.

right now I think that if I want a good picture of current state of non-string QG-----or one can say "nonperturbative/backgroundindependent" QG as the organizers of Loop05 say----that one way to make a good snapshot of the current situation is to look at the online programme abstracts, like those I copied earlier in this thread, and then TWEAK the picture slightly by thinking about this quoted passage from Baez.

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The preceding post has a significant quote from John Baez

elegance is an interesting standard. perhaps not entirely straightforward either, I think new and noble mathematics might sometimes sprout from a mere pragmatic contrivance---something cobbled together just to get it to work. I think the awkward could sometimes harbor the seeds of elegance. Maybe one just needs to understand the inelegant thing better.

Some famous mathematicians are apparently not shy about visiting a dusty attic or the junkyard---I learned from Arivero posts ("graviton" thread) that Alain Connes has been concerning himself with the infinitesimals of Calculus, with limits of sequences of numbers, with the Nonstandard Analysis of Abraham Robinson (goes back to the 1960s, didn't catch on much IIRC).

Maybe it is not appropriate to this context but I want to mention Rodolfo Gambini. here is what he is going to talk about at Loops conference:

http://loops05.aei.mpg.de/index_files/abstract_gambini.html

Prof. Rodolfo Gambini

Consistent discretizations of totally constrained systems

"I describe the consistent discretization scheme for the quantization of totally constrained systems and quantum gravity. As a first example I analyze the canonical formalism for Regge Calculus. I complete all the steps of the program for a reparameterization invariant system composed by a microscopic particle in a potential and a macroscopic clock. I define the physical Hilbert space of the discrete totally constrained theories, and I study the quantum evolution given by conditional probabilities defined in terms of quantum magnitudes associated to the clock and the system. I show how to recover the standard quantum mechanical continuum limit for the microscopic system."

If you want to follow up and see what his thing is, here is a recent paper

http://arxiv.org/abs/gr-qc/0505052

**Classical and quantum general relativity: a new paradigm**

Rodolfo Gambini, Jorge Pullin

8 pages, one figure, fifth prize of the Gravity Research Foundation 2005 essay competition

"We argue that recent developments in discretizations of classical and quantum gravity imply a new paradigm for doing research in these areas. The paradigm consists in discretizing the theory in such a way that the resulting discrete theory has no constraints. This solves many of the hard conceptual problems of quantum gravity. It also appears as a useful tool in some numerical simulations of interest in classical relativity. We outline some of the salient aspects and results of this new framework."

http://arxiv.org/abs/gr-qc/0409057

**Consistent discretization and loop quantum geometry**

Rodolfo Gambini, Jorge Pullin

4 pages

Phys.Rev.Lett. 94 (2005) 101302

"We apply the ``consistent discretization'' approach to general relativity leaving the spatial slices continuous. The resulting theory is free of the diffeomorphism and Hamiltonian constraints, but one can impose the diffeomorphism constraint to reduce its space of solutions and the constraint is preserved exactly under the discrete evolution. One ends up with a theory that has as physical space what is usually considered the kinematical space of loop quantum geometry, given by diffeomorphism invariant spin networks endowed with appropriate rigorously defined diffeomorphism invariant measures and inner products. The dynamics can be implemented as a unitary transformation and the problem of time explicitly solved or at least reduced to as a numerical problem. We exhibit the technique explicitly in 2+1 dimensional gravity."

To offer an unqualified opinion, always risky, GAMBINI's way might not have the distinguishing STYLISTIC features of 21st Century mathematics. It might not be what one expects, but it looks like it kind of WORKS and Gambini thinks so. It has one or two odd gimmicks but maybe that's actually a good thing.

AND THEN THERE IS MARTIN REUTER STUFF and it has signs of working too. And Loll. To generalize, all these approaches have the character of sticking very close to General Relativity, posssibly in some discretized lattice-like form, or some Regge triangles form, or taking the quantization that was the obvious thing to try in 1960 and stubbornly making the renormalization that didnt work then work now. But in whatever form they follow Gen Rel closely---and the indications are that these approaches might very well have the right classical limit, just because they stay so close to the original model.

And Loll. She is using the original Einstein action put into Regge triangles form (which has been around since 1960) so how far off from the classical limit can she be?

And Rovelli. Looks like he might have a handle on the vacuum and flat-limit gravitons and scattering amplitudes.

So all these various people are converging on home base from different directions. And JB says

**I see no reason why such a thing can’t be found if we drop the restriction on “mathematical elegance” ...**

Baez hunches have often been a good guide---a compass needle in the QG wilderness. So I'm beginning to feel some nearness to the right classical limit (the behavior in the large scale, low energy, not terribly curved case that Gen Rel says you should have).

And once SOMETHING works then even if it is awkward then its very awkwardness (when people confront and understand that) could point to some new mathematics.

Let's put links to all the ones mentioned here, to their conference abstracts:

http://loops05.aei.mpg.de/index_files/abstract_gambini.html

http://loops05.aei.mpg.de/index_files/abstract_reuter.html

http://loops05.aei.mpg.de/index_files/abstract_rovelli.html

http://loops05.aei.mpg.de/index_files/abstract_loll.html

---quote JB on Woit blog---

But, to answer your question, what could get me working harder on quantum gravity is some evidence that we can find a mathematically elegant background-free quantum theory that can reduce to general relativity in a suitable limit.I see no reason why such a thing can’t be found if we drop the restriction on “mathematical elegance” - but I like things that use beautiful math.

This is precisely why I mentioned Carlo Rovelli’s new paper...

---end quote---

elegance is an interesting standard. perhaps not entirely straightforward either, I think new and noble mathematics might sometimes sprout from a mere pragmatic contrivance---something cobbled together just to get it to work. I think the awkward could sometimes harbor the seeds of elegance. Maybe one just needs to understand the inelegant thing better.

Some famous mathematicians are apparently not shy about visiting a dusty attic or the junkyard---I learned from Arivero posts ("graviton" thread) that Alain Connes has been concerning himself with the infinitesimals of Calculus, with limits of sequences of numbers, with the Nonstandard Analysis of Abraham Robinson (goes back to the 1960s, didn't catch on much IIRC).

Maybe it is not appropriate to this context but I want to mention Rodolfo Gambini. here is what he is going to talk about at Loops conference:

http://loops05.aei.mpg.de/index_files/abstract_gambini.html

Prof. Rodolfo Gambini

Consistent discretizations of totally constrained systems

"I describe the consistent discretization scheme for the quantization of totally constrained systems and quantum gravity. As a first example I analyze the canonical formalism for Regge Calculus. I complete all the steps of the program for a reparameterization invariant system composed by a microscopic particle in a potential and a macroscopic clock. I define the physical Hilbert space of the discrete totally constrained theories, and I study the quantum evolution given by conditional probabilities defined in terms of quantum magnitudes associated to the clock and the system. I show how to recover the standard quantum mechanical continuum limit for the microscopic system."

If you want to follow up and see what his thing is, here is a recent paper

http://arxiv.org/abs/gr-qc/0505052

Rodolfo Gambini, Jorge Pullin

8 pages, one figure, fifth prize of the Gravity Research Foundation 2005 essay competition

"We argue that recent developments in discretizations of classical and quantum gravity imply a new paradigm for doing research in these areas. The paradigm consists in discretizing the theory in such a way that the resulting discrete theory has no constraints. This solves many of the hard conceptual problems of quantum gravity. It also appears as a useful tool in some numerical simulations of interest in classical relativity. We outline some of the salient aspects and results of this new framework."

http://arxiv.org/abs/gr-qc/0409057

Rodolfo Gambini, Jorge Pullin

4 pages

Phys.Rev.Lett. 94 (2005) 101302

"We apply the ``consistent discretization'' approach to general relativity leaving the spatial slices continuous. The resulting theory is free of the diffeomorphism and Hamiltonian constraints, but one can impose the diffeomorphism constraint to reduce its space of solutions and the constraint is preserved exactly under the discrete evolution. One ends up with a theory that has as physical space what is usually considered the kinematical space of loop quantum geometry, given by diffeomorphism invariant spin networks endowed with appropriate rigorously defined diffeomorphism invariant measures and inner products. The dynamics can be implemented as a unitary transformation and the problem of time explicitly solved or at least reduced to as a numerical problem. We exhibit the technique explicitly in 2+1 dimensional gravity."

To offer an unqualified opinion, always risky, GAMBINI's way might not have the distinguishing STYLISTIC features of 21st Century mathematics. It might not be what one expects, but it looks like it kind of WORKS and Gambini thinks so. It has one or two odd gimmicks but maybe that's actually a good thing.

AND THEN THERE IS MARTIN REUTER STUFF and it has signs of working too. And Loll. To generalize, all these approaches have the character of sticking very close to General Relativity, posssibly in some discretized lattice-like form, or some Regge triangles form, or taking the quantization that was the obvious thing to try in 1960 and stubbornly making the renormalization that didnt work then work now. But in whatever form they follow Gen Rel closely---and the indications are that these approaches might very well have the right classical limit, just because they stay so close to the original model.

And Loll. She is using the original Einstein action put into Regge triangles form (which has been around since 1960) so how far off from the classical limit can she be?

And Rovelli. Looks like he might have a handle on the vacuum and flat-limit gravitons and scattering amplitudes.

So all these various people are converging on home base from different directions. And JB says

Baez hunches have often been a good guide---a compass needle in the QG wilderness. So I'm beginning to feel some nearness to the right classical limit (the behavior in the large scale, low energy, not terribly curved case that Gen Rel says you should have).

And once SOMETHING works then even if it is awkward then its very awkwardness (when people confront and understand that) could point to some new mathematics.

Let's put links to all the ones mentioned here, to their conference abstracts:

http://loops05.aei.mpg.de/index_files/abstract_gambini.html

http://loops05.aei.mpg.de/index_files/abstract_reuter.html

http://loops05.aei.mpg.de/index_files/abstract_rovelli.html

http://loops05.aei.mpg.de/index_files/abstract_loll.html

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marcus said:

Marcus, one of the main things to consider is what Olaf Dreyer:http://loops05.aei.mpg.de/index_files/abstract_dreyer.html

appears to tackling?

I have one or two things I would be very interested in finding out regarding Quantum to Macro transitions, having my own ideas, I am going to see how close I am to what the Theorists predict, but I ll leave that till about two days before the programme gets under way.

Regarding the "new hypotheses" from Smolin,http://loops05.aei.mpg.de/index_files/abstract_smolin.html

I wonder what it could involve?..I have a number of calculated guesses, but,again this will go onto my website no later than two days before the start date.

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Spin_Network said:Marcus, one of the main things to consider is what Olaf Dreyer:http://loops05.aei.mpg.de/index_files/abstract_dreyer.html

appears to tackling?

I have one or two things I would be very interested in finding out regarding Quantum to Macro transitions, having my own ideas, I am going to see how close I am to what the Theorists predict, but I ll leave that till about two days before the programme gets under way.

Regarding the "new hypotheses" from Smolin,http://loops05.aei.mpg.de/index_files/abstract_smolin.html

I wonder what it could involve?..I have a number of calculated guesses, but,again this will go onto my website no later than two days before the start date.

Hi Spin, I may have missed something you said earlier. How will we know anything about what they say at the conference until much later if at all? I am unable to attend---comes at the wrong time for me and it's far away. It would be wonderful of course if someone were to attend and put reports up on a website.

I hope John Baez does a TWF soon after this exciting conference. But I have no certainty that we will hear an extensive report----may have missed something but I don't know of anyone who is, for example, live-blogging.

Do you?

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Chronos

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Since I have too much free time, here are the scheduled talks for Loops 05 day 1.

Speaker: Carlo Rovelli

Title: GENERAL RELATIVISTIC QUANTUM PHYSICS: Background independent scattering amplitudes, boundary formalism, local particles and partial observables

Speaker: Prof. Lee Smolin

Title: Persistent puzzles in background independent approaches to quantum gravity

Speaker: Dr. John Barrett

Title: Three-dimensional quantum gravity coupled to matter

Speaker: Stefan Theisen

Title:

Speaker: Karim Noui

Title: Creation and anhiliation of particles in 3D gravity: toward a background independant field theory

Speaker: Florian Girelli

Title: Higher dimensional perspective on DSR

Speaker: Tomasz Konopka

Title: A model of a scalar field theory with Lorentz-invariant energy scale

Speaker: Simone Speziale

Title: On the perturbative expansion of a quantum field theory around a topological sector

Speaker: Florian Conrady

Title: Yang-Mills theory and 3d gravity as string theories

Speaker: Dr. Dorothea Bahns

Title: The Invariant Charges of the Nambu-Goto String

Speaker: Seth Major

Title: A Discrete Machian Model: Fermions

Speaker: Dr. Daniel Terno

Title: The classical limit of a quantum tetrahedron

Speaker: Prof. Fotini Markopoulou

Title: Erorr-free quantum gravity

Speaker: Olaf Dreyer

Title: Classicality and Quantum Mechanics

Speaker: Yidun Wan

Title: 2-D Ising Model with non-local links - a study of nonlocality problem

Speaker: Pierre Martinetti

Title: An overview on the thermal time hypothesis

Speaker: Aureliano Skirzewski

Title: Effective Action From Geometric Formulation of Quantum Mechanics

Speaker: Rafael Porto

Title: Fundamental gravitational limitations to quantum computing [QIT]

Speaker: Dr. Robert Oeckl

Title: States on timelike hypersurfaces

Speaker: Bruno Hartmann

Title: An operational determination of the physical action

Speaker: Ruediger Vaas

Title: Before Time: Loop Quantum Cosmology and Philosophy of Science

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Scheduled talks for L05 day 2:

Speaker: Prof. John Baez

Title: New Progress in Spin Foam Models

Speaker: Prof. Robbert Dijkgraaf

Title: Quantum geometry and topological strings

Speaker: Dr. Laurent Freidel

Title: Effective Field theory from quantum gravity

Speaker: Alejandro Perez

Title:

Speaker: Prof. Dan Christensen

Title: Finiteness and Positivity for the Lorentzian partition function

Speaker: Dr. Josh Willis

Title: Computational Building Blocks for Lorentzian Spin Foams

Speaker: Dr. Suresh K Maran

Title: Classical and Spin Foam Quantum Gravity for Complex, Real Orthogonal Groups and the Square of Area Reality Constraint

Speaker: Dr. Etera Livine

Title: Black Hole Entropy, Entanglement, and Area Renormalisation in LQG

Speaker: Dr. Daniele Oriti

Title: Parametrised group field theories and quantum gravity transition amplitudes

Speaker: James Ryan

Title: A group field theory for 3d quantum gravity coupled to a scalar field

Speaker: Artem Starodubtsev

Title: Scattering problem in background independent quantum gravity

Speaker: Prof. Aleksandar Mikovic

Title: [I am going to describe how to construct a spin foam model of quantum gravity based on the path integral quantization of the Palatini action of General relativity].

Speaker: Dr. Kirill Krasnov

Title: Lambda<0 Quantum Gravity in 2+1 Dimensions

Speaker: Dr. Hendryk Pfeiffer

Title: Two-dimensional open-closed TQFT from a state sum model

Speaker: Federico Mattei

Title: From 3-geometry transition amplitudes to graviton states

Speaker: Aristide Baratin

Title: Hidden quantum gravity in Feymnan graphs

Speaker: Winston Fairbairn

Title: Large N QCD and spinfoam models

Speaker: Catherine Meusburger

Title: Grafting and Poisson structure in (2+1)-gravity with vanishing cosmological constant

Speaker: Eugeniu M. Popescu

Title: Topological matter models in (2+1)-dimensional gravity

Speaker: Jesper M. Grimstrup

Title: Spectral Triples of Holonomy Loops

Speaker: Dr. Mario Paschke

Title: Causal Parametrices and Lorentzian spectral geometries

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Speaker: Renate Loll

Title: Emergence of Quantum Spacetime from Causal Dynamical Triangulations

Abstract: In this talk, I will summarize and discuss some recent results on the emergent macro- and microstructure of quantum spacetime obtained in the Causal Dynamical Triangulations approach to nonperturbative quantum gravity. Particularly compelling is the finding of a dynamically generated scale-dependent dimension of this spacetime, interpolating between the classical value four on large scales and two at short distances.

Speaker: Prof. Martin Reuter

Title: Asymptotically safe quantum gravity: from the UV to the IR

Abstract: The construction of a microscopic theory of quantum gravity in the asymptotic safety scenario is reviewed, and the nonperturbative renormalization group flow of Quantum Einstein Gravity from the UV fixed point towards macroscopic scales is described. Furthermore, the emergence of a fractal spacetime structure will be discussed.

Speaker: Prof. Rafael D. Sorkin

Title: Some Insights for Quantum Gravity Derived from Work on Causal Sets

Abstract: Drawing on a range of results from the causet program, I try to extract some general lessons that can help guide future work on quantum gravity in general. Some of these insights are of long standing, but some are very recent, notably the indication that an "intermediate nonlocality scale" much greater than the Planck length might be required to make contact with local continuum physics.

Speaker: Dr. Fay Dowker

Title: Causal Set Phenomenology

Abstract: The hypothesis that the discrete substructure of spacetime is a causal set suggests a straightforward model building technique: invent phenomenological dynamics for matter (particles or fields) on a background causal set that is well approximated by our continuum spacetime. These models can be analysed to see if they predict observable deviations from continuum models. I will describe two examples of such models: "particle swerves" and a model of detector response to the scalar field of a scalar charge source.

that's all there is this day, because the organizers made Wednesday afternoon a free day with no talks.

that means people can go into Berlin, a pretty good cultural city, or they can hang out in small groups and discuss the implications of the talks they have heard so far, and maybe make some contacts for possible collaboration on future research. I guess Wednesday afternoon could be one of the nicest times of the conference.

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Day 1 looks like dynamite. Many interesting papers.

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Speaker: Prof. Jerzy Lewandowski

Title: Loop Quantum Gravity

Speaker: Dr. Alejandro Corichi

Title: Towards semi-classical physical states in LQG

Speaker: Dr. Rainer Verch

Title: Generally covariant quantum field theory

Speaker: Prof. Rodolfo Gambini

Title: Consistent discretizations of totally constrained systems

Speaker: Prof. Yongge Ma

Title: On the Master Constraint Operator for Loop Quantum Gravity

Speaker: Dr. Hanno Sahlmann

Title: On the measure theory underlying LQG

Speaker: Dr. Christian Fleischhack

Title: Stone-von Neumann Theorem in Quantum Geometry In ordinary quantum mechanics

Speaker: Dr. Jose A. Zapata

Title: Loop quantization as a continuum limit

Speaker: Kristina Giesel

Title: Consistency Check on Volume and Triad Operator Quantisation in Loop Quantum Gravity

Speaker: Bianca Dittrich

Title: Partial and Complete Observables for Canonical General Relativity

Speaker: Dr. Jorma Louko

Title: Group averaging, positive definiteness and superselection sectors

Speaker: Wayne Bomstad II

Title: Linearized Gravity as the Rosetta Stone of Quantization Techniques

Speaker: Hal Finkel

Title: Stochastic Evolution of Graphs using Local Moves

Speaker: Prof. Jorge Pullin

Title: Classical applications of consistent discretizations

Speaker: Dr. Gabriele Gionti

Title: Discrete Theory of Gravity as a local Theory of the Poincare Group in the First Order Formalism

Speaker: Dr. David Rideout

Title: Spatial Hypersurfaces in Causal Set Cosmology

Speaker: Dr. Sumati Surya

Title: Recovering spacetime topology from a causet

Speaker: Derek K. Wise

Title: Discrete p-form Electromagnetism as a Chain Field Theory

Speaker: Stefan Zohren

Title: Taming the cosmological constant in 2D causal quantum gravity with topology change

Speaker: Prof. Manfred Droste

Title: Universal homogeneous causal sets

Speaker: Dr. Jan-Markus Schwindt

Title: Is there a minimal length in asymptotically safe quantum gravity?

http://loops05.aei.mpg.de/index_files/abstract_schwindt.html

Thanks to Chronos for entering the list of talks for days 1 and 2.

I was intrigued by the abstract of J-M Schwindt's talk---it refers to Martin Reuter QEG (assympt. safe quantum GR, "quantum Einstein gravity") which has a sequence of metrics appropriate for finer and finer resolution, and so a sequence of "microscopes" with which to view spacetime. And Schwindt has the surprising result that there is a limit on the resolution of angle. with no-matter-how-fine a microscope one cannot see less than a certain angle. So he asks if there is a minimal length as well? weird. I did not expect this of Reuter QEG

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Chronos you started this and I think it is a good idea because seeing the talks listed like this let's one scan down and perhaps see some themes or shared ideas that one could otherwise have overlooked. So I will finish the listing by doing Friday 14 October talks. the source is

http://loops05.aei.mpg.de/index_files/Programme.html

http://loops05.aei.mpg.de

Speaker: Dr. Bernard L. Julia

Title: Provisional title Dualization of gravity

Speaker: Hugo A. Morales-Tecotl

Title: On the top-down approach to quantum gravity

Speaker: Prof. Roy Maartens

Title: COSMOLOGY AND QUANTUM GRAVITY

Abstract: I will review the basic features of the standard, classical model of cosmology, which is based on General Relativity, and how this model accounts for observed properties of the universe. Modifications to General Relativity that are inspired by quantum gravity

Speaker: Prof. Abhay Ashtekar

Title:

Abstract: General relativity provides a subtle and powerful interplay between gravity and geometry, thereby opening numerous possibilities for novel phenomena. However, this interplay also implies that the space-time itself ends when the gravitational field becomes singular. In loop quantum gravity, the interplay is elevated to the quantum regime through quantum geometry. I will present examples which strongly suggest that the

Speaker: Dr. Oliver Winkler

Title:

Abstract: We discuss a novel approach to a quantization of spherically symmetric black holes. We present the kinematical setup and focus in particular on a genuinely quantum definition of what a quantum black hole is which avoids fixing classical inputs such as boundaries a priori.

Speaker: Prof. Viqar Husain

Title:

Abstract: I will describe an approach to studying gravitational collapse in quantum gravity. The model studied is the spherically symmetric gravity-scalar system, which is a 2D field theory. An ADM Hamiltonian formalism is utilised, together with an alternative quantisation scheme with similarities to loop quantum gravity. The calculational scheme allows an initial quantum state of matter and metric excitations to be followed to black hole formation, and beyond.

Speaker: Dr. Stefan Hofmann

Title:

Abstract: We analyze fluctuations of a generic scalar field around its vacuum state in a singularity free quantum cosmology... In this framework we calculate the power spectrum and discuss its leading

Speaker: Johannes Brunnemann

Title:

Abstract: Loop Quantum Cosmology (LQC) has taken huge steps towards an answer to the question about sigularity (dis-)appearence in a quantum theory of gravity. The results look very promising, however they are usually obtained within the framework of symmetry reduced and thus simplified models...[comment: concern with BB and BH singularities, work with Thiemann]

Speaker: Dr. Leonardo Modesto

Title:

Abstract: We apply techniques introduced in quantum cosmology to the Schwarzschild metric inside the horizon. The space-time inside the horizon is the Kantowski-Sachs space-time with space section of topology R x S^2. In particular, we use a mini-superspace quantization procedure suggested by Loop Quantum Gravity and based on an alternative to the Schroedinger representation. Using this quantization procedure, we show that the black hole singularity disappears and spacetime can be dynamically extended beyond the classical singularity. At this point we introduce the matter and we study the gravitational collapse of a star. We study the dynamic of the space time in the interior of the Schwarzschild radius. In particular in our simple model we obtain the evolution of the matter inside the star and of the gravity outside the region where the matter is present. When we impose the boundary condition between the two region we identify an unique time coordinate inside and outside the star.

Speaker: Gregory Vereshchagin

Title:

Abstract: I will discuss basic phenomenological consequences of modification of Friedmann's equations in the semi-classical LQC. Such aspects as cosmic singularity problem, size of the Universe, generation of initial conditions for inflation and constraints on parameters of inflationary models will be considered. [comment: all these COSMOLOGY papers held until Friday!]

Speaker: Dr. Tomasz Pawlowski

Title:

Abstract: The self-adjoint form of the Hamiltonian constraint within Loop Quantum Cosmology framework is used to determine the evolution of an isotropic universe with massless scalar field. Its dynamics near classical big-bang singularity is analysed. In particular we are going to present the results on the evolution 'through' the singularity and analysis of the classicality of the universe re-emerging from deep quantum region.[comment: a co-author with Ashtekar, talking about the BB side of a BH-bounce, more cosmology, refers to future paper by Ashtekar-Pawlowski-Singh]

Speaker: Dr. Marek Szydlowski

Title:

[comment: more bounce]

Speaker: Dr. Hector Hugo Hernandez

Title:

[comment: more cosmology]

Speaker: Jonathan Engle

Title:

Abstract: In loop quantum cosmology, a key issue regards the relation of the reduced quantum model to the full quantum theory. In the ideal scenario, the reduced model is isomorphic to the ``symmetric sector'' of the full quantum theory...

Speaker: Kevin Vandersloot

Title:

Abstract: We discuss non-perturbative modifications to standard Friedmann dynamics implied by loop quantum cosmology. Particular focus is paid to effects arising from the discrete quantum geometry inherant in loop quantum cosmology. We show that these effects can imply bouncing behavior even in spatially flat models. We discuss the implications for pre big-bang and cyclic style models of string cosmology. Lastly we discuss how these effects can help provide sucessful initial conditions for inflation with a possibly

[comment: I was always wondering about this! LQC seems to want the universe FINITE and only almost flat, not perfectly flat. But Kevin now says it may be flat. Good. I notice again a concern for testing---an observable signature possibly in the Cosmic Microwave Background]

Speaker: Dr. Daniel Cartin

Title:

Abstract: Loop quantum cosmology is a simplified version of the full quantum geometry program,... the Hamiltonian constraint becomes a partial difference equation acting on the wave function. ...are not straightforward to solve; we discuss generic methods that can be used, and present results for various specific cases.

[Cartin works with Gaurav Khanna, they were both at Penn State. Khanna is at Dartmouth and does supercomputer,

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- #23

marcus

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Monday is Carloday

Tuesday is Freidelday

Wednesday is Lollday

Thursday is Otherday (Rodolfo plus Jerzy et al on math. foundations)

Friday is Abhayday

- #24

Chronos

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Terno and Markopoulou are giving presentations based on quantum information theory on Monday [Markopoulouday on my calender]. I'm a big fan of QIT. FM's presentation should be quite interesting

Title: Erorr-free quantum gravity

Abstract: We investigate the possibility that a background independent quantum theory of gravity is not a theory of quantum geometry. We provide a way for global spacetime symmetries to emerge from a background independent theory without geometry. In this, we use a quantum information theoretic formulation of quantum gravity and the method of noiseless subsystems in quantum error correction. This is also a method that can extract particles from a quantum geometric theory such as a spin foam model.

Instead of the usual 'sum of geometries' approach to quantum gravity, FM is suggesting the geometry itself may be emergent. How is that for background independence? This appears to build upon a presentation from late 2004. Here's a link if anyone is interested:

http://www.Newton.cam.ac.uk/webseminars/pg+ws/2004/qisw05/1216/markopoulou/

Title: Erorr-free quantum gravity

Abstract: We investigate the possibility that a background independent quantum theory of gravity is not a theory of quantum geometry. We provide a way for global spacetime symmetries to emerge from a background independent theory without geometry. In this, we use a quantum information theoretic formulation of quantum gravity and the method of noiseless subsystems in quantum error correction. This is also a method that can extract particles from a quantum geometric theory such as a spin foam model.

Instead of the usual 'sum of geometries' approach to quantum gravity, FM is suggesting the geometry itself may be emergent. How is that for background independence? This appears to build upon a presentation from late 2004. Here's a link if anyone is interested:

http://www.Newton.cam.ac.uk/webseminars/pg+ws/2004/qisw05/1216/markopoulou/

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- #25

Spin_Network

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Chronos said:Terno and Markopoulou are giving presentations based on quantum information theory on Monday [Markopoulouday on my calender]. I'm a big fan of QIT. FM's presentation should be quite interesting

Title: Erorr-free quantum gravity

Abstract: We investigate the possibility that a background independent quantum theory of gravity is not a theory of quantum geometry. We provide a way for global spacetime symmetries to emerge from a background independent theory without geometry. In this, we use a quantum information theoretic formulation of quantum gravity and the method of noiseless subsystems in quantum error correction. This is also a method that can extract particles from a quantum geometric theory such as a spin foam model.

Instead of the usual 'sum of geometries' approach to quantum gravity, FM is suggesting the geometry itself may be emergent. How is that for background independence? This appears to build upon a presentation from late 2004. Here's a link if anyone is interested:

http://www.Newton.cam.ac.uk/webseminars/pg+ws/2004/qisw05/1216/markopoulou/

Chronos and Marcus thanks for the great thread, keeping everything in check.

Chronus, the above link you have provided is really,really interesting!

Thanks

- #26

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Hi -

You can see the transparencies of my talk at Loops '05 here:

Towards a Spin Foam Model of Quantum Gravity

I've changed the title from what appears in http://loops05.aei.mpg.de/" [Broken]. I've also changed my abstract. Here it is:

Spin foam models include several different classes of physical theories: lattice gauge theories, dynamical triangulation models of quantum gravity, "chain mail" quantum field theories, and topological string theories. Is there a spin foam model of quantum gravity in 4 dimensions? To address this question, we review recent work on causal dynamical triangulations and the renormalization group. This suggests that quantum gravity is a well-defined theory with the curious property that spacetime is effectively 4-dimensional at large distance scales, but 2-dimensional at very short distance scales. This is just what one might expect from a spin foam model, since spacetime is fundamentally 2-dimensional in these theories. We discuss properties a spin foam model should have in order to approximate general relativity at large distance scales.

I refer to a bunch of papers in my talk, but you can more easily get to those papers "[URL [Broken] my webpage[/URL].

My grad students Jeffrey Morton, Derek Wise and I are flying to Berlin this Saturday... this is going to be fun!

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- #27

marcus

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john baez said:Hi -

You can see the transparencies of my talk at Loops '05 here:

Towards a Spin Foam Model of Quantum Gravity

I've changed the title from what appears in http://loops05.aei.mpg.de/" [Broken]. I've also changed my abstract. Here it is:

Spin foam models include several different classes of physical theories: lattice gauge theories, dynamical triangulation models of quantum gravity, "chain mail" quantum field theories, and topological string theories. Is there a spin foam model of quantum gravity in 4 dimensions? To address this question, we review recent work on causal dynamical triangulations and the renormalization group. This suggests that quantum gravity is a well-defined theory with the curious property that spacetime is effectively 4-dimensional at large distance scales, but 2-dimensional at very short distance scales. This is just what one might expect from a spin foam model, since spacetime is fundamentally 2-dimensional in these theories. We discuss properties a spin foam model should have in order to approximate general relativity at large distance scales.

I refer to a bunch of papers in my talk, but you can more easily get to those papers "[URL [Broken] my webpage[/URL].

My grad students Jeffrey Morton, Derek Wise and I are flying to Berlin this Saturday... this is going to be fun!

thanks for keeping us posted!

be sure to ship us all the tidbits you can. it is an historical conference, and we crave news of it

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- #28

marcus

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john baez said:... we review recent work on causal dynamical triangulations and the renormalization group. This suggests that quantum gravity is a well-defined theory with the curious property that spacetime is effectively 4-dimensional at large distance scales, but 2-dimensional at very short distance scales. This is just what one might expect from a spin foam model, since spacetime is fundamentally 2-dimensional in these theories...

you really have redirected! now it looks like focusing on work of Loll and Reuter and aligning spinfoam so it takes in CDT as a special case

my head is reeling slightly but it was not altogether unexpected

Bravo!

you will all have a great time for sure!

- #29

marcus

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I have started on your lecture notes

they get hard around page 13

and interesting too

I hope other people here at PF study your notes and we can make a collective effort of understanding

I see that Tullio Regge is the hero of your pages 16, 17, 18

here's a picture

http://www.cicap.org/enciclop/at101249.htm

he's one of my heroes too. Tully (also the English nickname of Cicero)

http://plagiarist.com/poetry/3381/ [Broken] Yeats said about the other Tully

Horace there by Homer stands,

Plato stands below,

And here is Tully's open page.

How many years ago

Were you and I unlettered lads

Mad as the mist and snow?

they get hard around page 13

and interesting too

I hope other people here at PF study your notes and we can make a collective effort of understanding

I see that Tullio Regge is the hero of your pages 16, 17, 18

here's a picture

http://www.cicap.org/enciclop/at101249.htm

he's one of my heroes too. Tully (also the English nickname of Cicero)

http://plagiarist.com/poetry/3381/ [Broken] Yeats said about the other Tully

Horace there by Homer stands,

Plato stands below,

And here is Tully's open page.

How many years ago

Were you and I unlettered lads

Mad as the mist and snow?

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- #30

marcus

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you should be going to Potsdam next week!

but I guess you are busy beginning university studies,

how do you like the university?

- #31

selfAdjoint

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marcus said:I

I see that Tullio Regge is the hero of your pages 16, 17, 18

here's a picture

http://www.cicap.org/enciclop/at101249.htm

he's one of my heroes too. Tully (also the English nickname of Cicero)

http://plagiarist.com/poetry/3381/ [Broken] Yeats said about the other Tully

Horace there by Homer stands,

Plato stands below,

And here is Tully's open page.

How many years ago

Were you and I unlettered lads

Mad as the mist and snow?

And also Browning's "Tully's every word" from his "The Bishop Orders his Tomb at St. Praxed's Church in Rome"

http://eir.library.utoronto.ca/rpo/display/poem265.html [Broken]

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- #32

selfAdjoint

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Marcus said:have started on your lecture notes

they get hard around page 13

and interesting too

I hope other people here at PF study your notes and we can make a collective effort of understanding

Which lecture notes are you reading? Can you give the link?

- #33

marcus

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selfAdjoint said:Which lecture notes are you reading? Can you give the link?

JB calls it a set of transparencies but I called it notes, it reads somewhat coherently, like 18 pages of lecture notes

it is here

http://math.ucr.edu/home/baez/loops05/

and you click on "Towards a..."

and IIRC you get this pdf

http://math.ucr.edu/home/baez/loops05/loops05.pdf

In fact you may already have been looking at this (which he calls his slides) and were confused by my calling it "lecture notes"

I didnt know that poem of Browning. It's good.

BTW won't you take a look at this site. some of the best translations into English that there are on the web I think:

http://www.brindin.com/main.htm

the site has over a dozen excellent translators working on the greatest poetry in over a dozen languages----often preserving some of the music (the rhyme, rhythm, general sound) of the original

the editors are an English couple, both verse translators themselves. the English seem to be more in touch with the European verse tradition than we are---maybe it is oldfashioned education. this site really deserves to be widely known

if you find some of the good stuff you will know what i mean

================

EDIT: I went to History and made a thread about it

https://www.physicsforums.com/showthread.php?p=777322#post777322

getting close to voices from the past is a way to approach the past

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- #34

Kea

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marcus said:JB calls it a set of transparencies but I called it notes, it reads somewhat coherently...

Hey guys, the references in JB's slides greatly expand the 18 pages.

- #35

marcus

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Kea said:Hey guys, the references in JB's slides greatly expand the 18 pages.

Indeed they do!

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