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

[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.