What is the recent development of Loop Quantum Gravity

[Karmerlo]

Recently, I am very interested in Loop Quantum Gravity. But I hope I can know more about the recent development of Loop Quantum Gravity. I mean the development from 2000 to 2011. Any conceptual or practical or technical development in this realm?


Further more, I do not know the relationship between Carlo Rovelli's Quantum Gravity and Thomas Thiemann's Modern Canonical Quantum General Relativity. They looks so different.

Thank you very much.

 

[Karmerlo]

I wish I could see some referent on recent development of this elegant theory.

 

[marcus]

There are some links here:
https://www.physicsforums.com/showthread.php?p=3597267#post3597267

A recent formulation of the theory is presented here, by Rovelli:
http://arxiv.org/abs/1102.3660

Another thing to look at, and see if you can find parts of it you can understand, is a 90-page account of recent spinfoam approach by Livine:
http://arxiv.org/abs/1101.5061
On the whole I think it is too hard to be an introduction for beginners, but you should know it is there (and a few parts might be helpful.)

There are simpler easier introductions than this, by other people. One for example by Hanno Sahlmann, he is a nice bright young fellow who got his PhD under the direction of Thomas Thiemann. As I recall he uses lots of pictures. I think his presentation is perhaps not so complete and not so hard as the one by Rovelli.

Do you know how to use ARXIV.ORG? To find a Sahlmann paper you go to
http://arxiv.org and you click on "search"
and you type Sahlmann into the author box.
It will give you Sahlmann's papers and there will be a recent one called something like "LQG, a short review"

There is a Sahlmann VIDEO about "new insights" in LQG. He was asked to give an opening talk at the regular Loops 2011 conference. Google "Loops 2011" and you will get the conference website, and click on "scientific program" and you will see a link to Sahlmann's talk. It might be helpful to watch the video. It reviews very recent work. He is a good clear presenter.

There is also a video talk by Rovelli at the same conference. You will see how to click on it if you decide you want to watch. It will be harder and more complete, but might still be helpful.

If you have any trouble, ask for help.

To save you time here is the link for Sahlmann "LQG, a short review" http://arxiv.org/abs/1001.4188

There are other introductions for beginners, like that, by others of the young people.
For example there is this:
http://arxiv.org/abs/1007.0402
called Introductory Lectures to LQG, by Dona and Speziale.
If Sahlmann is not right for you, and if Rovelli is too hard, then ask again and I or someone else will find other links. My personal opinion is that Sahlmann 1001.4188 is not up-to-date. I would only use something from 2011. But that is partly a personal bias of mine, and you might find his treatment to be just right for you. One has to begin somewhere!

 

[Karmerlo]

Hi Macus:

Do you have any recommendations about Loop Quantum Gravity Ph.D program? There are quite a lot of String/Brane research programs, but Loop Quantum Gravity research programs are rarely heard of. I only know Penn State, and Louisiana State in U.S, Waterloo University in Canada. Is there any more?

Thank you very much.

 

[marcus]

Do you have any recommendations about Loop Quantum Gravity Ph.D program? There are quite a lot of String/Brane research programs, but Loop Quantum Gravity research programs are rarely heard of. I only know Penn State, and Louisiana State in U.S, Waterloo University in Canada. Is there any more?
...

First of all where are you geographically and where do you want to be? I assume your first language is English. They use English at several good LQG centers in continental Europe. Plus there's Nottingham in the UK. Have you thought about grad school in Europe?

I'm guessing you are in Usa, and prefer staying in North America. Waterloo in Canada would be great.
If you like California there's Steve Carlip's eclectic QG program at UC Davis. He has phd students working in several different QG approaches. I have high respect for him and his approach to the subject.

Maybe I shouldn't try to answer in a complete way until I hear more from you.

Nottingham (John Barrett) has set up a one-year Masters program which can have a QG focus preparing you for LQG/spinfoam PhD research. The program just started this year, so I have not seen any results. But it seems like a very solid program, good way to get started.

Steve Carlip has somebody working in Shape Dynamics (close cousin of LQG) named Henrique Gomes. Gomes coauthors with Tim Koslowski at Perimeter. There is a fair amount of traffic between Perimeter and Davis. Carlip also has a PhD student working specifically in Loop, and I think also someone doing CDT, if I remember right.

You already know about the programs at Penn State and at LSU, I gather.

there are other smaller programs with just one main person but I won't try to give an exhaustive reply at this point. Still wondering where you are and what you want and why you aren't considering European centers as well.

 

[Karmerlo]

First of all where are you geographically and where do you want to be? I assume your first language is English. They use English at several good LQG centers in continental Europe. Plus there's Nottingham in the UK. Have you thought about grad school in Europe?

I'm guessing you are in Usa, and prefer staying in North America. Waterloo in Canada would be great.
If you like California there's Steve Carlip's eclectic QG program at UC Davis. He has phd students working in several different QG approaches. I have high respect for him and his approach to the subject.

Maybe I shouldn't try to answer in a complete way until I hear more from you.

Nottingham (John Barrett) has a one-year Masters which can have a QG focus preparing you for LQG/spinfoam PhD research. It's new so I have not seen any results. But it seems like a very solid program, good way to get started.

Steve Carlip has somebody working in Shape Dynamics (close cousin of LQG) named Henrique Gomes. Gomes coauthors with Tim Koslowski at Perimeter. There is a fair amount of traffic between Perimeter and Davis. Carlip also has a PhD student working specifically in Loop, and I think also someone doing CDT, if I remember right.

You already know about the programs at Penn State and at LSU, I gather.

there are other smaller programs with just one main person but I won't try to give an exhaustive reply at this point. Still wondering where you are and what you want and why you aren't considering European centers as well.

Thanks Macus. I still want to put Europe in my wishlist.

 

[marcus]

Thanks Macus. I still want to put Europe in my wishlist.

Great! Then you could start doing some research into what programs are offered and what you need in order to apply to get in.

Of course you could simply WRITE to Steve Carlip at UC Davis, and or others and ask what prerequisites they would like to see. It would be educational to learn what they are looking for, and also give you a chance to pick up courses that they think are valuable.

But some of that information is probably already online.

Have a look at the prerequisites for entering John Barrett's Masters program at Nottingham.
The link is in the "introduction to LQG" thread.
https://www.physicsforums.com/showthread.php?p=3597267#post3597267

Here is the link for the Quantum Gravity group at Nottingham:
http://www.nottingham.ac.uk/mathematics/research/groups/mathematical-physics/quantum-gravity.aspx

If you get into the oneyear masters program then I expect a large part of that year will be devoted to a "masters thesis" research paper that is done within the QG group. John Barrett leads that group. He is in contact with the whole LQG community and I imagine the master's program is good practice for succeeding in a PhD program in LQG.
My guess is that he would be a good person to place you in a PhD program that is right for you, whether in Europe, the UK, or North America. I can't really help since I don't know you.

Depending on where you are, there are people you could go talk with.

Jon Engle at Florida Atlantic
Jorge Pullin at LSU
Steve Carlin at Cal Davis
any of numerous great people at Perimeter and at Penn State

In fact there is this world map that Francesca created:
http://maps.google.com/maps/ms?ie=U...985216139270436.0004843830d27f3e6c50e&t=h&z=0

 

[tom.stoer]

Regarding conceptual issues have a look at
https://www.physicsforums.com/showthread.php?t=544728
https://www.physicsforums.com/showthread.php?t=545596

 

[marcus]

But the 2010 paper which is the main focus of those threads does not engage what I would call RECENT development of LQG.

The recent formulation I told Karmerlo about in post #3 is that in http://arxiv.org/abs/1102.3660
which does not come up in the 2010 paper you cite.

The first place I read about the new LQG formulation was in a March 2010 paper "A New Look at LQG". They might have discussed it, but didn't. Just included it in their bibliography as reference [10] and made an inaccurate passing reference on page 44.

 

[tom.stoer]

Let's not restart the discussion here, but ...

... the recent reformulation does not solve many old issues!

 

[marcus]

Let's not restart the discussion here, but ...

... the recent reformulation does not solve many old issues!

But how do we know that? I have not seen any critical analysis of 1102.3660 which lays out the issues which are not resolved. Except of course the reservations freely stated by the author himself right in the paper.

In particular Alex'ov and Roche paper does not seem relevant. I don't believe I have expressed my misgivings about it. It does not even seem honestly objective to me.

 

[tom.stoer]

Check

http://arxiv.org/abs/1111.1879
Discretisations, constraints and diffeomorphisms in quantum gravity
Authors: Benjamin Bahr, Rodolfo Gambini, Jorge Pullin
(Submitted on 8 Nov 2011)
Abstract: In this review we discuss the interplay between discretization, constraint implementation, and diffeomorphism symmetry in Loop Quantum Gravity and Spin Foam models. To this end we review the Consistent Discretizations approach, which is an application of the master constraint program to construct the physical Hilbert space of the canonical theory, as well as the Perfect Actions approach, which aims at finding a path integral measure with the correct symmetry behavior under diffeomorphisms.

and Thiemann's papers, of course.

There is a research direction focussing on application of the new models; you will not find discussion regarding conceptual issues by looking only at the applications.

 

[marcus]

http://arxiv.org/abs/1111.1879
Discretisations, constraints and diffeomorphisms in quantum gravity
Authors: Benjamin Bahr, Rodolfo Gambini, Jorge Pullin
(Submitted on 8 Nov 2011)

Now that is a good paper, I think! An even better up-to-date treatment that highlights some fascinating conceptual problems accessible to researchers is the Freidel et al I was discussing earlier.

http://arxiv.org/abs/1110.4833
Continuous formulation of the Loop Quantum Gravity phase space
Laurent Freidel, Marc Geiller, Jonathan Ziprick
(Submitted on 21 Oct 2011)

In this paper, we study the discrete classical phase space of loop gravity, which is expressed in terms of the holonomy-flux variables, and show how it is related to the continuous phase space of general relativity. In particular, we prove an isomorphism between the loop gravity discrete phase space and the symplectic reduction of the continuous phase space with respect to a flatness constraint. This gives for the first time a precise relationship between the continuum and holonomy-flux variables...
27 pages
​

There is a research direction focussing on application of the new models; you will not find discussion regarding conceptual issues by looking only at the applications.

I agree! But why would anyone "look only at the applications"? One of the reasons that LQG research is growing and rapidly attracting new people is precisely because it has a lot of accessible research problems---both conceptual and applied.

 

[tom.stoer]

I agree! But why would anyone "look only at the applications"? One of the reasons that LQG research is growing and rapidly attracting new people is precisely because it has a lot of accessible research problems---both conceptual and applied.

I think the situation is as follows: there are these 'new models' from which physical predictions can be derived; this atracts a lot of interest and researchers. Then there are conceptualdifficulties wihc are addressed by less people - but which are (at least) equally important.

I was in a similar situation when working in a QCD group 20 years ago. QCD was a well establsihed theory, everything was 'standard textbook' - nevertheless there was no convincing idea regardig confinement - and it soon became clear that the well-established quantization failed in the non-perturbative regime. That's why a couple of groups all over the world (Lenz in Erlangen - now Thiemann's chair, Jackiw, O'Rafferty, van Baal, ...) started to develop non-perturbative and canoncal methods.

The number of researchers and the list of publications was rather small compared to numerous people wasting time their in three-loop calculations ...

 

[marcus]

I can see one one could think by analogies. You recall a situation where a few, with vision, worked on the basic theoretical/conceptual problems (in QCD in this case) while there were any who spent a lot of time blindly doing laborious "3-loop calculations" and such like.

Different people will of course apply the analogy to the present situation in different ways.
For example, I see the Freidel Geiller Ziprick (FGZ) October 2011 paper as a conceptual breakthrough especially when coupled with Bianchi's 2009-2010 reformulation of LQG in terms of a flat manifold with topological defects. These developments involve fundamentally new ways of envisioning quantum geometry.

You talk about research that focuses on "applications". I am not sure what you mean by applications. The only active area of application I can think of is to cosmology and other areas where there are potential observations of QG effects. I don't imagine there is a very clear analogy here. I don't minimize the importance of calculating observational consequences that one can look for. The ESA apparently has plans for another CMB mission after the current Planck. The applied calculations are not like laborious 3-loop QCD calculations---not a big investment of man-days.

Anyway, the analogies with the present situation are not straightforward, so different people will probably see them differently.

BTW the Erlangen group has grown and seems pretty strong to me. I will post some links.

 

[marcus]

I think the situation is as follows: there are these 'new models' from which physical predictions can be derived; this atracts a lot of interest and researchers. Then there are conceptualdifficulties wihc are addressed by less people - but which are (at least) equally important.
...

The new models ARE the conceptual breakthroughs which are the fruit of a few people wrestling long and hard with the conceptual issues. The issue of how to think about geometry, in a quantum theory.

BTW if you have been paying attention to the new models (the "polytope", the "aharo-bohm", the "zako" to give them nicknames ) you may have noticed the key role played by Bianchi in all three.

Now I suppose we will see a growing emphasis at Erlangen on what we can call the "new models." There is an impressive bunch of people being gathered there. The QG group is in two parts, one in Math led by Catherine Meusburger, one in Physics led by Thiemann. I'll get some links.
Here is about Meusburger:
http://www.algeo.math.uni-erlangen.de/people/meusburger-catherine/prof-dr-catherine-meusburger/positions.html
She recently sent out this email announcement:
==excerpt==
A postdoc position will be available in the quantum gravity group within the algebra and geometry group at the Department of Mathematics, University of Erlangen-Nürnberg in Erlangen, Germany...
==endquote==
Deadline for application is 15 December. This group is distinct from Thiemann's group.
http://www.algeo.math.uni-erlangen.de/people/meusburger-catherine/prof-dr-catherine-meusburger/research-group.html
A postdoc in Meusburger's group is Winston Fairbairn whom you may know of as a Rovelli PhD and co-author.

Here is about the quantum gravity group in the Physics Department led by Thiemann:
http://theorie3.physik.uni-erlangen.de/people.html

This group has grown by the addition of some strong people who have had experience with various "new models" approaches. Maïté Dupuis who comes there from Lyon, for example.
She has co-authored a lot with Etera Livine, who was her PhD advisor and also one paper with Freidel.

Enrique Borja, who has co-authored with Etera Livine (several) and Freidel (one)

Emanuele Alesci, a Rovelli PhD and coauthor.

It's also interesting that John Baez' student Derek Wise is there.

 

[qsa]

Let's not restart the discussion here, but ...

... the recent reformulation does not solve many old issues!

so what was the purpose of the reformulation then.

 

[marcus]

so what was the purpose of the reformulation then.

Heh heh That may just be a rhetorical question. We'll have to see what Tom says.

 

[tom.stoer]

so what was the purpose of the reformulation then.

To avoid the notoriously difficult Hamiltonian and to provide a tractable formulation from which results (especially in the semiclassical regime) can be derived more easily. The problem is that the underlying conceptual issues are still there but show up in a different (and not so obvious) way.

One issue is this: usually the PI (including vertex and measure) is derived via the Hamiltonian; in the new models this derivation is avoided (intentionally b/c the Hamiltonian itself is still poorely understood). The question remains in which way the dynamics of the SF models is related to the original formulation (our understanding is restricted to the kinematical level).

 

[marcus]

To avoid the notoriously difficult Hamiltonian and to provide a tractable formulation from which results (especially in the semiclassical regime) can be derived more easily. The problem is that the underlying conceptual issues are still there but show up in a different (and not so obvious) way.

One issue is this: usually the PI (including vertex and measure) is derived via the Hamiltonian; in the new models this derivation is avoided (intentionally b/c the Hamiltonian itself is still poorely understood). The question remains in which way the dynamics of the SF models is related to the original formulation (our understanding is restricted to the kinematical level).

I think this is a fair account as far as it goes, but leaves off the conceptual/aesthetic motivation---which I think is a factor both with Bianchi and with Rovelli.

The drive to discover new ways to think the world---new ways to visualize geometry and how it responds to measurement---new quantum concepts of geometry in other words.

I mentioned that as I see it the new models we are talking about are aharo-bohm, polytopes, and zakopane.

A. The aharo-bohm model is based on topological defects embedded in a flat manifold. The curvature lives on the defects. Rovelli discussed it as a side aspect, possible alternate way to see things, in the zako lectures. It's exciting that Freidel adopts it in the FGZ paper.

B. The polytope model (e.g. work by Bianchi) has the nodes of the network be fuzzy indefinite uncertain polyhedra. I find it interesting to imagine space built of such things. Whenever theory has several versions it provides opportunity researchers to learn something by investigating the extent to which they are equivalent or not equivalent. Quantum relativists are growing a new area of imagination.

C. A key step in zako model dynamics, according to Rovelli, was presented at conference by Bianchi in January 2010. It has conceptual elegance. The boundary state is a labeled network of measurements, enclosing a labeled foam of process.
There is this injective map of SU(2) reps into SL(2,C) reps, which they simply denote by the letter f. This map f contains all the calculation. There is a remarkable mental economy here: All the clutter is removed so that one can readily see what is happening.======the rest of this post is just notes on sources========
polytope: http://arxiv.org/abs/1009.3402
Pirsa video: http://pirsa.org/10110052/ "Q'tum polyhedra in LQG"
polytope-related: http://arxiv.org/abs/1011.5628

aharo-bohm: http://arxiv.org/abs/0907.4388
Google "pirsa bianchi" and you get http://pirsa.org/11090125/
"Loop Gravity as the Dynamics of Topological Defects"
aharo-bohm related: http://arxiv.org/abs/1110.4833 (FGZ)

zako history: http://arxiv.org/abs/1004.1780
"I emphasize in particular the fact –pointed out by Eugenio Bianchi [2]– that the dynamics of the theory has a very simple and natural definition, largely determined by general physical principles. It is given by a natural immersion of SU(2) representations into SL(2,C) ones. A simple group theoretical construction (Eq. (45) below) appears to code the full Einstein equations.²"
Reference [2] is to Bianchi's talk at a January 2010 conference at the Sophia-Antipolis campus.
http://wwnpqft.inln.cnrs.fr/previous.html
http://wwnpqft.inln.cnrs.fr/pdf/Bianchi.pdf
"2 Note added in proofs: For a much simpler and straightforward presentation of the dynamics of the theory, which does not require the full intertwiner space machinery, see [3]."
Reference [3] http://arxiv.org/abs/1010.1939 is to a strip-down feynman-rules presentation developed in Moscow, see page 1 of “Simple model for quantum general relativity from loop quantum gravity.”

 

[tom.stoer]

marcus, nearly everything what you are saying is related to the kinematical properties (and this is exactly where canonical LQG and SF are equivalent); but the issues are due to unknown or not well defined properties of the full dynamics (Hamiltonian, vertex, measure, ...)

 

[marcus]

marcus, nearly everything what you are saying is related to the kinematical properties (and this is exactly where canonical LQG and SF are equivalent); but the issues are due to unknown or not well defined properties of the full dynamics (Hamiltonian, vertex, measure, ...)

zako history: http://arxiv.org/abs/1004.1780
"I emphasize in particular the fact –pointed out by Eugenio Bianchi [2]– that the dynamics of the theory has a very simple and natural definition, largely determined by general physical principles. It is given by a natural immersion of SU(2) representations into SL(2,C) ones. A simple group theoretical construction (Eq. (45) below) appears to code the full Einstein equations.²"
Reference [2] is to Bianchi's talk at a January 2010 conference at the Sophia-Antipolis campus.
http://wwnpqft.inln.cnrs.fr/previous.html
http://wwnpqft.inln.cnrs.fr/pdf/Bianchi.pdf
"2 Note added in proofs: For a much simpler and straightforward presentation of the dynamics of the theory, which does not require the full intertwiner space machinery, see [3]."
Reference [3] http://arxiv.org/abs/1010.1939 is to a strip-down feynman-rules presentation developed in Moscow, see page 1 of “Simple model for quantum general relativity from loop quantum gravity.”

The whole idea of the Moscow feynman-rule presentation (1010.1939) was to show how to do the dynamics in a really concise streamlined way. By dynamics, I mean calculate (generalized) transtion amplitudes.

The core idea in the zako formulation is again transition amplitude dynamics. That is the motivation for the setup where you have a labeled network boundary state enclosing a labeled foam process.

Perhaps the most remarkable thing about the zako formulation is that it captures the dynamics in such a simple way---with the injection f: SU(2) reps --> SL(2,C) reps.
I speculate that this was contributed by Bianchi. Rovelli seems to be crediting him with the idea on the first page (third paragraph) of 1004.1780.

 

[tom.stoer]

But the dynamics of SFs can neither be derived nor can it be shown to be equivalent to an underlying canonical structure. In that sense the two approaches are still incomplete.

The problem is that there are two possible ways to check (in a limited sense) whether dynamical structures are 'correct'. Either you can show their equivalence (which we can't for canonical LQG and SFs) or you can for at least one of them show that it agrees with experiments (which we can't, either). In addition the accessable semiclassical sector misses certain underlying features of full QG b/c the limit washes away hbar-corrections and may be blind for off-shell properties of the full theory.

That means that - yes - "... the dynamics of the theory has a very simple and natural definition, largely determined by general physical principles ... and appears to code the full Einstein equations" but that this is by far not sufficient to resolve all conceptual issues and prove that the QG regime itself is described correctly. The Einstein equations are nothing else but a preliminary test for consistency.

marcus, don't get me wrong: the new models are of course a major step forward, but they are still work in progress and it may very well be that they fail be provide a (mathematically) consistent and (physically) correct theory of quantum gravity.

 

[marcus]

marcus, don't get me wrong: the new models are of course a major step forward, but they are still work in progress and it may very well be that they fail be provide a (mathematically) consistent and (physically) correct theory of quantum gravity.

Just to make sure we understand each other, the new models I've been talking about are primarily the one that first appeared in April 2010 with the "New Look" paper that you quote in your post: 1004.1780. And then got a more thorough presentation in http://arxiv.org/abs/1102.3660 . I also mentioned two new approaches by Eugenio Bianchi that I called "aharo-bohm" and "polytopes". But these two are in very early stages. The main new model is the one that came out last year and is most fully presented in the Zakopane paper 1102.3660.

I think we both understand that this is not the same as what is often referred to as "EPRL" or as "EPRL-FK". It's what I've been calling "zako" for short

I agree that the zako formulation of LQG is a major step forward, as you say, and also fully agree that it is work in progress!

Indeed we do not know how physically correct it will turn out to be! Nature must decide that one. Rovelli, who is the main architect of this formulation says explicitly at the end of the main paper on it (1102.3660) that it may be wrong and he urges the researchers for whom the paper is written to try to show it wrong. But that is normal for him, in every survey talk he emphasizes the theory's tentativeness, and that there is plenty of work to be done on it.

That means that - yes - "... the dynamics of the theory has a very simple and natural definition, largely determined by general physical principles ... and appears to code the full Einstein equations" but that this is by far not sufficient to resolve all conceptual issues and prove that the QG regime itself is described correctly. The Einstein equations are nothing else but a preliminary test for consistency.

Indeed! Derivation from classical theory is no guarantee that a quantum theory is right.
And as you say consistency with the (classical) Einstein equations are no guarantee either!
As you say this is just a preliminary test.
That would also be true, obviously, if one were to formulate a canonical LQG, with hamiltonian, and show that it couid be derived from the classical by "quantizing" general relativity. All these things are merely preliminary tests.

The purpose of a theory is twofold---to give a new better way to think the world (in this case a new conceptual picture of geometry and cosmology) and to predict testable new phenomena. If it does not predict anything it is empty fantasy.

But I think you are not right when you say LQG cannot be tested. You say not by "experiments", but I assume you include cosmological observation on the same footing as ground-based experiment. The whole point of LQG, as far as I am concerned, is to predict future early universe observations as more and better instruments are launched into orbit.
That and of course to have a better way to think about (big bang, black hole and other) geometry--improved concepts.

http://arxiv.org/abs/1102.3660

 

[qsa]

but that this is by far not sufficient to resolve all conceptual issues and prove that the QG regime itself is described correctly. .

first, shouldn't the conceptual issues be an output of the theory. second, how do we know if QG regime is described correctly since there is nothing to compare. I guess you mean something else by those two things.

 

[marcus]

I mentioned the issue of predicting new observations. One reason that early universe phenomena are a good "LABORATORY" for LQG and quantum relativity more generally is that quantum effects are associated with high energy density and with small scale, while the early universe bang or bounce is a HUGE MICROSCOPE.

Quantum effects, fluctuations, are enormously magnified and spread across the sky.

So one wants a quantum theory of geometry (interacting with matter) which predicts what we see and will see, as new intruments go into orbit.

This is the reason for many of the papers found by this InSpire search:
http://inspirehep.net/search?ln=en&...Search&sf=&so=d&rm=citation&rg=100&sc=0&of=hb

Earlier I was using a Spires search, but Spires is being turned off and replaced by the new InSpire service. This search gets papers which appeared or were published in the four years 2008-2011.
In conjunction with LQG/LQC it uses the OR of categories "gravitational radiation", "inflation", "power spectrum", "cosmic background radiation", "primordial".
The papers are ranked by citation count.

I want to stress the Empiricism aspect. It is the final criterion. Of course we are happy that LQG seems able to reproduce predictions from the Einstein equation of GR---but ultimately this is of value only because the Einstein equation agrees with observations over a wide range of scales.

 

[qsa]

So what does lqg predict exactly that the new missions should be looking for.

 

[marcus]

So what does lqg predict exactly that the new missions should be looking for.

It doesn't have precise killer predictions on record yet! but it is getting there.
To see what some phenomenologists (not LQG people themselves but the uncommitted people whose business is figuring out how to test theories) have come up with, look at the most highly cited papers in that list that are by phenomenologists like Barrau, Grain, Wen Zhao, and their co-authors.

Do you remember what the next ESA CMB mission (after the current Planck mission) is called? I saw it discussed recently and lost the link.
[EDIT: It may have been canceled. I looked and could only find this:
http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=42839
There is no mission name attached. Just the bare study of a concept.]

Wen Zhao's paper refers to a proposed NASA mission tentatively called CMBpol, which as far as I know has not been funded. But Zhao et al also use data that has already been gathered (e.g. by WMAP) to see what constraints they can derive.

I'll suggest five sample papers, but these are not the most recent you can find using the InSpire search. Ranked by cite-count, these are #1, #7, #11, #12, and #21. Some I've read and some just barely glanced at and thought might interest you.

#1 (cited 43 times) http://inspirehep.net/record/812301?ln=en
Cosmological footprints of loop quantum gravity.
#7 (cited 21 times) http://inspirehep.net/record/798154
The gravitational wave background from super-inflation in Loop Quantum Cosmology.
#11 (cited 18 times) http://inspirehep.net/record/830146
Observational constraints on a power spectrum from super-inflation in Loop Quantum Cosmology.
#12 (cited 18 times) http://inspirehep.net/record/813856
Inverse volume corrections from loop quantum gravity and the primordial tensor power spectrum in slow-roll inflation.
#21 (cited 8 times) http://inspirehep.net/record/861191
Constraints on standard and non-standard early Universe models from CMB B-mode polarization.

BTW one robust prediction of Loop cosmology is a bounce with a period of faster-than-exponential growth called "superinflation" during which the Hubble parameter actually increases very quickly, rather than (as in ordinary inflation) remaining approximately steady or gradually declining. You find this discussed authoritatively in Ashtekar papers. It does not require an inflaton or any exotic physics, it is just built into the Loop bounce. So that is kind of distinctive and I see that a couple of the papers on the list explictly study the possible observational consequences of that superinflation bounce feature.

 

[tom.stoer]

Just to make sure we understand each other, the new models I've been talking about are primarily the one that first appeared in April 2010 with the "New Look" paper ...

OK

Indeed! Derivation from classical theory is no guarantee that a quantum theory is right.
And as you say consistency with the (classical) Einstein equations are no guarantee either!
As you say this is just a preliminary test.

OK

That would also be true, obviously, if one were to formulate a canonical LQG, with hamiltonian, ...

A second, independent derivation of the dynamics + formal proof of equivalence would be more than a preliminary test, it would be a breakthrough.

But I think you are not right when you say LQG cannot be tested. You say not by "experiments", but I assume you include cosmological observation ...

The problem is that most cosmological observation will not address the deep QG regime but the semiclassical one which cannot serve as a litmus test. But this is a generic problem for QG and indicates somehow a paradigm shift away from close interaction between theory and experiment towards predominantly mathematical considerations. This is a major problem in (certain domains of) modern physics and - as far as I can see - there is absolurely no way out!

 

[tom.stoer]

So what does lqg predict exactly that the new missions should be looking for.

Afaik - nothing!

- for high-energy cosmic radiation (GZK cutoff) there is no unambiguous, indisputable prediction (*)
- for energy dependent speed of light there is no unambiguous, indisputable prediction, either (*)
- for effects on light polarization there is no unambiguous, indisputable prediction, either
- LQG footprint in CMB due to primordial gravitational waves is only be derived within LQC (**)
- everything else (black holes, big bang) is neither fully understood nor directly testable

(* there were attempts to derive such effects but afaik the old models suffered from physically incorrect approximations, e.g. weave states which were not exactly in the kernel of the - largely unknwon - Hamiltonian)

(** this sems to be the most promising research area, but afaik it is unclear if full LQG will reproduce the LQC predictions)

 

[marcus]

...
- LQG footprint in CMB due to primordial gravitational waves is only be derived within LQC (**)
...
...

(** this sems to be the most promising research area, but afaik it is unclear if full LQG will reproduce the LQC predictions)

I agree that it is the most promising. It sounds to me like the big issue for you is whether full LQG will reproduce the LQC predictions.

I keep seeing research in that direction which seems to be making progress. So to satisfy you that LQG is as testable as LQC I need to start keeping better track of those particular papers.
==================================

BTW I should report that, contrary to my expectations and perhaps prejudices, when I had a look in the literature just now the FIRST author I found working on Lqg --> Lqc
was a guy working for Steve Carlip at UC Davis. Carlip is an excellent quantum relativist who has PhD students working in several QG including LQG, "shape dynamics", and CDT.

I heard Carlip talk on QG spontaneous dimensional reduction here at Berkeley and hold him in high regard. He has this student Chun-yen Lin.
PhD thesis --- http://arxiv.org/abs/0912.0554 (revised March 2011)
November 2011 followup --- http://arxiv.org/abs/1111.1766
Emergence of Loop Quantum Cosmology from Loop Quantum Gravity: Lowest Order in h
I don't know if the work is good, but I was surprised to see it uses the
canonical Lqg approach. You have to figure that Carlip (who is world class) is guiding this guy. He seems to be still at UC Davis even though probably now post-doc. Here is Chun-yen Lin's conclusion paragraph:
==quote 1111.1766==
This paper starts from the kinematical Hilbert space of loop quantum gravity, which describes the matter fields living in the dynamical quantum geometry of space. Using the model with a modified Hamiltonian constraint operator, we see that the dynamics of such a system reproduces FRW cosmology in the large scale limit. Further, the O(h⁰) corrections of the model for FRW cosmology conform with loop quantum cosmology in a specific scheme. Such a result is valuable, since it attributes the predictions of loop quantum cosmology to the fundamental principles in loop quantum gravity.
The result serves as a starting point to many possible future projects. First, one may explicitly construct the coherent states in the model to evaluate the emergent cosmology beyond O(h⁰), to get the quantum fluctuation corrections in the emergent cosmology. Second, one may try to derive more of the implications of loop quantum cosmology by applying the model to more realistic cosmological settings. Third, one may try to improve the model by incorporating the graph-topology changing feature in the Hamiltonian constraint operator, in the hope of deriving loop quantum cosmological models with μ = μ ̄.
==endquote==
Well nobody is saying that the job is finished! But I see he has made a little progress. He gets a bounce with his Lqg model and estimates the matter density and it higher than the density at bounce that Ashtekar gets in regular Lqc, but at least not grossly lower as one might have feared. Plenty of work left to do here.

And there are also other Lqg --> Lqc papers which I should gather to get an idea of how the research is going on this front.

 

[tom.stoer]

I agree that it is the most promising. It sounds to me like the big issue for you is whether full LQG will reproduce the LQC predictions.

Well, the issue is whether "LGQ: first quantize - then reduce symmetry" is (in a certain approximation) equivalent to "LQC: first reduce symmetry - then quantize".

 

[marcus]

...
- LQG footprint in CMB due to primordial gravitational waves is only be derived within LQC (**)
...

(** this sems to be the most promising research area, but afaik it is unclear if full LQG will reproduce the LQC predictions)

I agree that it is the most promising. It sounds to me like the big issue for you is whether full LQG will reproduce the LQC predictions.

I keep seeing research in that direction which seems to be making progress...

...papers which I should gather to get an idea of how the research is going on this front.

Well, the issue is whether "LGQ: first quantize - then reduce symmetry" is (in a certain approximation) equivalent to "LQC: first reduce symmetry - then quantize".

I'm not sure what you mean by "the" issue. There are probably several issues, some of greater importance. I think of vintage-2006 LQC as an heuristic--eventually to be replaced by full-LQG cosmology (or retained as an approximation if it can be shown useful in that role.)

I see that beginning to happen in a number of papers from the Ashtekar and Marseille groups. They already have some preliminary results indicating a bounce which is LIKE the usual LQC bounce (using spin foam or other LQG with some restrictions which one then tries to progressively relax.)

One does not have to symmetrize first! The assumptions of homogeneity and isotropy can be weakened, gradually. This is a common theme in current research as I expect you know. Various means are used to make the problem tractable.

I judge that it is a "done deal" that the full theory will yield bounce predictions which are, in any case, LIKE, those obtained from the usual LQC. I would not necessarily expect them to be precisely the same, just similar. The Loop bounce is robust. So then the phenomenologists can work with full-LQG cosmology and work out observational tests.

The restrictive version, LQC, would then be relegated to a secondary role---it might continue as a useful approximation or it might not---such details are hard to foresee.

But on conceptual grounds I would say that the "symmetrize first" issue you mention is of only passing or temporary importance. What I consider of prime importance are cosmological tests of the full theory. This involves ongoing work that a number of people are involved with where they use the full theory under restrictions which are (if all continues to go well) progressively relaxed.

In any case that's how I view the main conceptual issues here. I should gather some links to the work on spin foam cosmology and perhaps some of the other papers that relax the traditional uniformity assumptions (iso and homog).

 

[tom.stoer]

With "LQC: first reduce symmetry - then quantize" I mean that in LQC you first constrain the system from infinitly many to finitly many degrees of freedom which you then quantize. It is by no means clear whether the LQG approach where you have to study a symmetric subsector of the full theory containing infinitly many variables will lead to the same predictions.

 

[marcus]

With "LQC: first reduce symmetry - then quantize" I mean that in LQC you first constrain the system from infinitly many to finitly many degrees of freedom which you then quantize..

Of course I understand that, but it's good you mention it in case someone is reading who is new to the subject.

==quote continued==
It is by no means clear whether the LQG approach where you have to study a symmetric subsector of the full theory containing infinitly many variables will lead to the same predictions
==endquote==

Well the trend in LQG/spinfoam cosmology is to relax the symmetry requirement. They get away from string isotropy and homogeneity, and see whether they continue to see a bounce.

I'm not sure why you say you would have to study a "symmetric subsector" of the theory.
That would be if you thought it was important to imitate LQC with the full theory.
As I see it, the main thing is not to imitate or show equivalence to other versions, but to get definite testable predictions from some formulation (like Zako) of the full theory. The most obvious being if you get a bounce. That seems to be the way the research is going.

 

[tom.stoer]

I agree that in the end a symmetry reduction in LQG is not what you really want, but perhaps it's easier to motivate LQC by some clever trick than to derive a bounce in full LQG. If this is the case than the symmetry reduction in LQG can at least provide some hint that the bounce in LQC is reasonable.

A simple example why I think this is important: in QED applied to a hollow sphere you find a casimir force. In QM with a finite number of degrees of freedom no similar effect is known..

 

[marcus]

Recently, I am very interested in Loop Quantum Gravity. But I hope I can know more about the recent development of Loop Quantum Gravity. I mean the development from 2000 to 2011. Any conceptual or practical or technical development in this realm?
...

This is a good sort of question to be asking. What are some significant recent developments? I would not go back so far in history as Karmerlo does. I would focus on 2008-2011---just the past 3 or 4 years. So much has happened!

It may be that Karmerlo found out everything necessary and went away, but other people could be wondering about recent developments and have similar questions. So probably we should try to give at least some partial answers.

I'll try to list some significant advances, particularly in the past two years. Other people may want to suggest other things to put the spotlight on, that I miss.

It's important to see the recent advances as opening up new questions and new areas for researchers to investigate.

LQG is in an exciting phase of rapid growth---the best papers now do not provide final solutions so much as they reveal new thesis problems for PhD students to work on. They open up more questions than they resolve. They stir up more issues than they settle Anyway, that is my impression.

The recent growth in the number of people working in LQG, and the numbers of papers published, seems to support this impression. Objectively speaking the level of activity is several times what it was some 5 years ago. So it's more than just a subjective impression.

Let's try to think of some significant developments. Especially those that open up fresh problems for a new entrant PhD student or postdoc to work on.

 

[marcus]

http://arxiv.org/abs/1005.0817
A regularization of the hamiltonian constraint compatible with the spinfoam dynamics
Emanuele Alesci, Carlo Rovelli
(Submitted on 5 May 2010)
We introduce a new regularization for Thiemann's Hamiltonian constraint. The resulting constraint can generate the 1-4 Pachner moves and is therefore more compatible with the dynamics defined by the spinfoam formalism. We calculate its matrix elements and observe the appearence of the 15j Wigner symbol in these.
24 pages

http://arxiv.org/abs/1110.6150
Regularized Hamiltonians and Spinfoams
Emanuele Alesci
(Submitted on 27 Oct 2011)
We review a recent proposal for the regularization of the scalar constraint of General Relativity in the context of LQG. The resulting constraint presents strengths and weaknesses compared to Thiemann's prescription. The main improvement is that it can generate the 1-4 Pachner moves and its matrix elements contain 15j Wigner symbols, it is therefore compatible with the spinfoam formalism: the drawback is that Thiemann anomaly free proof is spoiled because the nodes that the constraint creates have volume.
4 pages, based on a talk given at Loops '11 in Madrid, to appear in Journal of Physics: Conference Series (JPCS)

A possible breakthough. IMHO. The previous hamiltonian has been unsatisfactory for a number of reasons, one being that it did not have a way for space to increase in volume. The nodes it created were only trivalent, could not have volume, and so were not very interesting.

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

So that is one significant development that happened just in the past couple of years---in response to Karmerlo's question. At last we have a proposed hamiltonian that looks amenable to showing a connection between canonical (i.e. hamiltonian) approach and spinfoam dynamics. This means work for people to do, elaborating and studying the new hamiltonian LQG, and investigating its relation to spinfoam LQG. Might even be equivalent.

 

[marcus]

quote from the "New Hamiltonian" paper of May 2010. It gives an idea of the potential importance of this recent development:

==quote 1005.0817==
...More precisely, instead of writing the curvature as a limit of the holonomy of the connection around a closed loop, we write it as a limit of the spin-network function of the connection, associated to a tetrahedral graph.
The consequences of this alternative regularization are multifold.

• First, the regularized operator appears to be more natural and more symmetric, especially when acting on four-valent nodes, where it admits a natural simplicial interpretation. In particular, the curvature is evaluated on a plane which appears to be natural from a geometric point of view.
  
• Second, and more importantly, when acting on a node the resulting quantum operator generates three new nodes, rather than two, as the old Hamiltonian operator. Therefore the constraint implements the 1-4 Pachner move [2], which is characteristic of the spinfoam dynamics.
  
• Third, the resulting operator creates 4-valent nodes, rather than 3-valent ones, as the old Hamiltonian operator. Since 3-valents nodes have zero volume, the new operator can create nodes with volume.
  
• Finally, when we compute matrix elements of this operator, we find 15j Wigner symbols, as well as fusion coefficients [7, 12], namely the basic building blocks of the spinfoam dynamics.

==endquote==

Alesci is now postdoc in Thiemann's group at Erlangen, so we will see what develops from this.

 

[marcus]

Alesci and Rovelli's papers offer the prospect of a better hamiltonian than has existed in LQG up to now. And clear signs that a connection with properly defined Spinfoam LQG transition amplitudes can be found.

On the other hand there is another quite recent development reported by Jon Engle (physics faculty at Florida Atlantic U.) which offers the possibility of a better spinfoam vertex formula. The spinfoam vertex formula is the key ingredient to calculating transition amplitudes. So this paper is another possible game-changer.

http://arxiv.org/abs/1111.2865
A proposed proper EPRL vertex amplitude
Jonathan Engle
(Submitted on 11 Nov 2011)
As established in a prior work of the author, the linear simplicity constraints used in the construction of the so-called 'new' spin-foam models mix three of the five sectors of Plebanski theory, only one of which is gravity in the usual sense, and this is the reason for certain 'unwanted' terms in the asymptotics of the EPRL vertex amplitude as calculated by Barrett et al.
In the present paper, an explicit classical discrete condition is derived that isolates the desired gravitational sector, which we call (II+), following other authors. This condition is quantized and used to modify the vertex amplitude, yielding what we call the 'proper EPRL vertex amplitude.' This vertex still depends only on standard SU(2) spin-network data on the boundary, is SU(2) gauge invariant, and is linear in the boundary state, as required. In addition, the asymptotics now consist in the single desired term of the form e^iS_Regge, and all degenerate configurations are exponentially suppressed.
25 pages

As Jon Engle explains in the conclusion paragraph, his new vertex amplitude might not be needed if another solution being tried by other people turns out to work, but that is something we can't know in advance, or I can't anyway. And Engle's vertex amplitude needs more work to extend it to a broader range of cases---to remove simplifying assumptions---the usual thing with new mathematical developments.

These are things I'd offer someone like Karmerlo who is considering PhD research in Loop and wants to know about recent developments in the field.

I'd say to read Rovelli's Zakopane tutorial survey 1102.3660. Check out the forthcoming special issue of the free e-journal SIGMA which is devoted to LQG/LQC. It will have many recent articles of which some have already been posted on arxiv. I don't know how it will turn out but it might be interesting.
And if he/she is looking for research topics, here are these new papers by Alesci and by Engle on possibly improved hamiltonian and spinfoam amplitude.

The proposed improvements involve the two main forms of LQG dynamics---there is plenty of research to be done examining extending and exploring their equivalence.

BTW here are links to SIGMA journal and the Loop gravity/cosmology special issue that is being assembled.
http://www.emis.de/journals/SIGMA/about.html
http://www.emis.de/journals/SIGMA/special_issues.html
http://www.emis.de/journals/SIGMA/LQGC.html

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 (180 found)

 

[tom.stoer]

The Alesci & Rovelli Hamiltonian is a big step forward.

 

[marcus]

Philipp Höhn mentions the Alesci-Rovelli hamiltonian* in the talk he gave yesterday (29 November) at the ILQGS
http://relativity.phys.lsu.edu/ilqgs/
Höhn's talk was on his effort to develop a causal dynamical triangulations hamiltonian
with Bianca Dittrich. No, that's wrong. He calls it "SIMPLICIAL GRAVITY" rather than CDT. It isn't really CDT but it is strongly reminiscent. Whatever Dittrich gets involved with tends to be interesting.

Here are the slides:
http://relativity.phys.lsu.edu/ilqgs/hoehn112911.pdf

It's probably a significant development in a close neighbor of LQG, which could be a stimulus to the Loop people. The title of the talk is:
A canonical formalism for simplicial gravity

*reference in the "outlook" slide #32 at the end.

 

[marcus]

Alesci and Rovelli's papers offer the prospect of a better hamiltonian than has existed in LQG up to now. And clear signs that a connection with properly defined Spinfoam LQG transition amplitudes can be found.

On the other hand there is another quite recent development reported by Jon Engle (physics faculty at Florida Atlantic U.) which offers the possibility of a better spinfoam vertex formula. The spinfoam vertex formula is the key ingredient to calculating transition amplitudes. So this paper is another possible game-changer.

http://arxiv.org/abs/1111.2865
A proposed proper EPRL vertex amplitude
Jonathan Engle
(Submitted on 11 Nov 2011)
...

The Alesci & Rovelli Hamiltonian is a big step forward.

I certainly agree about the Alesci-Rovelli Hamiltonian. Can't be quite as sure about Jon Engle's new spinfoam formula, but if anyone is interested in hearing his seminar talk about it, here's a link
SLIDES: http://relativity.phys.lsu.edu/ilqgs/engle111511.pdf
AUDIO: http://relativity.phys.lsu.edu/ilqgs/engle111511.wav
There are technical problems with the first 2 minutes 45 seconds of the audio. You can skip to minute 2:45 without missing anything.
For more info google "ILQGS" for international loop QG seminar.
==================================

Very interesting new paper re the Hamiltonian approach:

http://arxiv.org/abs/1111.7195
Spontaneously broken Lorentz symmetry for Hamiltonian gravity
Steffen Gielen, Derek K. Wise
(Submitted on 30 Nov 2011)
In Ashtekar's Hamiltonian formulation of general relativity, and in loop quantum gravity, Lorentz covariance is a subtle issue that has been strongly debated. Maintaining manifest Lorentz covariance seems to require introducing either complex-valued fields or second class constraints, and either option presents a significant obstacle to quantization. After reviewing the sources of difficulty, we present a Lorentz covariant, real formulation free of second class constraints. Rather than a foliation of spacetime, we use a gauge field y, interpreted as a field of observers, to break the SO(3,1) symmetry down to a subgroup SO(3)_y. This symmetry breaking plays a role analogous to that in MacDowell-Mansouri gravity, which is based on Cartan geometry, leading us to a picture of gravity as 'Cartan geometrodynamics.' We study both Lorentz gauge transformations and transformations of the observer field to show that the apparent breaking of SO(3,1) to SO(3) is not in conflict with Lorentz covariance.
10 pages

 

[tom.stoer]

Very interesting new paper re the Hamiltonian approach:

http://arxiv.org/abs/1111.7195
Spontaneously broken Lorentz symmetry for Hamiltonian gravity
Steffen Gielen, Derek K. Wise

Interesting!

It seems that the criticism expressed by Nicolao, Alexandrov and others is taken seriously. There is a handful of papers appeared over the last couple of weeks adressing spacetime foliation, gauge fixing / no gauge fixing, breaking diff.-inv., hamiltonian analysis and constraint algebra, 1st vs. 2nd-class constraints, discretization and singular geometry, re-deriving Ashtekar-Barbero formulation in several different ways, SU(2) vs. SO(3,1) vs SL(2,C) etc. (I guess marcus has a rather exhaustive list)

The new papers shed some light on problems already present in the classical symplectic structure (especially foliation, gauge fixing and constraint algebra). Rovelli's new Hamiltonian may be a hint how to proceed with the genuine quantum issues (regularization, operator topology and convergence, operator algebra and anomalies, renormalization). There is an increasing number of indications that the canonical LQG formulation may be on the right track - although we understand afterwards why some well-known 'drivations' can be justified.

So the general situation may be not as bad as I expressed it a couple of weeks ago.

 

[nates]

without starting a new thread, is LQG and ST/M Theory still the front runners these days? has one moved past the other?

 

[marcus]

without starting a new thread, is LQG and ST/M Theory still the front runners these days? has one moved past the other?

That's a research trends statistics question. Changing research fashions, job demographics, citation standings etc have only indirect bearing on the ultimate validity of math models. But they can give at least a partial picture. Here for example:
Loop and String research trends as of 6 December:
http://howlonguntil.net/ 339/365 of year elapsed

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 (201 annualized from 187 found)

STRING,MEMBRANE,AdS/CFT RESEARCH BY YEAR
(search terms "string model", "membrane model" and "AdS/CFT correspondence")
2005 http://inspirebeta.net/search?ln=en...2y=2005&sf=&so=a&rm=citation&rg=10&sc=0&of=hb (988 found)
2006 http://inspirebeta.net/search?ln=en...2y=2006&sf=&so=a&rm=citation&rg=10&sc=0&of=hb (1029 found)
2007 http://inspirebeta.net/search?ln=en...2y=2007&sf=&so=a&rm=citation&rg=10&sc=0&of=hb (1050 found)
2008 http://inspirebeta.net/search?ln=en...2y=2008&sf=&so=a&rm=citation&rg=10&sc=0&of=hb (1128 found)
2009 http://inspirebeta.net/search?ln=en...2y=2009&sf=&so=a&rm=citation&rg=10&sc=0&of=hb (1132 found)
2010 http://inspirebeta.net/search?ln=en...2y=2010&sf=&so=a&rm=citation&rg=10&sc=0&of=hb (1046 found)
2011 http://inspirebeta.net/search?ln=en...2y=2011&sf=&so=a&rm=citation&rg=10&sc=0&of=hb (946 annualized from 879 found)
========

You probably see how the annualized figure is gotten. 339/365 of the year has elapsed and so far 187 research papers were posted, so if that rate continues for the rest of the year one would have 187*365/339 = 201 papers.

The searches are imperfect, so the absolute numbers probably matter less than whatever change or non-change one sees by repeating the same identical search for each consecutive year.

People's subjective judgments differ as to the ultimate prospects of different lines of research leading to testable predictions of new phenomena, and ultimately to a successful new vision of nature, explaining the big bang etc.

It ordinarily doesn't mean much just to hear people make authoritative-sounding pronouncements about what they think "most physicists" think. You can listen to a certain amount of that but I also believe in watching actual behavior. Departments are hiring fewer string theorists these days (than say 5 years ago) and there is a noticeable shift of people out of research on stringy unification into other areas---some into areas with no clear connection to string at all.
===============

I think it's important to realize that string research and LQG are not rivals in any direct sense. They are very different paths to the physics of the future. And they are far from the only paths being explored!

LQG research traditionally aims to re-envision geometry, to successfully demonstrate a quantum theory of spacetime geometry, and then to invite the matter fields to come and live in that new version of space and time.

It is far less concerned with explaining the matter fields of the current standard model---as if thinking "first let's get the world's uncertain changing geometry right."

By contrast, the String program has traditionally been concerned with matter defined on some fixed geometry. You could say that in some respects it is far more ambitious, because of its wider scope, but also in another respect less ambitious (in the quantum geometry department.)

So it's apples and oranges. There is no clear set rivalry to achieve a unique goal. And there are also important bananas. Other paths being pursued towards the physics of the future.

Loop is one of several programs going after several different goals. It's an interesting one to watch. And the whole scene is interesting, partly because it is so indefinite and unpredictable.
==================================
I've thought of another way to answer your question
Here is a long beautifully written paper by Richard Woodard (a particle theorist who studied under Sidney Coleman at Harvard)
http://arxiv.org/abs/0907.4238
It explains the whole complicated situation discussing SEVERAL approaches in what I think is a calm unbiased way and explaining just why the problems are difficult and what has caused the different programs to veer off in various directions.
It is 105 pages, but it is written in a helpful pedagogical style, so beginners can get something from it even if they just skip around and grab a section or two here and there. If you just understand 20 pages you will have gotten something worthwhile out of it.

He is not betting on this or that pet project. He is trying to help you understand the whole complex enterprise and the obstacles that Nature has set up for us. I'd like to meet the guy. It doesn't get any better, in my opinion. And I say this understanding probably less than half.

Of course it's out of date. 2009. Quite a lot has happened since. That's the breaks. It's like surveying an expanding universe---you can't.

 

[marcus]

Still another approach to answering your question, Nate.

Look at this very recent top-level workshop that brought together leading people from a number of different research programs (string, early universe cosmo, LQG, asym safe QG, causal triangulations QG, group field theory, simplicial QG...)

http://www.physics.ntua.gr/cosmo11/Naxos2011/sci_prog.html

Many speakers' slides PDF files are accessible online.
The workshop was held in September 2011. Richard Woodard's talks were on Friday and Saturday 16 and 17 Sept.

The island of Naxos in the Aegean. The organizers gathered the best people from all the fields of research that they thought could have something to say about quantum gravity and quantum cosmology. It's the right thing to do. A synthesis will emerge that we cannot envision ahead of time.

 

[nates]

as always, you couldn't be more helpful, marcus.

thanks!

 

[marcus]

...
Very interesting new paper re the Hamiltonian approach:

http://arxiv.org/abs/1111.7195
Spontaneously broken Lorentz symmetry for Hamiltonian gravity
Steffen Gielen, Derek K. Wise
(Submitted on 30 Nov 2011)

Interesting!

It seems that the criticism expressed by Nicolao, Alexandrov and others is taken seriously. There is a handful of papers appeared over the last couple of weeks adressing spacetime foliation, gauge fixing / no gauge fixing, breaking diff.-inv., hamiltonian analysis and constraint algebra, 1st vs. 2nd-class constraints, discretization and singular geometry, re-deriving Ashtekar-Barbero formulation in several different ways, SU(2) vs. SO(3,1) vs SL(2,C) etc. ...

The new papers shed some light on problems already present in the classical symplectic structure (especially foliation, gauge fixing and constraint algebra). Rovelli's new Hamiltonian may be a hint how to proceed with the genuine quantum issues (regularization, operator topology and convergence, operator algebra and anomalies, renormalization). There is an increasing number of indications that the canonical LQG formulation may be on the right track - although we understand afterwards why some well-known 'drivations' can be justified.

So the general situation may be not as bad as I expressed it a couple of weeks ago.

Continuing this discussion, a couple of papers appeared yesterday which brought problems with EPRL into sharper focus and suggested a possible improvement to the spin foam vertex amplitude.

Noui and Geiller are exploring (in 3d toy version) some idea they have for additional constraints.

http://arxiv.org/abs/1112.1965
Testing the imposition of the Spin Foam Simplicity Constraints
Marc Geiller, Karim Noui
(Submitted on 8 Dec 2011)
We introduce a three-dimensional Plebanski action for the gauge group SO(4). In this model, the B field satisfies quadratic simplicity constraints similar to that of the four-dimensional Plebanski theory, but with the difference that the B field is now a one-form. We exhibit a natural notion of "simple one-form", and identify a gravitational sector, a topological sector and a degenerate sector in the space of solutions to the simplicity constraints. Classically, in the gravitational sector, the action is shown to be equivalent to that of three-dimensional first order Riemannian gravity. This enables us to perform the complete spin foam quantization of the theory once the simplicity constraints are solved at the classical level, and to compare this result with the various models that have been proposed for the implementation of the constraints after quantization. In particular, we impose the simplicity constraints following the prescriptions of the so-called BC and EPRL models. We observe that the BC prescription cannot lead to the proper vertex amplitude. The EPRL prescription allows to recover the expected result when, in this three-dimensional model, it is supplemented with additional secondary second class constraints.
30 pages. 18 figures

Marc Geiller is at Paris 7 (Univ of Paris "Diderot" campus) and it looks like Noui is joining him there. Noui is a former coauthor with Alejandro Perez, one of the Marseille group, who is mentioned in the acknowledgments. Geiller and Noui have another paper just posted coauthored with Sergei Alexandrov, to be included in the special LQG issue of SIGMA.

http://arxiv.org/abs/1112.1961
Spin Foams and Canonical Quantization
Sergei Alexandrov, Marc Geiller, Karim Noui
(Submitted on 8 Dec 2011)
This review is devoted to the analysis of the mutual consistency of the spin foam and canonical loop quantizations in three and four spacetime dimensions. In the three-dimensional context, where the two approaches are in good agreement, we show how the canonical quantization à la Witten of Riemannian gravity with a positive cosmological constant is related to the Turaev-Viro spin foam model, and how the Ponzano-Regge amplitudes are related to the physical scalar product of Riemannian loop quantum gravity without cosmological constant. In the four-dimensional case, we recall a Lorentz-covariant formulation of loop quantum gravity using projected spin networks, compare it with the new spin foam models, and identify interesting relations and their pitfalls. Finally, we discuss the properties which a spin foam model is expected to possesses in order to be consistent with the canonical quantization, and suggest a new model illustrating these results.
88 pages. Invited review for SIGMA Special Issue "Loop Quantum Gravity and Cosmology"

 

[marcus]

without starting a new thread, is LQG and ST/M Theory still the front runners these days? has one moved past the other?

Part of understanding the situation and status of the LQG program is to keep in mind what other active approaches to QG there are and it's not just front runners (although they are important!) I tried to list them a few days ago in a different context https://www.physicsforums.com/showpost.php?p=3673983&postcount=9 but the list might be useful here. Not a complete list, just those that immediately came to mind:

Asymptotic Safe QG is one possibility. (Steven Weinberg's idea now much expanded by Reuter Percacci and friends)

Triangulations QG is another (see Renate Loll's SciAm I told you about, link in my sig)

Simplicial QG approach (a team of young researchers led by Bianca Dittrich)

A new thing called Shape Dynamics (see papers by Gomes and Koslowski)

There are several approaches to gravity inspired by condensed matter physics (see e.g. X.G. Wen's papers, and a recent one by Liberati Finazzi Sidoni).

Group Field Theory (Oriti and co-workers)

The quantization of Cartan's GR (new papers by Derek Wise and Steffen Gielen)

Erik Verlinde's entropic gravity (2009 not so much lately)

Petr Horava's anisotropic gravity (2009 not so much lately)

Verlinde and Horava were formerly prominent string theorists. Their alternative non-string QG approaches got a lot of attention a couple of years ago but not so much now.

Maybe its fair to say there could be many ways up the mountain and several separate parties of climbers seem to be making progress towards the summit.