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:
http://physicsforums.com/showthread....67#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

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

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

marcus

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.
http://physicsforums.com/showthread....67#post3597267

Here is the link for the Quantum Gravity group at Nottingham:
http://www.nottingham.ac.uk/mathemat...m-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=UT...e6c50e&t=h&z=0

tom.stoer

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

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

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

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 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

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.d...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.d...rch-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

so what was the purpose of the reformulation then.