What's happening with Loop? (new potential challenges)

[tom.stoer]

The FGZ paper is a great attempt to discuss as much as possible on the classical level, i.e. identifiying issues already at the classical level, and seperating them clearly from quantization issues. It shows clearly that there is a kind of discretization not related to quanization. It shows how to reconstruct spacetime.

It explains that there are two gauges, namey the LQG and the SF gauge! This questions the LOST theorem b/c
a) there may be even more gauge choices available
b) they may be missed by LOST just as the SF gauge was (which sems to be uncritical as this is a kind of duality)
c) there may very well be different and inequivalent quantizations

Looking at standard QFTs usually one fixes the dynamical variables, fixes a local symmery i.e. implements some constraints and quantizes the theory (or the other way round). In GR / LQG it seems that there are several steps where different dynamical variables are used, several steps where local symmetres are fixed (gauge, spacelike diff., ...) and it is by o means clear how many different choices and orders there are, at which point in time one should quantize and whether all these different steps and limits do commute.

 

[Diffeomorphic]

Well, I sure hope Shape Dynamics turns out as it seems to have a bright future. I had the privilege to talk with Sean Gryb for a few days when he was at PI and he was really excited about what him and the other people like Tim Koslowski had planned for it. He was really was astounded that within the SD formalism GR became re-normalizable, and he was talking about constructing a quantum theory of SD with some collaborators. I'm pretty excited that I get to publish a paper with him. :D

 

[marcus]

Well, I sure hope Shape Dynamics turns out as it seems to have a bright future. I had the privilege to talk with Sean Gryb for a few days when he was at PI and he was really excited about what him and the other people like Tim Koslowski had planned for it. He was really was astounded that within the SD formalism GR became re-normalizable, and he was talking about constructing a quantum theory of SD with some collaborators. I'm pretty excited that I get to publish a paper with him. :D

That's absolutely wonderful! What great news! Here's wishing you every possible success in your collaboration with Sean Gryb.

BTW, since other people may not have seen this, I recall that you introduced yourself in a Academic or Career thread (I forget which) as having a dual honors major at U. Calgary in Astrophysics and Appl. Math. and after finishing first year you went to Perimeter summer program. So it you and Sean would have had some conversations at PI this summer. Is there a PIRSA talk that describes the *SD renormalizable* result which Sean was excited about? Any particular PIRSA video you want to recommend, or a specific part of a particular SD paper? I can often use pointers. And about SD I don't know alot.

 

[tom.stoer]

Besides SD we should add AS.

There are recent attempts to apply AS to Ashtekar-like / first-order formalism of GR including the Holst-term and to study the scaling of G, Lambda and the IP.

 

[marcus]

Besides SD we should add AS.

There are recent attempts to apply AS to Ashtekar-like / first-order formalism of GR including the Holst-term and to study the scaling of G, Lambda and the IP.

Good point. A paper that appeared yesterday illustrates what you said about the Immirzi parameter (IP) coming up in Asymptotic Safety gravity:

http://arxiv.org/abs/1111.1000
Running Immirzi Parameter and Asymptotic Safety
Jan-Eric Daum, Martin Reuter
(Submitted on 3 Nov 2011)
We explore the renormalization group (RG) properties of quantum gravity, using the vielbein and the spin connection as the fundamental field variables. We require the effective action to be invariant under the semidirect product of spacetime diffeomorphisms and local frame rotations. Starting from the corresponding functional integral we review the construction of an appropriate theory space and an exact funtional RG equation operating on it. We then solve this equation on a truncated space defined by a three parameter family of Holst-type actions which involve a running Immirzi parameter. We find evidence for the existence of an asymptotically safe fundamental theory. It is probably inequivalent to metric quantum gravity constructed in the same way.
To appear in the proceedings of CORFU 2010

 

[tom.stoer]

It's interesting that the IP seems to run to 0 or infinity in AS. This seems to be natural when defining LQG - but then something is strange with area eigenvalues (no observable quantities!) and BH entropy (which scales with the IP).

In the end I wouldn't trust the area law as a physical prediction; it's an interesting property of the eigenvalue spectrum, but the area operator is not a physical (Dirac) observable. In the same sense I wouldn't trust the isolated horizon construction for BHs. A careful investigation of the rG flow in LQG is still missing (here AS seems to be ahead of LQG).

...

 

[tom.stoer]

...

An interesting challenge to LQG could be this:

a) There are several different approaches (can. LQG, SF, AS, CDT, ...) with comparable or nearly equivalent predictions in the UV which are NOT testable (running of spacetime dimension to D=2, ...)
b) All these approaches reproduce GR in the IR

Then there seems to be no good reason to believe in one specific approach as being fundamental; fapp one would use the simplest one - and it could very well be that this is NOT LQG!

 

[marcus]

...

An interesting challenge to LQG could be this:

a) There are several different approaches (can. LQG, SF, AS, CDT, ...) with comparable or nearly equivalent predictions in the UV which are NOT testable (running of spacetime dimension to D=2, ...)

Perhaps you disagree for some reason that would be interesting to hear! but I see several different approaches as markedly different in the UV.

By the UV, I mean how they handle the BB and inflation.

When I think of the UV predictions of canLQG (HamiltonianLQG) it is certainly not the first thing that comes to mind to think of "running of spacetime dimensions to D=2"

I think of Ashtekar et al work on the naturalness of inflation (partial independence from exotic "inflaton" field). Detailed prediction of the Hubble constant behavior through a regime where it momentarily achieves Planck scale levels. Relating LQG bounce to WMAP data. Some further prediction by Grain Barrau and others.

CDT does not do the bounce.

What about AsymSafe? Well Stephen Weinberg tried working on AS Cosmology. I was hoping he would get somewhere! He sounded hopeful in 2009. He reported frustration in 2010 (at the College Station meeting). We heard nothing from him about it in 2011.

In 2009 it was possible he would have come up with an AS bounce cosmology, with predictions of a footprint in CMB which might have been testably different from the Loop bounce footprint. But so far I have heard no result.

Research output in CDT seems to have declined. AsymSafe is showing little progress, or anyway such is my impression.

It is always nice to have rival theories! LQG has had a changing cast of rivals. In 2005-2007 Reuter and Loll starred with rival AS and CDT. But now suddenly the featured rival is Shape Dynamics---that is what made the big splash at Loops 2011. Hard to predict how this all will develop.

I am very interested to know how Shape Dynamics will be quantized, and when it is quantized how it handles the BB!

 

[tom.stoer]

marcus, LQG is about 25 years old, all the other approaches are (much) younger and less developped (due to less researchers); don't be too hasty.

I am still not convinced that there is one unique LQG theory; there are indications that LQG/SF is a family of related but not necessarily equivalent theories.

 

[marcus]

marcus, LQG is about 25 years old, all the other approaches are (much) younger and less developped (due to less researchers); don't be too hasty.

I am still not convinced that there is one unique LQG theory; there are indications that LQG/SF is a family of related but not necessarily equivalent theories.

I have never said, or thought, there was one UNIQUE LQG theory, myself. It seems obvious there is not, to me at least.
I have noted that there is one DEFINITE theory that anyone can define in about one page. And that has been done. One gives a concise formulation and after about one page one says "That is the theory". So there is something to test.

So Tom, I am not surprised that you are "still not convinced" there is a unique. It would really astonish me if you said you were convinced!

As I recall both CDT and AsymSafe go back to about 1998. The first Ambjorn Loll collaboration, the first Martin Reuter paper.
What you say about "due to less researchers" is very significant, I think.
I have been watching both since around 2005. and much of that time have had a lively hopeful interest. I could easily now be wrong, but my sense of them is that they have made valuable discoveries and contributed to understanding and both programs are in decline.
Having gone about as far as they can. Most likely I am wrong, but that is how it seems to me. Interest shifting to Shape Dynamics, and possibly Group Field Theory/"pregeometry" (whatever that means!)

Good advice not to be hasty. I will try not to be

 

[tom.stoer]

I have never said, or thought, there was one UNIQUE LQG theory, myself. ... I have noted that there is one DEFINITE theory ...

OK, good point, I got it!

... that they have made valuable discoveries and contributed to understanding and both programs are in decline ...

Difficult to say ...

Interest shifting to Shape Dynamics, ...

I have to learn more about that approach.