With all due respect, is there going to be mud wrestling?

[marcus]

in a post today on Woit's blog, Smolin gave some indications what to expect at Loops 05 conference:

https://www.physicsforums.com/showthread.php?p=734165#post734165

"I agree that background independent quantum theories of gravity, including LQG, must address the problem of unification. We have some new ideas and results about this that I’m pretty excited about which will be announced at the Loops 05 conference."

[I guess one of the papers he means could likely be Freidel "Perturbative expansion for the 3D Yang-Mills theory coupled to quantum gravity in the spinfoam formalism"]

"We must show that the ground state is semiclassical, by solving the dynamics. This is a hard problem, analogous to showing that the ground state of water is a solid. But as this is the focus of attention there are beginning to be significant, non-trivial results on how classical spacetime can emerge from a background independent quantum theory. The best so far are not in LQG, they are the Ambjorn-Jurkiewicz-Loll results on CDT, http://arxiv.org/hep-th/0404156 . More is coming, I know of 3 papers in preparation by different authors that contain interesting new approaches or results on this problem. So stay tuned and (to John) don’t lose heart."

So he is talking on one hand about UNIFICATION (combining matter into the picture with gravity and spacetime geometry, defining broader goals for nonperturbative QG) and on the other hand he is talking about verifying that for some of the QG approaches the largescale, or classical, or semiclassical LIMIT is right. And he is talking about somebody besides Renate Loll, who has made noteworthy progress on that recently. He says THREE PAPERS in preparation by people different from Loll. So it gives an idea of what to be looking out for.

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

Another issue Smolin raised in that post was modified Poincaré (DSR), and he had something to say about string claims:

-------Smolin post on Woit blog-----
Lee Smolin; September 3rd, 2005 at 10:03 am
...
As for string theory being the unique UV completion, the claimed uniqueness requires imposing two physically unjustified assumptions,

1) that it makes sense to an arbitrarily high energy to separate the spacetime geometry into a fixed background and gravitons of arbitrarily high energy and

2) those graviton states transform under the ordinary Poincare transformations, no matter how high the energy.

The first appears false in any consistent non-perturbative unification of gravity and quantum theory including CDT and LQG.

The second is much less compelling since we learned that Poincaré invariance may be deformed, as in deformed or doubly special relativity theories. These allow the relativity of inertial frames to be consistent with energy and/or momentum cutoffs. At least in 2+1 gravity coupled to matter, we know this is how the theory achieves consistency. And there are indications (far from proofs) that the same will be true in 3+1 when we get the low energy limit sorted out.

Of course, the best news is that AUGER and GLAST will in only a few years tell us the fate of Lorentz invariance. The need to firm up our predictcions before the experiments report is what keeps us working hard on these problems.
-------end quote------

I have bolded a reference to some interesting recent work that includes these Freidel papers.
http://arxiv.org/hep-th/0502106

http://arxiv.org/gr-qc/0506067

 

[marcus]

"...At least in 2+1 gravity coupled to matter, we know this is how the theory achieves consistency. And there are indications (far from proofs) that the same will be true in 3+1 when we get the low energy limit sorted out..."

I have bolded a reference to some interesting recent work that includes these Freidel papers.
http://arxiv.org/hep-th/0502106

http://arxiv.org/gr-qc/0506067

The implication here is that the 2+1 case IS sorted out and that it depends on modified Poincaré invariance. Let's take a look at these Freidel papers, which conclude that.

http://arxiv.org/hep-th/0502106
Ponzano-Regge model revisited III: Feynman diagrams and Effective field theory
Laurent Freidel, Etera R. Livine
46 pages
"We study the no gravity limit G_{N}-> 0 of the Ponzano-Regge amplitudes with massive particles and show that we recover in this limit Feynman graph amplitudes (with Hadamard propagator) expressed as an abelian spin foam model. We show how the G_{N} expansion of the Ponzano-Regge amplitudes can be resummed. This leads to the conclusion that the dynamics of quantum particles coupled to quantum 3d gravity can be expressed in terms of an effective new non commutative field theory which respects the principles of doubly special relativity. We discuss the construction of Lorentzian spin foam models including Feynman propagators"

http://arxiv.org/gr-qc/0506067
A group field theory for 3d quantum gravity coupled to a scalar field
Laurent Freidel, Daniele Oriti, James Ryan
11 pages
"We present a new group field theory model, generalising the Boulatov model, which incorporates both 3-dimensional gravity and matter coupled to gravity. We show that the Feynman diagram amplitudes of this model are given by Riemannian quantum gravity spin foam amplitudes coupled to a scalar matter field. We briefly discuss the features of this model and its possible generalisations."

---quote from the introduction---
"Spin foam models [1, 2] represent a purely combinatorial and algebraic implementation of the sum-over-histories approach to quantum gravity, in any signature and spacetime dimension, with an abstract 2-complex playing the role of a discrete spacetime, and algebraic data from the representation theory of the Lorentz group playing the role of geometric data assigned to it.

This approach has recently been developed to a great extent in the 3-dimensional case. It is now clear that it provides a full quantisation of pure gravity[3], whose relation with the one obtained by other approaches is well understood[4, 5].

Moreover, matter can be consistently included in the picture[3, 6], providing a link between spin foam models and effective field theory[7] living on a non-commutative geometry. This picture allows us to naturally address the semi-classical limit of spin foam models and shows that quantum gravity in dimension 3 effectively follows the principle of the so-called deformed (or doubly) special relativity[8].

The group field theory formalism[9] represents a generalisation of matrix models of 2-dimensional quantum gravity [10]. It is a universal structure lying behind any spin foam model for quantum gravity[11, 12], providing a third quantisation point of view on gravity[9] and allowing us to sum over pure quantum gravity amplitudes associated with different topologies[13].

In this picture, spin foams, and thus spacetime itself, appear as (higher-dimensional analogues of) Feynman diagrams of a field theory defined on a group manifold and spin foam amplitudes are simply the Feynman amplitudes weighting the different graphs in the perturbative expansion of the quantum field theory.

On the other hand, we can construct a noncommutative field theory whose Feynman diagram amplitudes reproduce the coupling of matter fields to 3d quantum gravity for a trivial topology of spacetime[7]. Remarkably, the momenta of the fields are labelled also by group elements.

Moreover, in three dimensions there is a duality between matter and geometry, and the insertion of matter can be understood as the insertion of a topological defect charged under the Poincaré group[3].

This suggests that one should be able to treat the third quantisation of gravity and the second quantisation of matter fields in one stroke (see[14] for an early attempt). The purpose of this paper is to study how the coupling of matter to quantum gravity is realized in the group field theory, and whether it is possible to write down a group field theory for gravity and particles that reproduces the amplitudes derived in [3] coupling quantum matter to quantum geometry. This is what we achieve in the present work.

The way the correct amplitudes are generated as Feynman amplitudes of the group field theory is highly non-trivial. It requires an extension of the usual group field theory (gft) formalism to a higher number of field variables, and produces an interesting intertwining of gravity and matter degrees of freedom, as we are going to discuss in the following...
---endquote---
More at
https://www.physicsforums.com/showthread.php?p=602668#post602668

A related paper that appeared about the same time:

http://arxiv.org/abs/hep-th/0505174
Quantum Gravity with Matter via Group Field Theory
Kirill Krasnov
43 pages, many figures

 

[marcus]

Without being able to follow all the steps, I am trying to see what Smolin is saying about DSR. I think he is saying that IN THE 2+1 CASE of spinfoams, the inclusion of matter and confirming the classical limit has been moved along far enough ( e.g. by Freidel and by Krasnov) to show that it only works with modified Poincaré invariance.

Freidel makes a strong assertion about this. He says DSR has been popular and the idea appealed to a lot of people, but he is now showing for the first time that it is necessary to make the theory consistent. This is still just in 2+1.

Now Smolin makes an educated guess and says it's possible when the 3+1 case is worked out that it will also turn out to require DSR. This would constitute a hard prediction that gammaray observations could test. So he thinks that spinfoams will have a falsifiable prediction ready before GLAST is launched, and that there will be a test.

He also extends from 2+1 case success with coupling to matter and classical limit, and concludes that a modified Lorentz invariance at small scale is physically possible. So that elevates to the status of an (empirically unconfirmed) ASSUMPTION the notion that there is no deviation from Lorentz invariance even at very small scale. I think that is what underlies his statement about string uniqueness claims. Maybe someone else can help clarify.

In my list of guesses a couple of posts back, as to what some of the topics of the talks might be, I didn't list anyone talking about DSR and tests of various Quantum Gravity models. That is something Smolin might talk about instead of the case for B.I.

 

[marcus]

As an exercise to see how well (if at all) I understand the nonpert. QG scene, given that the Loops 05 organizers have published a tentative list of speakers but not the titles of the talks, I expanded the list into a possible program. The first attempt at this was a few posts back.

As indicated earlier, in cases where person has recently come out with SEVERAL papers and i cannot choose, or where for whatever reason there is no clear pick, then I leave that speaker name in BRACKETS.

Does anyone have any inside information or guesses they want to add?
Do any of the speakers themselves want to help out and actually tell us?

This is reckless speculation of course, they could change the list of speakers from what i found at the site, and it is risky trying to guess anything to do with research, including what people might talk about at a conference, but nevertheless here is my current Loops 05 Ersatz Programme:

Abhay Ashtekar http://www.arxiv.org/abs/gr-qc/0504029
Black Hole Evaporation

[John Baez]

John Barrett http://www.arxiv.org/abs/gr-qc/0502048
Feynman diagams coupled to three-dimensional quantum gravity

Alejandro Corichi http://www.arxiv.org/abs/gr-qc/0504052
Semiclassical States for Constrained Systems

Robbert Dijkgraaf http://www.arxiv.org/abs/hep-th/0411073
Topological M-theory as Unification of Form Theories of Gravity

Fay Dowker http://arxiv.org/gr-qc/0508109
Causal sets and the deep structure of spacetime

Laurent Freidel http://arxiv.org/abs/hep-th/0505016
Group Field Theory: An overview

[Karel Kuchar]
Jurek Lewandowski http://www.arxiv.org/gr-qc/0508091
Background independent quantizations: the scalar field I

Renate Loll http://www.arxiv.org/hep-th/0509010
The Universe from Scratch

Roy Maartens http://www.arxiv.org/abs/gr-qc/0503041
A black hole mass threshold from non-singular quantum gravitational collapse

[Hugo Morales Tecotl
Alejandro Perez
Jorge Pullin]
Martin Reuter http://www.arxiv.org/hep-th/0508202
Fractal Spacetime Structure in Asymptotically Safe Gravity

Carlo Rovelli http://arxiv.org/gr-qc/0508124
Graviton propagator from background-independent quantum gravity

Lee Smolin http://www.arxiv.org/hep-th/0507235
The Case for Background Independence

[Rafael Sorkin
Stefan Theisen
Rainer Verch]


============
although not listed, to keep in mind is "Perturbative expansion for the 3D Yang-Mills theory coupled to quantum gravity in the spinfoam formalism"
to appear, cited in http://www.arxiv.org/abs/gr-qc/0508106)

 

[marcus]

In the preceding post there is a reference to the Martin Reuter paper "Fractal Spacetime Structure"


From his "QEG" quantization of General Relativity ("quantum Einstein gravity") he conjectures in a different direction that there could be some possible long distance effects that might account for some of the postulated "Dark Matter"

this just came out today:
http://www.arxiv.org/abs/astro-ph/0509163
Do we Observe Quantum Gravity Effects at Galactic Scales?
M. Reuter, H. Weyer
6 pages, 1 figure. Talk given by M.R. at the 21st IAP meeting "Mass Profiles and Shapes of Cosmological Structures", Paris, July 4-9, 2005; to appear in the proceedings

"The nonperturbative renormalization group flow of Quantum Einstein Gravity (QEG) is reviewed. It is argued that there could be strong renormalization effects at large distances, in particular a scale dependent Newton constant, which mimic the presence of dark matter at galactic and cosmological scales."

 

[john baez]

Loops '05

I believe the job of the Loops05 conference at Potsdam in October is to forge an alliance of research programs into a field called "Nonperturbative Quantum Gravity" so that there can be faculty positions in NQG in physics departments at various major universities.

Really? Who told you that? I'm on the organizing committee and I never heard anything about that. I thought we were just going to talk about physics.

Well that sounds serious and dignified, and even historical, but everybody knows the stakes are high and in its own way it means there is going to be a circus maximus.

I think the conference will be interesting, but I doubt there will be "mud wrestling" of the sort you find on blogs and newsgroups. Most of the people working on alternatives to string theory know each other, and a lot of us are old friends. To take an example, Renate Loll was part of the quantum gravity team at Syracuse University back in 1992 when I first visited Ashtekar, Lewandowski, and Smolin there - back before they moved to Penn State! We've all shared plenty of beers since.

So, while some of us have left loop quantum gravity and set up approaches that are slightly different (spin foams) or more drastically so (causal dynamical triangulations), the arguments are usually friendly. Not always, but usually.

On the other hand, if it would help more people get seriously interested in physics, I'd be glad to challenge Carlo Rovelli to a duel at dawn on Potsdamer Platz. I think higher categories are more important than group field theory for a deep understanding of spin foam models. He thinks the opposite. Clearly we should fight to the death.

The difference is Fay does not go to the limit.

You mean she's unwilling to die for her cause?

She has all these, like, beads or beans of spacetime with each one being exactly equal to the Planck volume, arranged in a partial ordering (causality) with a certain finiteness condition (which is automatically satisfied in Renate's triangulated manifolds). So it is like she was going to do Renate's thing but at a certain size or scale of simplex, at a certain point, she STOPPED MAKING IT SMALLER.

Oh, right. Yes, Fay Dowker does not go to the continuum limit. Historically speaking, this is because she's a follower of Rafael Sorkin's approach, which he takes locally finite causal sets as fundamental - basically for philosophical reasons. But, Sorkin taught at Syracuse when Ashtekar, Lewandowski and Smolin were there - in fact Sorkin is still there - and Smolin has advertised Sorkin's virtues in "Three Roads to Quantum Gravity". So don't expect Fay Dowker and Renate Loll to start a fist-fight in the auditorium aisles.

I think we should try to understand this issue. Why is it so important to so many Quantum Gravitists that spacetime be in some fashion or other discrete?
What is at the root of this urge or drive or intuition or feeling that it ought to be that way?

For a few people like Rafael Sorkin it's a philosophical conviction born of many old problems with the concept of infinity.

For most of the loop quantum gravity folks, it's the result of doing calculations and discovering that observables like area have a discrete spectrum in certain models! This lent credence to the idea that quantum theory could make certain quantities discrete in quantum gravity, just as it did for atomic spectra. It's a cool idea which is easy to like, since it uses the same math as the quantum mechanics of angular momentum: the stuff we all learned in grad school. But, it doesn't come from an inherent distaste for the continuum. Indeed, quantum theory uses the complex numbers all over the place. And if the rotation group gets replaced by the Lorentz group (as it does in the Barrett-Crane model) and the spectra turn out to be continuous in the end (as they might), well, that's fine as long as we get a background-free quantum theory that reduces to general relativity in the classical limit.

It's this last goal, I believe, that unifies most people who'll be at Loops '05.

Actually didn't Newton and Leibniz have it out over some issue or other?

Newton hated Leibniz's guts because he thought Leibniz stole the idea of calculus from him:

Newton vs. Leibniz: the Calculus Controversy

Newton could be quite nasty. He quarrelled with many people, including Hooke and Halley.

 

[john baez]

Loops '05

As an exercise to see how well (if at all) I understand the nonperturbative QG scene, given that the Loops 05 organizers have published a tentative list of speakers but not the titles of the talks, I expanded the list into a possible program. The first attempt at this was a few posts back.

As indicated earlier, in cases where person has recently come out with SEVERAL papers and I cannot choose, or where for whatever reason there is no clear pick, then I leave that speaker name in BRACKETS.

Does anyone have any inside information or guesses they want to add?
Do any of the speakers themselves want to help out and actually tell us?

Sure, why not?

This is reckless speculation of course, they could change the list of speakers from what i found at the site...

The list of speakers has been settled on, and the program went up today, with abstracts - at least for those speakers who provided them:

LOOPS 'O5- THE PROGRAMME

So, check it out.

Since I'm one of your bracketed "mystery speakers", take a look at my abstract. However, I may change the slant of my talk somewhat, to spend time explaining Rovelli's new work (getting gravitons out of spin foams) and some calculations Greg Egan has just done.

Basically, I want to explain the most important new work on spin foams, show people how it fits together, and talk about where we stand on the big nasty hard open problem - namely, getting a theory that reduces to general relativity in the large-scale limit.

 

[Kea]

I'd be glad to challenge Carlo Rovelli to a duel at dawn on Potsdamer Platz. I think higher categories are more important than group field theory for a deep understanding of spin foam models. He thinks the opposite. Clearly we should fight to the death.

Swords or pistols? Let's see. Spectators on the Baez side, I'm guessing, include Barrett, Blau, Dijkgraaf, Dowker, Girelli ... and cheering for Rovelli: Ashtekhar, Bojowald, Dittrich, Freidel, Lewandowski ... Mmm. More even than I would have guessed. Great! Quite a number of potential Cat Theory supporters at Loops 05.

 

[Kea]

debate at Loops 05

John

Perhaps Loops 05 could timetable a debate or panel discussion, like they had at Strings 05. Topics might include "GFT vs Cats"!

Cheers
Kea

 

[john baez]

Loops '05

Perhaps Loops 05 could timetable a debate or panel discussion, like they had at Strings 05.

Cheers
Kea

Lee Smolin suggested devoting Wednesday afternoon to a general discussion of "where do we go from here?" We had something similar at the conference last year at the Perimeter Institute. But Thomas Thiemann, representing the local organizers, pointed out that it will be a grueling week. This afternoon has been set aside for relaxation and tourism.

I suspect that Lee and others will try to organize some smaller informal discussions during this time. That'll probably be more productive, in fact.

 

[Chronos]

Lee Smolin suggested devoting Wednesday afternoon to a general discussion of "where do we go from here?" We had something similar at the conference last year at the Perimeter Institute. But Thomas Thiemann, representing the local organizers, pointed out that it will be a grueling week. This afternoon has been set aside for relaxation and tourism.

I suspect that Lee and others will try to organize some smaller informal discussions during this time. That'll probably be more productive, in fact.

I think you hit the nail on the head, Dr. Baez. I really admire Dr. Smolin's work. I think he has made a tremendous effort to bring the best minds from all schools of thought into the same room. I think you have too, albeit in a more subtle fashion. Hooray Beer!

 

[marcus]

Re the spirit of cooperation between lines of research (instead of competition) this post by John Baez yesterday on Peter Woit's blog is too a propos not to quote:

----quote JB on NEW---
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---

if anyone would like to check out the whole discussion it is at
http://www.math.columbia.edu/~woit/wordpress/?p=251#comments
where this post is #53 in the comments.

the links JB gave there, which I have colored blue, are as follows.

"this" is The Universe from Scratch (we have a thread on that at PF) 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 report from Marseille conference, which I think we also have a thread on that he started.
http://math.ucr.edu/home/baez/week206.html