What is the Current Status of Loop Quantum Gravity and Its Open Issues?

[atyy]

I know that Dittrich et al is largely about Migdal-Kadanoff method and they say a lot about the ability to calculate. What I am curious about is calculating WHAT? Can you or anyone help me imagine what kind of massive lattice calculation might be in view? Can you see how this might connect up with CMB observations, for instance, or with some other data?

Within Rovellian aesthetics (which I'm not fond of), it'd be used to calculate the semiclassical limit - the limit that Bianchi, Perini, Magliaro, Ding etc are now trying to look at by taking the Immirzi parameter to zero. Rovelli's latest review indicates he understands this is kludgey, and a more proper method is needed.

The other thing philosophy which Dittrich seems open to - a more Smolinesque or at least Perimeterish inclination - is that maybe there's a link between the http://arxiv.org/abs/1010.5437" proposals.

BTW Atyy, Eugenio Bianchi in his 21 September PIRSA talk referred to something of XG Wen.
It is on one of those slides. I checked: slide 23/24 on page 46/48 of the PDF

Thanks, I'll check it out!

 

[marcus]

PIRSA has fixed the link to Flash video of Bianchi's 21 September talk.Or at least for some reason I can now watch. However my connection seems to be slow. I have to give the streaming a "head start" by pausing. Perhaps this video is experiencing heavy demand.

This is a new way of developing LQG. It is more interesting than I realized earlier, in 2009, when Eugenio first proposed it. It is a new answer to the question "Why loops?" A new rationale for proceding towards QG this way.

Here is the Flash link, if you are interested:
http://pirsa.org/displayFlash.php?id=11090125

There is also an audio MP3 and a slides PDF. So one can forego the video and just listen and scroll through the slides. Here is my earlier post, when I had only seen the slides.

...
http://arxiv.org/abs/0907.4388 . it may be something that you noticed and commented on two years ago, but I did not realize at the time was interesting.

What sparked my interest in this alternative Lqg formulation is this recent talk:
http://pirsa.org/11090125
given 21 September, titled Loop Gravity as the Dynamics of Topological Defects

...Loop states measure the flux of the gravitational magnetic field through a defect line. A feature of this reformulation is that the space of states of Loop Gravity can be derived from an ordinary QFT quantization of a classical diffeomorphism-invariant theory defined on a manifold. I'll discuss the role quantum geometry operators play in this picture, and the [prospect] of formulating the Spin Foam dynamics as the local interaction of topological defects.​

This talk is based on 0907.4388 and has 24 very good slides (in fact my computer cannot get the Flash video so all I can see are the slides!) Slide 22 has an interesting point:

Appealing scenario for Quantum Gravity
No trans-Planckian d.o.f. because topological (and therefore finite) at small scales
at larger scales, finitely many d.o.f. which can be described effectively in terms of a local quantum field theory.​

In the Perimeter pdf, slides are repeated so there are 48 pdf pages and this slide #22 does not appear until page 41. So you have to scroll down a lot to get there.
...

The talk is only 45 minutes. It is followed by 13 minutes of Q/A. For me the audio is not very loud, so I cannot follow the questions. I can only hear the answers. Some of the questions are quite long, so I am ready to dispense with the final 13 minutes. But the 45 minute talk is very good and I will certainly watch it again.

 

[tom.stoer]

Actually there is more to quantum gravity than UV problems, as certain problems do not depend on the UV completion at all. Moreover it is not even clear whether there are serious UV problems in the first place - due to the phenomenon of classicalization. Some of these issues are going to be discussed here:
http://ph-dep-th.web.cern.ch/ph-dep-th/content2/THInstitutes/2011/QG11/QG11.html

I am still trying to collect further information regarding Nicolai's talk. Can you comment on some aspects?