Exploring the Current Status and Open Issues in LQG: A Comprehensive Review

[tom.stoer]

I try to read all relevant papers regarding LQG, but of course for an outsider this is no longer not possible, therefore somehow I lost track when it comes to certain special topics.

Some years ago authors like Smolin wrote review articles regarding current status, open issues and research topics. In addition there where of course the two books by Rovelli and Thiemann which provided an excellent introduction and a a thorough exposition of the formalism.

This is something I am missing today.

Rovelli has written some overview articles, e.g. his "Zakopane lectures", but these are not very exhaustive. For special topics there are no review articles available, especially articles already taking into account the spin foam formalism.

So my question is whether there is a recent review article (or a new book in preparation) covering (some of) the following topics including relevant open issues:
- definition and status of path integral and canonical formalism plus their relation
- open (or solved) issues regarding Hamiltonian, dynamics, regularization, off-shell closure of constraint algebra, ...
- meaning, value, ... of the Immirzi parameter
- different classes of spin networks, different vertices / intertwiners (higher SU(N) and/or higher dimension)
- different graphes w/ and w/o dual triangulation
- quantum deformation, status of the cosmological constant
- matter coupling for fermions and gauge fields, gauge fixing for other gauge fields, SUSY / SUGRA
- renormalization, summing / refining, "block-spin" method
- horizons, surface Hilbert spaces, holographic principle
- definition of observables
- construction of coherent states, semiclassical limit, propagators, ...
- phenomenology

 

[marcus]

In a period of rapid development it can be that the "current status of LQG" is most clearly shown in the biannual conference. Perhaps this can partially take the place of a long review article, or an up-to-date book.

All the Madrid plenary talks, plus the round-table discussion held on Saturday 28 May, have video mp4 available, but I cannot get the sound to work on the computer i am using.
http://www.iem.csic.es/loops11/

Maybe you would have better luck with the sound. Please let me know if you get sound with the video.

All the Madrid talks have the slides PDFs available---not only the plenary but also the parallel session talks.
========================

I have just been reading the slides PDF of plenary talks by Thomas Thiemann, Hanno Sahlmann, and Jerzy Lewandowski.

I find Sahlmann's slides very interesting. The organizers had a good a reason to make his talk the first on the first day (Monday 23 May). Fortunately the slides communicate well even without the other media.

Thiemann's slides also have understandable content. He writes the important statements out in full on the slide, like the motivation for doing higher dimensions and SUGRA in LQG. He says that Sugra is actually the "stepchild" of LQG. He gives a lot of history going back more than 10 years.

It is possible that Thiemann's recent series of 7 or 8 papers will change the field by enabling linkage with developments beyond SM in QFT.

Lewandowski's slides by themselves do not communicate much to me. I expect he had important things to say but instead of writing out what he wants to say he makes the slides serve as an outline of topics: listing what he is going to talk about. To understand anything, you would have to hear the talk (which I cannot) as well as viewing the slides. However I would say there is clearly a shift in the field back towards the canonical side, and Lewandowski's talk is about this, and you can guess some things by looking at the slides even if they are just a skeleton. Anyway that's what I think.

 

[marcus]

I put two Thiemann papers on the MIP poll. Each one actually stands for a series of papers that came out at around the same time. I could not put all 7 or 8 papers in the poll.
One is
Towards Loop Quantum Supergravity (LQSG)

(I think this is important. There are two other papers in the LQSG series.)

The other Thiemann paper on the poll is
New Variables for Classical and Quantum Gravity in all Dimensions I. Hamiltonian Analysis

It is the first in a series of four. The others are
New Variables for Classical and Quantum Gravity in all Dimensions II. Lagrangian Analysis
New Variables for Classical and Quantum Gravity in all Dimensions III. Quantum Theory
New Variables for Classical and Quantum Gravity in all Dimensions IV. Matter Coupling
https://www.physicsforums.com/showthread.php?t=512103

This see-saw dialog between the covariant and the canononical----the spinfoam and the Hamiltonian---reminds me of the homely analogy of walking with two feet. To make progress you don't always have the same foot in front.

BTW there is also something very interesting happening on the spinfoam side that did not appear at the May conference. There is a paper by Jon Engle that just came out this month. It could precipitate a change the EPRL, which he co-authored as Mr "E".
http://arxiv.org/abs/1107.0709

I am often reminded (by what I see happening) of the longterm goal of Lqg program which is to discover how to do backgroundless-QFT. Rovelli made that explicit in his 2004 book, also in his talk at strings-2008, and I suppose on many other occasions.
The short-range goal is gravity/geometry, but matter is gradually being put into the picture (Lewandowski shows particular and persistent interest in this.)
I think that longterm goal will influence what happens. This is one reason that Wilczek's talk interested me so much.
If an abbreviation is needed for "no prior geometry" QFT would this do? [STRIKE]PG[/STRIKE]QFT
or just !QFT.

 

[mitchell porter]

Can LQG describe a particle moving through space?

 

[marcus]

Can LQG describe a particle moving through space?

Only if the particle is a graviton The graviton propagator (2-point function) has been derived. The most recent paper on this dealt with 3-point functions. Tom Stoer may know more.
This paper has references to earlier work:
http://arxiv.org/abs/1105.0566
Euclidean three-point function in loop and perturbative gravity
Carlo Rovelli, Mingyi Zhang
(Submitted on 3 May 2011)
We compute the leading order of the three-point function in loop quantum gravity, using the vertex expansion of the Euclidean version of the new spin foam dynamics, in the region of gamma<1. We find results consistent with Regge calculus in the limit gamma->0 and j->infinity. We also compute the tree-level three-point function of perturbative quantum general relativity in position space, and discuss the possibility of directly comparing the two results.
16 pages

BTW the link to the MIP poll that I gave in the preceding post
https://www.physicsforums.com/showthread.php?t=512103
has this, among other things:
Towards Loop Quantization of Plane Gravitational Waves
Franz Hinterleitner, Seth Major
http://arxiv.org/abs/1106.1448
The polarized Gowdy model in terms of Ashtekar-Barbero variables is further reduced by including the Killing equations for plane-fronted parallel gravitational waves with parallel rays. The resulting constraint algebra, including one constraint derived from the Killing equations in addition to the standard ones of General Relativity, are shown to form a set of first-class constraints. Using earlier work by Banerjee and Date the constraints are expressed in terms of classical quantities that have an operator equivalent in Loop Quantum Gravity, making space-times with pp-waves accessible to loop quantization techniques.
14 pages

It's faintly suggestive, but the answer is that I can't say with complete confidence. Of course you meant matter particles. I suspect anything in that direction so far is very rudimentary.

 

[tom.stoer]

marcus, don't get me wrong: I am not asking for research papers but for one review article covering these topics.

 

[mitchell porter]

Of course you meant matter particles.

I did, but the graviton propagator might still be useful for my dabbling in "loop Vasiliev gravity".

 

[marcus]

marcus, don't get me wrong: I am not asking for research papers but for one review article covering these topics.

You mentioned the books by Rovelli and by Thiemann, and the comprehensive review articles that one used to see. The field was moving more slowly then, with fewer active people. I don't know of any comparable book or review article that is up-to-date.

I tried to respond to your request in post #2.
The best overview I can think of is to look at the Loops 2011 conference itself. Especially the invited talks. If the organizers do a good job, a conference can be like an up-to-date review article that surveys all the active research areas of a field, with representative talks. The organizers of a conference play a role very much like editors compiling a book.

I mentioned the videos of Sahlmann's talk, Thiemann's... Lewandowski's...
Have you tried to watch any of them? For me it's frustrating because I still can't get the audio on my computer. The slides are interesting but I would like to hear the talk that went along with them.

There is clearly a lot of new research. Not covered by the latest survey article which I think is 1012.4707. The best substitute for a review article would be to listen to half a dozen or so of the plenary lectures from that conference!

 

[marcus]

I tried to suggest this in post #2 but apparently was not very clear about it: since there is currently no comprehensive survey article or book covering all of the research in the Loop program we have to make do with the Madrid conference as a substitute.
http://www.iem.csic.es/loops11/

There are 19 invited speakers. These 19 invited talks are like the chapters of a book.
As with any anthology, each person will pick what chapters they want to read. With my present interests, I would pick perhaps ten "chapters" to look at.

Note that in each case the video and slides PDF are available onine. Also the proceedings will be published online (I think also free of charge but I could be mistaken about that)
http://iopscience.iop.org/1742-6596

Here is a sample of eleven "chapters" of the book Loops 2011. I leave out interesting talks by several people---e.g. Loll on CDT, Oriti on GFT---with a few exceptions this is only a bare minimum concentrated specifically on LQG and LQC topics.

Abhay Ashtekar (Penn State University, USA).
Recent Advances in Loop Quantum Cosmology.
Bianca Dittrich (Albert Einstein Institute, Germany).
Towards a large scale limit of (quantum gravity) models: baby spin foams and nets.
Kristina Giesel (Louisiana State University, USA).
Dynamics of Loop Quantum Gravity.
Jerzy Lewandowski (Warsaw University, Poland).
Recent Advances in the Canonical LQG.
Tomasz Pawlowski (University of New Brunswick, Canada).
Quantum dynamics and semiclassicality in LQC.
Alejandro Pérez (Univ. Méditerranée, Marseille, France).
Black Hole Entropy and SU(2) Chern Simons Theory.
Carlo Rovelli (Univ. Méditerranée, Marseille, France).
The covariant version of Loop Quantum Gravity: definition of the theory, results, open problems
Hanno Sahlmann (Asia Pacific Center for Theoretical Physics, Korea).
New insights in quantum geometry.
Lee Smolin (Perimeter Institute, Canada).
The Principle of Relative Locality.
Thomas Thiemann (Univ. Erlangen-Nurnberg, Germany).
New Variables for Classical and Quantum (Super)gravity (LQSG) in all Dimensions.
Madhavan Varadarajan (Raman Research Institute, India).
The Diffeomorphism Constraint operator in Loop Quantum Gravity.

We can further pare this down if we exclude applications such as Cosmology, Black Holes, simplified models such as those studied by Dittrich and Giesel, Smolin's Relative Locality, and anything that is not recognizable as narrowly defined core LQG. That brings it down to 5 talks. I guess these five taken together could serve as a LQG "review article":

Jerzy Lewandowski (Warsaw University, Poland).
Recent Advances in the Canonical LQG.
Carlo Rovelli (Univ. Méditerranée, Marseille, France).
The covariant version of Loop Quantum Gravity: definition of the theory, results, open problems
Hanno Sahlmann (Asia Pacific Center for Theoretical Physics, Korea).
New insights in quantum geometry.
Thomas Thiemann (Univ. Erlangen-Nurnberg, Germany).
New Variables for Classical and Quantum (Super)gravity (LQSG) in all Dimensions.
Madhavan Varadarajan (Raman Research Institute, India).
The Diffeomorphism Constraint operator in Loop Quantum Gravity.

 

[tom.stoer]

marcus, thanks a lot, but this is not how I try to study these topics; I want to have some paper at hand that I can read, e.g. while travelling; I will listen to some talks, but honestly, I really need some old-fashioned sheet of paper where I can make notes etc.

 

[marcus]

marcus, thanks a lot, but this is not how I try to study these topics; I want to have some paper at hand that I can read, e.g. while travelling; I will listen to some talks, but honestly, I really need some old-fashioned sheet of paper where I can make notes etc.

I like doing that too. I write in the margins and underline and draw arrows etc. Paper is good! You can write page references and reminders on the front and back covers.

I know that there is a Russian Journal called SIGMA that plans to bring out a Special Issue devoted to LQG and LQC. The deadline for submission is August 31, 2011.
http://www.emis.de/journals/SIGMA/
This might contain one or more LQG review articles. Then one could download the PDF and print it out and take it on the train or go sit under a shady tree and mark it up with a ballpoint pen.
The call for papers is here
http://www.emis.de/journals/SIGMA/LQGC.html

But it is a long time to wait for the Special Issue of SIGMA. I suppose it could take 6 months after August 2011, or longer. I will look for some alternative.

BTW I think Rovelli's most recent LQG (which is 1012.4707) was written for a special "cluster" issue of the journal CQG. Maybe that special issue would be a place to look.
This is not the "Zakopane Lectures" you understand, but the December 2010 review article.
http://arxiv.org/abs/1012.4707
(it would already not be covering some new developments because limitations of space and also over 6 months old.)
Here is the final publication in CQG:
http://iopscience.iop.org/0264-9381/28/15/153002
The publication date is 21 June 2011. It is a FREE article, at least for now.

This "cluster" issue has some string QG too. It has various related QG approaches, not only Loop.
It is called CQG volume 28 issue 15 (August).

It could be argued that this 1012.4707 is the best most recent LQG review article since it was invited by CQG to be the LQG review article for its "cluster" issue. But I will still try to see if there is something more recent in a downloadable printout article form.

 

[Jakeman002]

E(8)XE(8) = quantum gravity

 

[qsa]

Why do i feel that all the presentation talks lack luster. Shaky and unsure of themselves and recite like a stale news. And the questions are always answered in a very twisted way. I guess it is just me.

 

[marcus]

Why do i feel that all the presentation talks lack luster. Shaky and unsure of themselves and recite like a stale news. And the questions are always answered in a very twisted way. I guess it is just me.

I haven't been able to get audio yet. It sounds like you have. My two computers are by apple and the browser is Safari. What kind of computer and browser do you use?

When did you download? Maybe there originally was a bug which they fixed.

Which presentations have you watched so far? I can't say anything about "lusterless, shaky, unsure, and stale" because I haven't seen them. Some variation in presentation quality is to be expected, for sure. But before judging I would just like to watch.

http://www.iem.csic.es/loops11/

=============================
EDIT TO REPLY TO UNUSUALNAME following post.

Thanks for the pointer to Sahlmann's review paper. I looked at it early in 2010 and it had slipped my mind. It is quite brief (20 pages plus bibliography/references) but gives a clear-sighted vision. I didn't print it out before---I will now.

 

[unusualname]

tom, you can download the video files and play them back while traveling (with vlc or similar) (eg use videocacheview on windows, or just google for ways to do it on other oses)

Otherwise the proceedings will be freely available sometime after November 1 (deadline for submission) at the iop site marcus linked to. (There are thousands of freely available proceedings at that site btw)

Sahlmann updated his short review article in february this year, not sure if it covers many of your points but it does contain over a hundred references mostly available free too.

http://arxiv.org/abs/1001.4188

 

[qsa]

I haven't been able to get audio yet. It sounds like you have. My two computers are by apple and the browser is Safari. What kind of computer and browser do you use?

When did you download? Maybe there originally was a bug which they fixed.

Which presentations have you watched so far? I can't say anything about "lusterless, shaky, unsure, and stale" because I haven't seen them. Some variation in presentation quality is to be expected, for sure. But before judging I would just like to watch.

http://www.iem.csic.es/loops11/

Sorry, I was just commenting generally. That is why I prefer papers since I can check the details , references and so on. when I watch these presentation I don't feel I get Anything more than the papers, I can skim four papers in the time I watch one of those.

 

[marcus]

Sorry,...

My questions to you were about what kind of computer/browser. I am using apple/safari and I can't get the audio part. I would like to know other people's experience as it may offer clues to what the problem is.

Anybody?

Is anybody else using safari and experiencing/not experiencing problems with the audio? This problem is confined to the Madrid conference, for me.
http://www.iem.csic.es/loops11/

tom, you can download the video files and play them back while traveling (with vlc or similar) (eg use videocacheview on windows, or just google for ways to do it on other oses)

Otherwise the proceedings will be freely available sometime after November 1 (deadline for submission) at the iop site marcus linked to. (There are thousands of freely available proceedings at that site btw)

Sahlmann updated his short review article in february this year, not sure if it covers many of your points but it does contain over a hundred references mostly available free too.

http://arxiv.org/abs/1001.4188

Sahlmann's short review of LQG 1001.4188 looks good! Thanks again for reminding us of it.

 

[atyy]

BTW there is also something very interesting happening on the spinfoam side that did not appear at the May conference. There is a paper by Jon Engle that just came out this month. It could precipitate a change the EPRL, which he co-authored as Mr "E".
http://arxiv.org/abs/1107.0709

Indeed. The meaning of the "semi-classical" results from Barrett et al and Freidel and Conrady have been already in dispute for quite some time - and not by different groups:
http://arxiv.org/abs/0905.4082
http://arxiv.org/abs/1004.4550

After seeing the different sorts of "gravity" produced by string theory and higher spin gravity, I'd really like to know whether http://arxiv.org/abs/1010.5437" exists.

http://arxiv.org/abs/1105.1334" indicates Bahr is going to have a paper on this.

 

[qsa]

My questions to you were about what kind of computer/browser. I am using apple/safari and I can't get the audio part. I would like to know other people's experience as it may offer clues to what the problem is.

to get a better help you must mention version numbers and all. but you can always google for the problem like

apple safari windows media audio problem

something like this can come up

http://hintsforums.macworld.com/archive/index.php/t-44077.html

 

[marcus]

@qsa, thanks for the suggestion. It puzzles me that I have only experienced this problem with the Madrid conference site.

BTW Perimeter just posted this video about incuding matter in the spinfoam picture:
http://pirsa.org/11070005/
Spinfoam Fermions
Elena Magliaro
Abstract: A serious shortcoming of spinfoam loop gravity is the absence of matter. I present a minimal and surprisingly simple coupling of a chiral fermion field in the framework of spinfoam quantum gravity. This result resonates with similar ones in early canonical loop theory...
(6 July 2011)

Most of the questioning from the audience was from Lee Smolin and Laurent Freidel. I thought the probing and challenging was useful.

I was reminded of the gradual/incremental approach to unification by this post of OHWILLEKE in another thread, which I want to keep tabs on as illustrating a kind of reality-based view of progress with simiarities to that presented in a recent talk by Frank Wilczek.

I've been really impressed with the progress that can be made within the Standard Model with a quite minor extension to include four rather than three generations of Standard Model fermions, something that makes possible something very close to unificiation without elaborate and undiscovered SUSY particles or extra dimensions within existing experimental boundaries using nothing more elaborate than an SU(5) framework.

It is one of the most straight foward ways to explain excess CP violation where it is observed and also comports with evidence from the MINOS conference by two different methodologies supporting the existing of more than three generations of neutrinos. P.Q. Hung has made this point in a number of articles among the earliest of which is this one: http://arxiv.org/abs/hep-ph/9712338 entitled "Minimal SU(5) Resuscitated by Long-Lived Quarks and Leptons." In this scenario: "SU(3)xSU(2)xU(1) gauge couplings converge to a common point of approximately 3.5x10^{15} GeV (corresponding to a proton lifetime of approximately 10^{34 plus/minus 1} years)." Updates to this original insight in this 1997 paper by Hung have been made by Hung and others to reflect theoretical refinements and new experimental data at least as recently as 2011 in this paper: http://arxiv.org/PS_cache/arxiv/pdf/1102/1102.3997v1.pdf

An example of how this framework could explain excess CP violation in B and kaon decay is found at http://arxiv.org/PS_cache/arxiv/pdf/1011/1011.2634v1.pdf

Along the same lines, a five generation model could explain neutrino mass in a satisfactory way. http://arxiv.org/PS_cache/arxiv/pdf/1106/1106.0415v2.pdf

I am also impressed with the prospects for finding links between the CKM and PMNS mixing matrixes and the masses of the fermions (perhaps via relationships between the square roots of these masses), that could give us better insights into the fundamentals at work in both of these constants, as explored, for example, in this doctoral dissertation: http://cp3.irmp.ucl.ac.be/upload/theses/phd/goffinet.pdf

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

The downside of relying on a brief review paper is that you can get the impression that LQG is purely about gravity/geometry. Indeed the short-term program goal is completely focused on gravity/geometry (as Tom has pointed out ) e.g. Carlo Rovelli has repeatedly made that point.

However it's equally obvious that the longterm goal is to include matter and geometry in the same LQG Hamiltonian and in the same Spinfoam amplitude. Who can say what will happen, or will not happen, when this is done? The point of proceeding this way is that it keeps the unification program simple, gradual, reality-based, and geared as much as possible to experiment and cosmological observation.

 

[marcus]

tom, you can download the video files and play them back while traveling (with vlc or similar) (eg use videocacheview on windows, or just google for ways to do it on other oses)

Otherwise the proceedings will be freely available sometime after November 1 (deadline for submission) at the iop site marcus linked to. (There are thousands of freely available proceedings at that site btw)

Sahlmann updated his short review article in february this year, not sure if it covers many of your points but it does contain over a hundred references mostly available free too.

http://arxiv.org/abs/1001.4188

Tom asked an important question. I'm afraid I gave a scattered and impractical answer at first, partly because I had been unable to listen to the video lectures from the May conference. This has been remedied: Now it is much more clear how to respond. Because Rovelli's Madrid talk is so complete (if you use both the video and the slides PDF) one only needs that one talk.

Thanks to Qsa and Unusualname and others for helping with the audio (there was a simple solution called VLC). I have now downloaded and listened to five of the Madrid talks. Their thumbnail symbols sit like icons on the "desktop" screen of my laptop. The VLC symbol sits in the "dock" at the bottom of the screen. I can instantly watch any desired part of any lecture just by dragging it to the VLC. It automatically opens, and I slide the time-button to the desired place on the timebar. It is like flipping through pages of a printed article---very convenient.

For a review of Current status of LQG, which Tom asked about, one only needs to study the one talk by Rovelli. That is what it is: an authoritative (multimedia) review article. To have paper printout in addition, to go along with it, it would be nice if we could print out the PDF file of the slides. That way one would have something to mark up. But so far I cannot find a reasonable way to print out the slides.

If one had a printout of the slides then at any especially important page of the PDF, one could write down the corresponding time (like minute 25) in the talk that corresponds to it and then easily return to that point in the talk for review. If one had the slides PDF printed out one could also annotate, and write in arxiv numbers of those articles referenced in the talk which might be of particular interest.

I have watched four other talks (by Ashtekar, Sahlmann, Thiemann, Lewandowski) and can recommend them for special topics e.g. Ashtekar reviews Loop cosmology, Thiemann D>4, ... But if one simply wants a balanced authoritative review of the current status of LQG it is enough to study the one talk by Rovelli. The presentation is exceptionally clear and well-thought---this is essentially what Laurent Freidel remarked immediately afterwards, in his comment from the audience.

 

[unusualname]

Just to clarify, the Rovelli talk is this one:

The covariant version of Loop Quantum Gravity: definition of the theory, results, open problems.

mp4 and pdf download links are provided (which is nice :-) ) (right-click and choose "save link as" or similar)

The other talks are here (some have video links, most have pdf links to slides)

 

[tom.stoer]

Looking at Rovelli's main open problems on slide #62 I find some of my questions - but Rovelli's list is much shorter than mine.

OK; I have to check individual papers and wait for further reviews.

 

[marcus]

Just to clarify, the Rovelli talk is this one:

The covariant version of Loop Quantum Gravity: definition of the theory, results, open problems.

mp4 and pdf download links are provided (which is nice :-) ) (right-click and choose "save link as" or similar)

The other talks are here (some have video links, most have pdf links to slides)

I found more concise forms of the direct-link URLs. The ones we got from the site have a lot of redundant information which is ignored. These "lean" thinned down URLs give the same results:

Ashtekar's talk (for a Loop cosmology review):
http://loops11.iem.csic.es/loops11/index.php?option=com_content&view=article&id=181
Rovelli's talk (for status and open problems of the full LQG theory):
http://loops11.iem.csic.es/loops11/index.php?option=com_content&view=article&id=76

Both of these have links to both the slides PDF and the video.

For the listing of all the talks (linked to an abstract for each):
http://loops11.iem.csic.es/loops11/index.php?option=com_content&view=article&id=75

All you need to write down the URL is to know the number of the talk:
Thiemann 179
Lewandowski 180
Ashtekar 181
Corichi 186
Sahlmann 172
Dittrich 178
Rovelli 76
...
...

 

[marcus]

I see that we have forgotten to update this thread Tom Stoer started even though it has the title "Current Status" and people consult it now and then. Just today I noticed this happening.

I hope others will help update--it could be a useful thread. I'll try to add some things to bring it up to date.

The main thing now, I think, would be to mention two August papers, or maybe four or five (now I think of it!)

One would definitely the new (August) version of the current review article on Lqg.
Zakopane lectures http://arxiv.org/abs/1102.3660

This paper cites the new Magliaro Perini that was recently published in EPL and deals with the double scaling limit.
Emergence of gravity from spinfoams http://arxiv.org/abs/1108.2258

Another paper that is new and making considerable use of work on the double scaling limit is
On the structure...http://arxiv.org/abs/1108.0832

Another paper, which is potentially important but where we need to wait and see if the results are confirmed, is this one by Ghosh Perez
Black hole entropy and isolated horizon thermodynamics http://arxiv.org/abs/1107.1320

 

[marcus]

Tom particularly asked for papers he could print out, mark up, and read on the train. In other words, the video of Madrid conference talks don't suffice. This makes sense to me (I like to mark up printout, write in the margins etc.)

When this thread was started I did not know of a satisfactory bunch of papers that would give a summary of the current status. But now the situation has improved. In the preceding post I gave links to papers that either appeared first in July-August 2011, or if they appeared earlier in 2011 had been substantially revised in July-August. Based thereon I would say briefly:

1. Lqg now has a definitive formulation.

2. Evidence is mounting that this definitive version has the right limits.

3. Considerable effort is directed at seeing how to test loop cosmology with early universe data.

I think the most valuable part of this thread, so far, is the post#1 list of questions to be answered. Tom is right that the Zakopane lectures are not exhaustive---they cover some but not all of these questions---and some, like the meaning of the Immirzi parameter, are the subject of interesting current research! Maybe we can find answers to some of these questions, and indicate ones that involve work in progress. Here is Tom's list:

==quote==

So my question is whether there is a recent review article (or a new book in preparation) covering (some of) the following topics including relevant open issues:
- definition and status of path integral and canonical formalism plus their relation
- open (or solved) issues regarding Hamiltonian, dynamics, regularization, off-shell closure of constraint algebra, ...
- meaning, value, ... of the Immirzi parameter
- different classes of spin networks, different vertices / intertwiners (higher SU(N) and/or higher dimension)
- different graphs w/ and w/o dual triangulation
- quantum deformation, status of the cosmological constant
- matter coupling for fermions and gauge fields, gauge fixing for other gauge fields, SUSY / SUGRA
- renormalization, summing / refining, "block-spin" method
- horizons, surface Hilbert spaces, holographic principle
- definition of observables
- construction of coherent states, semiclassical limit, propagators, ...
- phenomenology
==endquote==

 

[marcus]

Tom asked about the current status of LQG and I've pointed mainly to the expanded August version of the current review of loop gravity: http://arxiv.org/abs/1102.3660
and mentioned some shorter auxiliary "helper" papers that go along with that review. (the Magliaro Perini is just 9 pages.)

I'd like to add another suggested auxiliary resource. This is in two parts. Winston Fairbairn's talk at the ILQGS about the quantum group introduction of the cosmo constant.
The interesting and timely thing is that Hanno Sahlmann just posted a essay on the work of Fairbairn and Meusburger this month (3 August) at the ILQGS blog. It gives some extra intuition about what is an important recent development. (finiteness, convergence as well as incorporating the cosmological constant.)

Here is the page that has both Hanno's essay and links to the AUDIO and SLIDES PDF for Winston's talk:
Quantum Deformation of 4D Spin Foam Models
http://ilqgs.blogspot.com/2011/08/quantum-deformations-of-4d-spin-foam.html

The Fairbairn Meusburger paper this talk is based on is Rovelli's reference [11] in his current review of loop gravity:
http://arxiv.org/abs/1012.4784
Quantum deformation of two four-dimensional spin foam models
Reference [12] is to subsequent work by Muxin Han in the same area:
http://arxiv.org/abs/1105.2212
Cosmological Constant in LQG Vertex Amplitude
==quote Muxin Han conclusions==
To summarize, in this paper we propose a new q-deformation of the Euclidean EPRL/FK spinfoam vertex amplitude. The concrete construction uses the evaluation of the Vassiliev invariant from 4-simplex graph. We also show that the asymptotics of the q-deformed vertex amplitude gives the Regge gravity with a cosmological constant (from Regge calculus using flat 4-simplices) in the regime that the physical scale of the 4-simplex is much greater than the Planck scale l_p but much smaller than the cosmological length l_c.
==endquote==
For anyone not familiar with it, the cosmological length l_c, given by Λ = 1/l_c², is the length scale associated with the cosmo constant Λ.

Like the Magliaro Perini paper just published in EPL, that I mentioned in post #25, Muxin Han's paper is only 6 pages, so maybe we'll take those as our add-on helpers to the main review 1102.3660. I want to keep any additional material brief.

That would make the combined essential "current status" review be
1102.3660+1108.2258+1105.2212
Zakopane lectures+Emergence of gravity+Cosmological constant
Rovelli +Magliaro Perini + Han
33 pages +6 pages +6 pages
On the basis of this overview, I'd sum up the essentials by saying loop is now a definite theory and evidently finite with the right limits. The loop research community has grown in size and shows an active interest in testing.

 

[marcus]

I wonder if Carlo Rovelli's few brief postings on PHYSICS STACK EXCHANGE could be a useful window on the current status of Lqg.

Here is the link:
http://physics.stackexchange.com/users/1613/carlo-rovelli

 

[Fra]

==quote==
So my question is whether there is a recent review article (or a new book in preparation) covering (some of) the following topics including relevant open issues:
- definition and status of path integral and canonical formalism plus their relation
- open (or solved) issues regarding Hamiltonian, dynamics, regularization, off-shell closure of constraint algebra, ...
- meaning, value, ... of the Immirzi parameter
- different classes of spin networks, different vertices / intertwiners (higher SU(N) and/or higher dimension)
- different graphs w/ and w/o dual triangulation
- quantum deformation, status of the cosmological constant
- matter coupling for fermions and gauge fields, gauge fixing for other gauge fields, SUSY / SUGRA
- renormalization, summing / refining, "block-spin" method
- horizons, surface Hilbert spaces, holographic principle
- definition of observables
- construction of coherent states, semiclassical limit, propagators, ...
- phenomenology
==endquote==

What do you guys think about the way the open questions are connected, and which that are more fundamental (and thus might resolve the others as spin offs once solved)?

I find it interesting that all research fields tend to list "open questions" where some of them are reall conditional upon wether you've comitted to the program already. While some "open questions" implicitly may question the program itself.

I'm curious what you guys think are the key questions in LQG? Ie. the ones that is better focused on first?

My own view is that I always get hung up on Rovellis IMO incomplete analysis of the observers role in a theory, and in particular the inferential status of the observer invariants in the context of a scienne. How to define observables not only mathematically, but in a way that is inferrable and computable by an actual inside observer.

Indeed ST and ordinary QFT also has problems with this, but it's different.

I just feel that some questions are of such profound importance, since all the other questions are built upon them that I think more focus should be brought to them.

For example a simple think: What exactly is the observational connection of the transition amplitudes between spin network states in LQG? More specifically, how is the probability and probability space encoded by the observing system? It seems, it isn't. And I recall that Rovelli for some reason thinks this isn't important. But I really don't understand how one can be comfortable in that position. It freaks me out.

/Fredrik

 

[marcus]

Hi Fra,
I just posted this at N.E.W.
The issue is that so far LHC sees no sign of susy or extra dimensions and one could say that this is good news for loop gravity because the current version of the theory is distinctly 4D and not-sugra. Does that make sense to you?

Here is what I posted, in response to a comments by Shantanu and Giotis:

==quote==
Shantanu: So Marcus, how then does LQG address the dark matter problem? or is the dark matter have nothing to do with electroweak scale according to LQG?
==========================
Giotis: It is widely accepted that LQG is not incompatible with SUSY/SUGRA and the papers you mentioned just prove this point…
LQG has nothing to say about physics at LHC; it can’t even derive GR at the classical limit.
==========================
There is now a kind of standard formulation, by far the most prevalent in terms of the number of researchers using it. This has developed since 2007. Thiemann’s “Towards LQSG” papers (towards loop sugra and extra dimensions) are along a different line and are preliminary.
I merely watch the field and don’t speak with authority. All I can say is that LHC seeing extra dimensions (and I think also supersymmetry) would be a severe setback for the program—equivalent to losing 4 or 5 years of work.

I’m talking about the current *prevailing version* of LQG summarized here:
1102.3660+1108.2258+1105.2212
Zakopane lectures+Emergence of gravity+Cosmological constant
Rovelli +Magliaro Perini + Han
33 pages +6 pages +6 pages
On the basis of this I’d say this version of loop is now a definite theory and much evidence points to it being finite with the right limits.

Current loop gravity has a definitive concise (one-page) formulation which is explicitly 4D and which is not sugra. If it had to include extraD and sugra it would probably have to be drastically modified. That is why Thiemann’s 2011 series of a half dozen papers says “towards”. I would accept that the basic ideas and philosophy of loop gravity could be be adapted in a new formulation. That’s credible. But it would mean throwing out the current version that a lot of people have worked on developing over the past 4 or 5 years.

Shantanu, thanks for asking about Dark Matter. I haven’t heard anything conjectured about DM from the loop gravity researchers—the people whose research is actually focused primarily on loop—that I can recall. You can see the progress that has been made in including fermions and Yang-Mills fields if you look at the August update of 1102.3660, the current review paper. Maybe the approach could accommodate susy matter but not sugra! I don’t know how that would be resolved. I think from a loop standpoint DM is simply somebody else’s problem. But I just watch from the sidelines. If I’m missing something (and DM is being addressed within loop context) please let me know.

If I were doing loop gravity research I would be feeling relieved and happy that LHC is not seeing either susy or extra-D. It increases the chances that the current approach is on the right track.
==endquote==

 

[Lee Smolin]

Dear Marcus,

I was about to post on NEW to reply to you. Often I am sympathetic to your comments, but this time I am afraid I agree with the others. Supersymmetry and supergravity are very easily included in LQG and spin foam models and were a long time ago. N=1 supersymmetry and supergravity are completely straightforward, there is no difficulty, nor does there seem to be any new result that requires N=1 supersymmetry. This is why the topic has not been much pursued. The literature on the inclusion of supergravity into LQG began with an early paper of Jacobson extending our action for the Ashtekar variables to supergravity. There are papers by Pullin and collaborators which were followed by several papers around 2000 by Yi Ling and myself extending spin networks to N=1 supergravity. We also made progress on 11 dimensional supergravity. I don't right now recall who wrote the several papers on extending spin foam models to supergravity.

Historically LQG has roots in supergravity. The Ashtekar-Sen form of the constraints was first found by Sen studying supergravity. An early very significant use of the Ashtekar connection is in Witten's proof of positive energy in general relativity, which was partly inspired by arguments of Deser and others (if I recall right) on the positivity of the hamiltonian in supergravity.

The really interesting question would be extending LQG and spin foam models to extended supersymmetry,and supergravity ie N=2 and higher, where the algebras are much more interesting and more constraining. This would be necessary to compare directly results on black hole entropy with string theory. The only one I know who has worked on this is Yi Ling, but his results remained unpublished.

There are several ideas which have been studied to incorporate the standard model in some interesting way in LQG and spin foam models. To my knowledge none of them so far make any predictions for the LHC.

Thanks, Lee

 

[Fra]

The issue is that so far LHC sees no sign of susy or extra dimensions and one could say that this is good news for loop gravity because the current version of the theory is distinctly 4D and not-sugra. Does that make sense to you?

I suppose it does but from my perspective those issue while certainly unimportant still comes out as problems built on questionable stances to deeper questions - this is what disturb me. And I'm not even sure these questions would appear once the deeper stances are made. This is why I am more motivated to start with what I think are core problems.

From a pure inference point, it seems dimensionality should be explained. After all, all it is, is an index for abstract distinguishable events. But I don't think starting at 10 or 11 and compactify to 4 is the way. I rather think that we should start from 0, let the continuum emerge and then dimensions. Sometime like causal set style starting points.

/Fredrik

 

[Fra]

Dear Smolin, it's very nice to see you post here!

I've very much enjoyed some directions you engaged in, I'm thinking about your thinking wrt evolving law and your cooperation with R. Unger. Your two perimeer talks on the subject I'm aware of have been extremely thought provocing in a good way.

I must say I find a lot of that, and in particular a lot of Ungers points to be at face with a lot of the structural realism in LQG.

Since you worked in both, how do you merge this two apparently diverging research directions? I find this somewhat paradoxal. Are they simply two diverging views that you like to entertain, or is there hidden connection I haven't understood?

/Fredrik

 

[tom.stoer]

Dear Lee!

it's a pleasure to see you here in the 'beyond forum'!

Just a short note: the guy recently working on n-dim. SFs and LQG with SUGRA is Thiemann from Erlangen, Germany.

Tom

 

[marcus]

Since you mentioned Thiemann's current work on D+1>4 Spinfoams, let me quote from the abstract of http://arxiv.org/abs/1105.3703
"Loop Quantum Gravity heavily relies on ... Unfortunately, this method is restricted to D+1 = 4 spacetime dimensions".

And from page 2:
"... Of course, a connection formulation is also forced on us if we want to treat fermionic matter as well. A connection formulation for gravity in D + 1 > 4 that can be satisfactorily quantised, even in the vacuum case, has not been given so far. For the case D + 1 = 4, it was only in 1986 that Ashtekar..."

And from page 3.
"In this paper, we will derive a connection formulation for higher dimensional General Relativity by using a different extension of the ADM phase space than the one employed in [13, 25] and which generalises to arbitrary spacetime dimension D + 1 for D > 1. It is based in part on Peldan’s seminal work [26] on the possibility of using higher dimensional gauge groups for gravity as well as on his concept of a hybrid spin connection..."

Setting the question of SUSY aside, what I was saying in my previous post #20 was Yes LQG could be adapted to higher D if we did see evidence of extra dimensions, but it would be a SETBACK---I guessed it would be like losing the last 4 or 5 years of work.
(see my post https://www.physicsforums.com/showthread.php?p=3476917#post3476917 )
So loop researchers can express LEGITIMATE SATISFACTION that evidence of extraD has not shown up.

Now I may be wrong when I make a similar guess about SUSY! But I am skeptical of any suggestion that incorporating supersymmetry in the current version of spinfoam LQG would be automatic.

If I remember right I've seen papers from before 2005 that stated that LQG, as the author conceived of it then, would accept supersymmetry. But the theory has changed remarkably in the past 4 years of so, and has reached a definitive formulation (1102.3660) with considerable evidence indicating it has the right limits.
I have to allow for the possibility that the present formulation, representing some 4 years of work, would have some catch or present some stumbling block to SUSYfication.

So absent some published research to the contrary, I have to remain skeptical of what I think Lee is saying. LQG has not stayed the same. Just because somebody back before 2005, say, thought there would be no problem formulating LQG (as he imagined it then) with arbitrary D and with supersymmetry, does not mean that you could do that with the version which has developed over the years 2007-2011.

So I can understand how, despite what Lee says, a currently active loop researcher could find encouragement in the fact that there are no signs of SUSY yet. That was basically my point at NEW.