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

[marcus]

Another aspect of the current status of the program is the rate of research publication. New researchers have been getting in and the rate increasing:
Here's another index we've been tracking:
LOOP RESEARCH BY YEAR (loop quantum gravity, loop quantum cosmology, spin foam)

2005 http://inspirebeta.net/search?ln=en...2y=2005&sf=&so=a&rm=citation&rg=25&sc=0&of=hb (42 found)
2006 http://inspirebeta.net/search?ln=en...2y=2006&sf=&so=a&rm=citation&rg=25&sc=0&of=hb (77 found)
2007 http://inspirebeta.net/search?ln=en...2y=2007&sf=&so=a&rm=citation&rg=25&sc=0&of=hb (120 found)
2008 http://inspirebeta.net/search?ln=en...2y=2008&sf=&so=a&rm=citation&rg=25&sc=0&of=hb (142 found)
2009 http://inspirebeta.net/search?ln=en...2y=2009&sf=&so=a&rm=citation&rg=25&sc=0&of=hb (145 found)
2010 http://inspirebeta.net/search?ln=en...2y=2010&sf=&so=a&rm=citation&rg=25&sc=0&of=hb (152 found)
2011 http://inspirebeta.net/search?ln=en...2y=2011&sf=&so=a&rm=citation&rg=25&sc=0&of=hb (139 as of 26 Sept, annualized > 180)

To illustrate, http://howlonguntil.net/ 268 days, so in this case annualized rate is 139*365/268 = 189

 

[marcus]

...

I think we have to add this August paper "On the structure" to our list. I hate to increase the number of pages of the "review of current status" but it is only 7 pages so here we are

That would make the combined essential "current status" review be
1102.3660 + 1108.2258 +1105.2212 + 1108.0832
Zakopane lectures+Emergence of gravity+Cosmological constant + On the structure
Rovelli + Magliaro Perini + Han + Rovelli
33 pages +6 pages +6 pages + 7 pages

That brings us up to 52 pages. I have been thinking for some time that I should include "On the structure" in our current status review. But Tom said at the outset that he did not want a LIST of papers, he wanted something like a single review paper compact package. So I was reluctant to include this one. Four papers begins to look like a list
...

In September two more papers appeared which significantly advance the current status. Now we have a problem. Are there any of the original four that we can drop because now superseded? How to keep this current status picture compact?

The two important new papers are by Dittrich et al and by Bianchi Ding.

Bianchi Ding seems to pretty much take care of the Regge limit of LQG. Could it maybe replace the Magliaro Perini "Emergence of gravity" 1108.2258 that we have here? Let's not do that. Let's include ALL these papers for the time being and then maybe edit the list down later.

Dittrich et al pursues the ideas of coarse-graining and numerical analysis---basically understanding how to do extensive lattice computations with spin nets and foams.
http://arxiv.org/abs/1109.4927" Coarse graining methods for spin net and spin foam models
It is a beautiful paper: thoughtful and lucid, and at the same time driving forward. Or so I think anyway---just a bystander's impression.

First let's look at Bianchi Ding http://arxiv.org/abs/1109.6538" Lorentzian spinfoam propagator. A key paragraph is at the bottom of page 1:
"Our main result is the following. We consider the limit, introduced in [13] and discussed in [9, 10], where the Barbero-Immirzi parameter is taken to zero γ → 0, and the spin of the boundary state is taken to infinity j → ∞, keeping the size of the quantum geometry A ∼ γj finite and fixed. This limit corresponds to neglecting Planck scale discreteness and twisting effects, at large finite distances. In this limit, the two-point function we obtain exactly matches the one obtained from Lorentzian Regge calculus [38]. We therefore extend to Lorentzian signature the results of [13]."
Reference [13] is a 2009 paper by Bianchi Magliaro Perini.

 

[marcus]

For the moment I'm finding it hard to cover the current LQG status in a concise compact way. Given the potential importance of the September paper by Eugenio Bianchi and You Ding, I want to add it to our short list of papers. We may be able to edit the list down later but at present I do not see how.

That makes the combined essential "current status" review consist of:
1102.3660 + 1108.2258 + 1109.6538 + 1105.2212 + 1108.0832
http://arxiv.org/abs/1102.3660"
Rovelli + Magliaro Perini + Bianchi Ding + Han + Rovelli
33 pages +6 pages +13 pages +6 pages +7 pages

and brings us up to 65 pages.

 

[tom.stoer]

marcus, thanks for the time spent for discussing and compiling this list.

You are right, when looking at you September poll it becomes clear that it's hard to vote for one specific LQG paper (in the past it was hard to vote for one single QG paper, now even for one specific approach there are many interesting new aspects).

Besides the papers you already have in your list I would add Coarse graining methods (which is the first attempt towards Kadanoff's block spin approach in the LQG context; I was waiting for something like that for years), Emergent Braided Matter (which is still an active but unfortunately small and slow research project) and of course Thiemann's papers trying to link spin foams and the canonical approach.

And of course Han's paper on the cc - especially b/c it shows that even the basic algebraic structure to be used is still under discussion.

 

[marcus]

hurrah! It's good to have your perspective. It is 12:30 here and I am falling asleep, so I will not try to respond. I'd like to ask for some help imagining what sort of calculations might arise using Kadanoff method in LqG context. What might people be calculating, or proving analytically. I am looking forward to re-reading this in the morning.

 

[tom.stoer]

marcus, honestly: do you really think that this is the Current status of LQG? It seems that it is a very active research program, but at the same time the big picture is (partially) missing. I think we don't know (yet) how to fit these puzzle pieces together:
- canonical and covariant formulation
- renormalization in the canonical approach (what is H?), renormalization a la Kadanoff, ...
- asymptotic safety
- cc as running parameter in the asymptotic safety approach, cc as a quantum deformation
- matter on top of LQG vs. emerging braided matter ...
- ...

I am afraid that the situation becomes comparable to string theory: plenty of indications, little hard evidence, no experimental facts. Maybe we are simpy not able to do physics w/o experiments!

 

[atyy]

I am afraid that the situation becomes comparable to string theory: plenty of indications, little hard evidence, no experimental facts. Maybe we are simpy not able to do physics w/o experiments!

I think there has to be an exploration of language so that when new data does come we'll be more ready to describe it.

 

[atyy]

hurrah! It's good to have your perspective. It is 12:30 here and I am falling asleep, so I will not try to respond. I'd like to ask for some help imagining what sort of calculations might arise using Kadanoff method in LqG context. What might people be calculating, or proving analytically. I am looking forward to re-reading this in the morning.

http://arxiv.org/abs/1109.4927" which describes the link between the two: "We describe a simple real space renormalization group technique for two dimensional classical lattice models. The approach is similar in spirit to block spin methods, but at the same time it is fundamentally based on the theory of quantum entanglement." Dittrich et al explicitly say "In this work we will therefore apply the Migdal-Kadanoff scheme [59, 60] and the tensor network renormalization (TNR) method [61, 62]."

 

[marcus]

marcus, thanks for the time spent for discussing and compiling this list.

You are right, when looking at you September poll it becomes clear that it's hard to vote for one specific LQG paper (in the past it was hard to vote for one single QG paper, now even for one specific approach there are many interesting new aspects).

Besides the papers you already have in your list I would add Coarse graining methods (which is the first attempt towards Kadanoff's block spin approach in the LQG context; I was waiting for something like that for years), Emergent Braided Matter (which is still an active but unfortunately small and slow research project) and of course Thiemann's papers trying to link spin foams and the canonical approach.

And of course Han's paper on the cc - especially b/c it shows that even the basic algebraic structure to be used is still under discussion.

marcus, honestly: do you really think that this is the Current status of LQG? It seems that it is a very active research program, but at the same time the big picture is (partially) missing. I think we don't know (yet) how to fit these puzzle pieces together:
- canonical and covariant formulation
- renormalization in the canonical approach (what is H?), renormalization a la Kadanoff, ...
- asymptotic safety
- cc as running parameter in the asymptotic safety approach, cc as a quantum deformation
- matter on top of LQG vs. emerging braided matter ...
- ...

I am afraid that the situation becomes comparable to string theory: plenty of indications, little hard evidence, no experimental facts. Maybe we are simpy not able to do physics w/o experiments!

There is a lot of truth in what you say. First of all, as you indicate, LQG is a research program. When we try to describe the current status of LQG we are talking about the status of that program

That covers a number of different initiatives, some more active than others. Some approaches can drop out of sight for a while---seem hardly to exist---and then regain prominence and importance.

For me, the picture goes through periods when it looks focussed and coherent, and then other times when it seems more fragmented and in flux.

I can't serve as anything more than an onlooker with my own personal impressions, so you mustn't take it too seriously when I say that I don't see much future for some things that both you and other smart informed people see as interesting. But I see OTHER new formulations that I think could challenge the dominant "Zakopane" version.

I think the expression is "dark horse"----the contender nobody noticed was even in the race.
This alternative formulation intrigues me right now: 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.

I suppose that this Topological Defect approach is not automatically unfriendly to braids. But one could also see them as two rival approaches, both very much on the periphery with only a few people currently attending to them. Bad luck that PIRSA, out of all its great collection of video lectures, just managed to lose or spoil this one, or that it just happens to be the one that my computer cannot read.

 

[marcus]

I think there has to be an exploration of language so that when new data does come we'll be more ready to describe it.

http://arxiv.org/abs/1109.4927" are in the spirit of Kadanoff's "block spin renormalization"...

I think you are right about being more ready. It does not matter if the LQG program has several different approaches being worked on. Not all the pieces need to be connected all times. What matters is that somebody has an approach which they are willing to say "This is the theory." and which they can calculate with and confront with observation.

In the case of Loop Gravity, I expect modeling of the cosmological bounce, calculating features of the CMB ancient light, and confrontation with polarization data from whatever mission comes after Planck. (Or possibly even with data from the current Planck mission.)

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?

Maybe it could lead to simulations of the bounce? Or or of black hole collapse?

I'm convinced that Dittrich's work will play a critical role, I just wonder what that role might be, more specifically. How do you picture it?

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

 

[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?