What is the recent development of Loop Quantum Gravity

[tom.stoer]

marcus, there may very well be n-valent nodes which do not correspond to triangulations but which may describe Voronoi-cell-like structures; but I think that not even this structure need always be sufficient. I am afraid that an arbitrary graph need not comply with any cell-like structure embedded in low-dimensional manifolds.

 

[marcus]

I am afraid that an arbitrary graph need not comply with any cell-like structure embedded in low-dimensional manifolds.

Is that important?
I was responding to your talking about triangulations. The overwhelming majority of graphs, of any given size, are NOT dual to a triangulation. So I wanted to agree with emphasis!

I think you can probably extend that to a division of a 3D manifold into 3D cells which are NOT simplices. Is this the kind of thing you mean? Most graphs would not be dual to that sort of structure either. Or so I believe (haven't thought about it.)

I was puzzled by your saying you are afraid such and such might not be so. Don't see why it matters.

 

[marcus]

Since the topic of polyhedra has come up, I'll mention some recent work in that area:
http://arxiv.org/abs/1009.3402 (google "bianchi polyhedra")
Polyhedra in loop quantum gravity
Eugenio Bianchi, Pietro Dona', Simone Speziale
32 pages

As it happens, I see that Eugenio Bianchi is at UC Berkeley this week giving a couple of talks. He has a co-author in the physics department so maybe they are working on something. Anyway there is this paper about quantum polyhedra. A quantum polyhedron (state space a space of intertwiners) can be thought of as a blur of possible classic polyhedra. Volume may be specified, also number of sides and areas. But shapes of sides may be indeterminate.

A quantum state of geometry might be imagined as a collection of quantum polyhedra, with adjacency relations. You aren't guaranteed the ability to match the faces.

The loop literature does not say something naive like space IS a bunch of quantum polyhedra, that is just one way to think about the theory. There are various ways of approaching and visualizing that give intuition. Use them if they help you but don't get hung up on them.

Another way, also worked out primarily by Eugenio, is to think of it as a quantum theory of topological defects. All the geometry, the curvature etc, is concentrated on the cracks and crevasses between chunks, which are flat (everything is flat except at the defects where they meet.)

This also is a way to visualize LQG, a guantum theory of the defects between otherwise flat chunks of space. The Freidel Geiller Ziprick paper takes off from Bianchi's work on this and, as you probably recall, develops it further.
http://arxiv.org/abs/0907.4388 (google "bianchi aharonov")
Loop Quantum Gravity a la Aharonov-Bohm
Eugenio Bianchi
19 pages

 

[torsten]

Hi Marcus, hi Tom,

my original goal of the last post was to say thank you for the good discussion.
But to meet the goal of this thread, here are some general remarks or better my motivation:

Also for me the basic requirement is a clear testable version of QG that reproduces classical geometry (where applicable) and resolves the cosmo singularity.
(like you Marcus) But more must be possible: an explainantion of dark matter / energy and inflation.
Currently, LQG is one of the best candidates to meet all these criteria.

So, from the QG point of view I'm rather a 'LQG follower'. But that don't prevent me from a critique of some aspects of the current research, like Tom does.
I never start my own QG program. I started with the investigaton of 4-dimensional smooth manifolds to understand general aspects of dynamics.
Currently there is a lot of work to find the Hamiltonian via trial and error (my opinion). So I miss a general concept for the next steps.
The large number of workers on that field is a great advantage.

My own philosophy is a little bit different: I agree to produce a testable version reproducing known theories in some limits.
Since 20 years ago I learn in my first topology lecture at the university of the existence of exotic R^4. So immediatly I wa interested.
What is the relevance of exotic smoothness for physics? The first results came from Carl Brans (I met him in 1995). Then we are both occupied with the book project.
The idea was very simple: two referenece systems (or systems of charts, i.e. an atlas) are equivalent if both a diffeomorphic to each other.
But then two non-equivalent reference systems (representing different physics) are non-diffeomorphic. In 4 dimensions it can be indepedent of the topology.
Therefore exotic spacetimes can be seen as different physical systems (of a spacetime with fixed topology).
My own investigations began around 1995 (when I thought to have studied enough differential topology) but with classical relativity theory by showing that exotic smoothness can be the source of a gravitational field.
Nearly 10 years later we found the first relation to quantum mechanics by constructing a factor II_1 von Neumann algebra (the Fock space of a fermion). You maybe remember on the discussion in 2005 in this forum.
There are only very few people working in this field. A student of M. Marcolli, Christopher Duston, joined our community and began to calculate the Euclidean path integral for different exotic smoothness structures.
It was folklore that exotic smoothness contributes (or better dominates) the path integral but no one showed it. Chris was the first to tackle this problem by perturbatively calculate it.
His results inspired me to calculate also the Lorentz case. In two papers we calculate (non-perturbatively) the exotic smothness part to show area quantization as a result (confirming LQG).
In parallel we try to find another description of exotic R^4's (without infinite handlebodies) and end with an amazing relation to codiemnsion-1 foliations. This relation brought us back to think about QG.
The space of leafs of a foliation was one of the first examples of a non-commutative space and geometry by Connes. In case of our foliation we obatin a factor III_1 von Neumann algebra also known as observablen algebra of a QFT (in the algebraic sense).
Currently we also find relations to Connes-Kreimer renormalization theory and to the Tree QFT of Rivasseau (arXiv:0807.4122).

But enough about history, my real motivation for this work is the relation between geometry and physics. Especially the question, what is quantum geometry? The simple answer, the quantization of the spacetime, is not correct.
(I will have a lookinto Bianchis polytop theory soon.)
So from the philosophical point of view, I'm interested in the relation between geometry and quantum theory, especially which one is the primary principle. Because of exotic smoothness, I believe it is geometry.
But then I have to understand the measurement process etc also from a geoemtrical point view. Another driving force is the naturalness, i.e. to derive the expressions for the Dirac action, the standard model etc. from geometrical expressions.
This brings me back to your discussion here. I miss the guiding principle in the current constructions in LQG. Of course there are excepts (Freidel is one, sometimes Rovelli). Everyone speaks about unification but currently there are alwyas two entities: the spin network and the dynamical spacetime (or the string and the background).
A real unification should end with one entity.


But now to your interesting questions:
what is the fundamental structure of (L)QG:
1) PL or smooth manifolds with diffeomorphisms factored away - resuting in triangulations?
2) generic spin networks?

As I tell in my previous post, I'm impressed by Marcollis topspin model. Then the spin network (as 1-dimensional complex) produces the 3-manifold as branched cover. Then we have one entity (the network) producing the space.
The spin network (as the expression of holonomies) has a topological interpretation: every closed loop in the network must be corespond to one element of the fundamental group of the 3-manifold. After the solution of Poincare conjecture we know that the fundamental group characterizes a 3-manifold uniquely.
Therefore (in my opinion) the two cases 1) and 2) are more connected then anybody thought.

The second question: in (L)QG, do we have to use a 3-dim. or a 4-dim manifold to start with?
is much harder to comment.
Usually one starts with a globally hyperbolic 4-manifold (SxR, S Cauchy surface) and one has to discuss only the topology of the Cauchy surface. Otherwise later one speaks about fluctuating geometries (by quantum fluctuations) which can be result in a topology change (at the Planck level).
But a topology change destroys the global hyperbolicity (now naked singularities appear). So, at first one has to discuss the global hyperbolicity condition. Even in the exotic smoothness case one lost this condition (see http://arxiv.org/abs/1201.6070).
But did we really need it? The main reason for its introduction were causility question. But now we know (after some work of Dowker about causal continuity) that topology change is possible.
Naked singularities seem bad at the first view but we need them (to prevent the horror of Parmenides block universe, i.e. a complete determinism). Such a singularity separates the past from the future. Then we cannot completely determine the trajectory of a particle
That is for me a necessary condition to implement quantum mechanics.
Therefore for my opinion, one should start with a spacetime (4dim) and should look for codim 1 subspaces (the 3dim space).

 

[marcus]

Thanks Torsten, this is one of the most thoughtful and interesting posts in my experience here at the BTSM forum! I appreciate your care in laying out your thoughts on QG and different smooth structures.

I just heard a 90 minute presentation at the UC physics department by Eugenio Bianchi which had some suggestive parallels with your research focus. He was talking about the dynamics of topological defects (as an alternative formulation of Loop gravity.)

There were questions and discussion during and after so it took the full two hours. Steve Carlip participated quite a lot. Good talk.

The slides overlapped some with those in the PIRSA video which you can watch if you wish:
Google "pirsa bianchi" and get http://pirsa.org/11090125/

PIRSA:11090125
Loop Gravity as the Dynamics of Topological Defects
Speaker(s): Eugenio Bianchi
Abstract: A charged particle can detect the presence of a magnetic field confined into a solenoid. The strength of the effect depends only on the phase shift experienced by the particle's wave function, as dictated by the Wilson loop of the Maxwell connection around the solenoid. In this seminar I'll show that Loop Gravity has a structure analogous to the one relevant in the Aharonov-Bohm effect described above: it is a quantum theory of connections with curvature vanishing everywhere, except on a 1d network of topological defects. Loop states measure the flux of the gravitational magnetic field through a defect line. A feature of this reformulation is that the space of states of Loop Gravity can be derived from an ordinary QFT quantization of a classical diffeomorphism-invariant theory defined on a manifold. I'll discuss the role quantum geometry operators play in this picture, and the perspective of formulating the Spin Foam dynamics as the local interaction of topological defects.
Date: 21/09/2011

As I say, many of the slides are the same as those of today's talk, but there seem to be new results, and I got more out of it the second time---either today's presentation contained more intuition and insight or else the questions by Carlip and Littlejohn helped bring out stuff. Anyway great!

I can't help suspecting that there is some kinship between the dynamics of topological defects and your investigation of differential structures.

One obvious difference from the September PIRSA talk was that this came after the October Freidel Geiller Ziprick paper in effect laying out a "constrain first then quantize" approach, developing the "Loop Classical Gravity" concept. There were several references to FGZ http://arxiv.org/abs/1110.4833 .

 

[marcus]

Looking over the schedule of the April meeting of the American Physical Society, one sees that there will be an invited talk reviewing current research in Spinfoam and Loop QG
http://meetings.aps.org/Meeting/APR12/Event/170161
Eugenio Bianchi (Perimeter Institute)
Loop Quantum Gravity, Spin Foams, and gravitons
Loop Quantum Gravity provides a candidate description for the quantum degrees of freedom of gravity at the Planck scale. In this talk, I review recent progress in formulating its covariant dynamics in terms of Spin Foams. In particular, I discuss the main assumptions behind this approach, its relation with classical General Relativity, and its low-energy description in terms of an effective quantum field theory of gravitons.

The session of invited QG talks is chaired by Jorge Pullin, who also chairs the regular session
"Quantum Aspects of Gravitation"
http://meetings.aps.org/Meeting/APR12/SessionIndex2/?SessionEventID=172413

Here are a few of the talks scheduled for the regular QG session.

http://meetings.aps.org/Meeting/APR12/Event/170104
Hal Haggard (UC Berkeley)
Volume dynamics and quantum gravity
Polyhedral grains of space can be given a dynamical structure. In recent work it was shown that Bohr-Sommerfeld quantization of the volume of a tetrahedral grain of space results in a spectrum in excellent agreement with loop gravity. Here we present preliminary investigations of the volume of a 5-faced convex polyhedron. We give for the first time a constructive method for finding these polyhedra given their face areas and normals to the faces and find an explicit formula for the volume. In particular, we are interested in discovering whether the evolution generated by this volume is chaotic or integrable which has important consequences for loop gravity: If the classical volume generates a chaotic flow then the corresponding quantum spectrum will generically be non-degenerate and the volume eigenvalue continues to act as a good label for spin network states. On the other hand, if the volume flow is classically integrable then the degeneracy of the corresponding quantum spectrum will have to be lifted by another observable. We report on progress distinguishing these two cases. Either of these outcomes will impact the direction of future research into volume operators in quantum gravity.

http://meetings.aps.org/Meeting/APR12/Event/170098
Rodolfo Gambini, Nestor Alvarez, Jorge Pullin (Montevideo, LSU)
A local Hamiltonian for spherically symmetric gravity coupled to a scalar field
Using Ashtekar's new variables we present a gauge fixing that achieves the longstanding goal of making gravity coupled to a scalar field in spherical symmetry endowed with a local Hamiltonian. It opens the possibility of direct quantization for a system that can accommodate black hole evaporation. The gauge fixing can be applied to other systems as well.
[my comment: related paper= http://arxiv.org/abs/1111.4962 ]

http://meetings.aps.org/Meeting/APR12/Event/170100
Jacopo Diaz-Polo, Aurelien Barrau, Thomas Cailleteau, Xiangyu Cao, Julien Grain (LSU, CRNS Paris)
Probing loop quantum gravity with evaporating black holes
Our goal is to show that the observation of evaporating black holes should allow the standard Hawking behavior to be distinguished from Loop Quantum Gravity (LQG) expectations. We present a Monte Carlo simulation of the evaporation of microscopic black holes in LQG and perform statistical tests that discriminate between competing models. We conclude that the discreteness of the area in LQG leads to characteristic features that qualify evaporating black holes as objects that could reveal specific quantum gravity footprints.
[my comment: related paper= http://arxiv.org/abs/1109.4239 ]

http://meetings.aps.org/Meeting/APR12/Event/170102
Seth Major (Hamilton College)
Coherent States and Quantum Geometry Phenomenology
The combinatorics of quantum geometry can raise the effective scale of the spatial geometry granularity predicted loop quantum gravity. However the sharply peaked properties of states built from SU(2) coherent states challenge the idea that such a combinatorial lever arm might lift the scale of spatial discreteness to an observationally accessible scale. For instance, the Livine-Speziale semi-coherent states exhibit no such lever arm. In this talk I discuss how an operational point of view suggests a different class of coherent states that are not built from states with microscopic classical geometry. These states are introduced, compared to previous coherent states, and the status of the combinatoric lever arm is discussed.

 

[torsten]

Thanks a lot for your words, Marcus
As usual, I like your recommendations. So I will have a look into Bianchi's article. The lecture is really interesting. It seems we share the same passion...
Maybe one thought which is independent of exotic smoothness:
In our article about topological D-branes
http://arxiv.org/abs/1105.1557
we discussed wild embeddings to use it as a quantum version of D branes.
An embedding is a map i:N->M so that i(N) is homeomorphic to N. The embedding is called tame if i(N) is represented by a finite polyhedron. Examples are Alexanders horned sphere or Antoines necklace. One of the main characteristica of a wild embedding is that the complement M\i(N) is mostly a non-simple connected space. Other examples of wild embeddings are also called fractals...
In section 5.3 we describe a wild embedding by using Connes non-commutative geometry, i.e. we associate a C* algebra to the wild embedding. All the resulst of this paper seem to imply that a quantum version of a D brane is a wild embedded D-brane. Maybe also any other quantum geometry is of this kind.
But now I will study your recommendations...

 

[tom.stoer]

torsten, I strongly believe that you miss one important point regarding your own work; it seems to me that (once you succeed with your program ;-) you will be able to explain why we live in a four-dim. spacetime!

 

[torsten]

Good point Tom, that was one of the reasons I began to study exotic smoothness. When I heard from this result, I studied superstring theory. But then I changed to differential topology to understand this result.

 

[marcus]

Yes good point: the multitude of structures does make D=4 special, or is one of the things that makes it special. Another thing to note is the suggestion of spontaneous dimensional reduction at extremely small scale which has appeared in several separate theory contexts as reviewed by Steve Carlip. BTW I forgot to mention another invited Loop talk at the April APS meeting.
There is a session called Advances in Quantum Gravity
consisting of three invited talks. One of the these, already mentioned, is by Eugenio Bianchi: a review of recent advances in Loop QG Spin Foams and gravitons. Abstract: http://meetings.aps.org/Meeting/APR12/Event/170161

I overlooked another Loop invited talk to be given by Ivan Agullo from Penn State:
http://meetings.aps.org/Meeting/APR12/Event/170160
Beyond the standard inflationary paradigm

The inflationary paradigm provides a compelling argument to account for the origin of the cosmic inhomogeneities that we observe in the CMB and galaxy distribution. In this talk we introduce a completion of the inflationary paradigm from a (loop) quantum gravity point of view, by addressing gravitational issues that have been open both for the background geometry and perturbations. These include a quantum gravity treatment of the Planck regime from which inflation arises, and a clarification of what the trans-Planckian problems are and what they are not. In addition, this approach provides examples of effects that may have observational implications, that may provide a window to test the basic quantum gravity principles employed here.

I hope to find something on arxiv that can give more of an idea what this will be about. I could not find anything the first try. It's late, have to look further tomorrow.

=======EDIT======
Well, I looked again this morning and couldn't find anything on arxiv that I could recognize as a clear indication of what this talk might be about. Ivan Agullo has worked a lot with Leonard Parker. He was at Parker's institution and is now with Ashtekar group at Penn State, I think. He brings a lot of non-Loop cosmology to Loop, or so it seems to me. I woujld like to see a paper co-authored by Agullo and Ashtekar, but I can't find one so far.

I will check ILQGS for a talk by Agullo.

Inflation is important because a some previous approaches to inflation bring on the multiverse ailment. You invent an inflaton field and then spend the rest of your life trying to make excuses for it.

 

[marcus]

Ah hah! I found a March 2011 ILQGS talk by Agullo
http://relativity.phys.lsu.edu/ilqgs/agullo032911.pdf
Observational signatures in LQC

He refers to work in progress by Ashtekar, William Nelson, and himself. This is precisely what is apparently not yet written up so I can't find on arxiv. It would be the basis of his invited presentation at the American Physical Society April meeting in Atlanta:

http://meetings.aps.org/Meeting/APR12/Event/170160
Beyond the standard inflationary paradigm
Ivan Agullo (Penn State)
... In this talk we introduce a completion of the inflationary paradigm from a (loop) quantum gravity point of view, by addressing gravitational issues that have been open both for the background geometry and perturbations. These include a quantum gravity treatment of the Planck regime from which inflation arises... In addition, this approach provides examples of effects that may have observational implications, that may provide a window to test the basic quantum gravity principles...


BTW today (28 February) the ILQGS will have a talk by Marc Geiller, one of the co-authors of the FGZ paper. This paper topped our fourth quarter MIP poll last year. It offers a new approach to constructing LQG as a "quantization" of a classical theory.

Google "ILQGS" and get http://relativity.phys.lsu.edu/ilqgs/

Scroll down to March 2011 to get links to audio and slides of Agullo's talk.

Geiller's talk about the FGZ research is currently at the top of the same page:
http://relativity.phys.lsu.edu/ilqgs/geiller022812.pdf
Continuous formulation of the loop quantum gravity phase space
He's at "Paris-Diderot": the Diderot campus of the University of Paris, on the right bank near the city's southeast edge.

 

[marcus]

Tomorrow 29 February the Perimeter QG group has talk by Wolfgang Wieland
which I hope will be online as video and slides. Postdocs at PI get to bring visitors to the Institute. I think Wolfgang is coming as Eugene Bianchi's guest. His home-base at this point is Marseille.
http://pirsa.org/12020129
Spinor Quantisation for Complex Ashtekar Variables
Wolfgang Wieland
Abstract: During the last couple of years Dupuis, Freidel, Livine, Speziale and Tambornino developed a twistorial formulation for loop quantum gravity.
Constructed from Ashtekar--Barbero variables, the formalism is restricted to SU(2) gauge transformations.
In this talk, I perform the generalisation to the full Lorentzian case, that is the group SL(2,C).
The phase space of SL(2,C) (i.e. complex or selfdual) Ashtekar variables on a spinnetwork graph is decomposed in terms of twistorial variables. To every link there are two twistors---one to each boundary point---attached. The formalism provides a clean derivation of the solution space of the reality conditions of loop quantum gravity.
Key features of the EPRL spinfoam model are perfectly recovered.
If there is still time, I'll sketch my current project concerning a twistorial path integral for spinfoam gravity as well.
29/02/2012 - 4:00 pm

Wieland's 29 February talk (available online at ILQGS) will evidently be based on this paper:
http://arxiv.org/abs/1107.5002
Twistorial phase space for complex Ashtekar variables
Wolfgang M. Wieland
(Submitted on 25 Jul 2011, last revised 24 Jan 2012)
We generalise the SU(2) spinor framework of twisted geometries developed by Dupuis, Freidel, Livine, Speziale and Tambornino to the Lorentzian case, that is the group SL(2,C). We show that the phase space for complex valued Ashtekar variables on a spinnetwork graph can be decomposed in terms of twistorial variables. To every link there are two twistors---one to each boundary point---attached. The formalism provides a new derivation of the solution space of the simplicity constraints of loop quantum gravity. Key properties of the EPRL spinfoam model are perfectly recovered.
18 pages, Classical and Quantum Gravity 29 (2012) 045007

 

[torsten]

Thanks marcus for your effort.
Yes inflation is indeed important. But one of the current problems is the infinity of the inflation process, i.e. if the inflation process (with the inflaton) is started then there is no known process which stops the inflation.
The second problem is agin the naturalness: there ar ean infinity of possibilities for the potential of the inflaton field.

Here we made also progress with exotic smoothness at the beginning of the year.
http://arxiv.org/abs/1201.3787
An exotic S^3xR can be partly described by a cobordism between the 3-sphere and a homology 3-sphere (Poincare sphere for instance) and vice versa. Except for the Poincare sphere, all other homology 3-spheres are negatively curved (I mean at least one component of the curvature tensor is negatively curved), a corrolary of Perelmans work.
Therefore we get the change:
postive curvature -> negative curvature -> positive curvature
For this case we explicitely solve the Friedman equations including the dust matter (p=0) and obtain inflation (I mean an exponential increase) which stops.

Also one word about the interesting claims of Carlip.
It is an amazing fact from general manifold theory that the simple 2-disk is one of the important tools. (I recommend a proceeding article of Michael Freedman "Working and playing wit the 2-disk") Therefore the dominance of 2-dimensional objects around the Planck scale was not amazing for me (I remember Loll et.al. got also this result in CDT).

 

[marcus]

Wonderful video talk by Wolfgang Wieland!
http://pirsa.org/12020129/
Goes back to the original complex Ashtekar variables and goes forward to the new double spinor version of Loop developed by Dupuis, Freidel, Livine, Speziale, Tambornino...

Maïte Dupuis is currently a visitor at Perimeter, there seems to be a convergence of people interested in this "twistorial" or dual spinor version of Loop.

I've been watching Wieland's lecture and was quite impressed. See what you think.

Torsten, you are pointing out suggestive parallels with the differential topology approach you have in progress. It would certainly be remarkable if there proved to be a solid bridge.

At first it seemed very strange to be going back to the original complex version of the Ashtekar variables. But he makes it look like a convincing move, and somehow the immirzi parameter reappears as a real number, which I would never have expected!

 

[marcus]

It's clear that there are some shifts going on. Generational, geographical, and even (on a minor level) formal.

Loop is fast moving. It isn't easy to keep in one's sights. The "target" that one is trying to describe and follow is evolving rapidly.

Generationally, we have to watch more carefully some younger representatives of the mainstream Loop.
Wolfgang Wieland, Eugenio Bianchi, Maite Dupuis, Simone Speziale, Etera Livine... (not a complete list.)
Also we should notice first-time faculty positions, some in comparatively new places, for people who were only recently postdocs:
Engle made faculty at Florida Atlantic
Sahlmann, Giesel and Meusberger made faculty at Erlangen
Singh faculty at LSU
Dittrich faculty at Perimeter

Bianchi, Haggard, Agullo are giving talks at the April APS in Atlanta. These are comparatively young researchers. Two of the talks are invited. These are not the only Loop talks at the APS meeting--I just mention these three because of the generational angle.
There seems to be some increased activity at UC Berkeley. Bianchi was just here and gave two talks.

In the formalism department, you could say that the "paradigm" of Loop is shifting towards what Dupuis, Speziale, Tambornino describe in their January 2012 paper
Spinors and Twistors in Loop gravity and Spin Foams

For me, the paper which best characterizes the new Loop wave is Wieland's
Twistorial phase space for complex Ashtekar variables
together with his PIRSA talk of 29 February. I have now viewed the whole 80 minutes, including the questons and discussion and I think it is a "must watch".

Geographically, there seems to be a shift from Europe to North America. Part of this is that Perimeter is so strong. It grabs many of the creative young people and if it does not get them on a longer term basis then it brings them there for one month visits to collaborate with people there already. For instance: Maite Dupuis, Marc Geiller and Wolfgang Wieland are all three currently visiting. There is some kind of critical mass effect. The next biannual Loops conference, Loops 2013, will be at Perimeter. Plus another factor is that the Usa has some catching up to do in Loop, which means faculty openings and growth at the newer centers south of the border.

Here's an informal window to help follow geographical movement:
http://sites.google.com/site/grqcrumourmill/
Sample postdoc moves in 2012:
Ed Wilson-Ewing/ Marseille -> LSU
Marc Geiller/ Paris -> Penn State
Thomas Cailleteau/ Grenoble -> Penn State
Philipp Höhn/ Utrecht -> Perimeter
Faculty:
Hanno Sahlmann/ Pohang -> Erlangen
Renate Loll/ Utrecht -> Nijmegen
Note that three of the postdoc moves are in the general direction Europe-->Usa

 

[marcus]

Earlier I was trying to find a write up that would preview the content of Agullo's invited talk at the April meeting of the American Physical Society. The best source, I now realize, is this set of ILQGS slides by William Nelson:
http://relativity.phys.lsu.edu/ilqgs/nelson101811.pdf
and the corresponding audio
http://relativity.phys.lsu.edu/ilqgs/nelson101811.wav
or
http://relativity.phys.lsu.edu/ilqgs/nelson101811.aif

Ah hah! I found a March 2011 ILQGS talk by Agullo
http://relativity.phys.lsu.edu/ilqgs/agullo032911.pdf
Observational signatures in LQC

He refers to work in progress by Ashtekar, William Nelson, and himself. This is precisely what is apparently not yet written up so I can't find on arxiv. It would be the basis of his invited presentation at the American Physical Society April meeting in Atlanta:

http://meetings.aps.org/Meeting/APR12/Event/170160
Beyond the standard inflationary paradigm
Ivan Agullo (Penn State)
... In this talk we introduce a completion of the inflationary paradigm from a (loop) quantum gravity point of view, by addressing gravitational issues that have been open both for the background geometry and perturbations. These include a quantum gravity treatment of the Planck regime from which inflation arises... In addition, this approach provides examples of effects that may have observational implications, that may provide a window to test the basic quantum gravity principles...

Google "ILQGS" and get http://relativity.phys.lsu.edu/ilqgs/
...Scroll down to March 2011 to get links to audio and slides of Agullo's talk.
...

And scroll down to October 2011 for Nelson's

William Nelson's talk is a "must-hear". It's some very good work (as Lee Smolin comments at the end) and is going to change how we view cosmology. It is joint work by Agullo, Ashteker, Nelson, and it just happens that Nelson gave the ILQGS presentation and Agullo will present it at the April APS in Atlanta.

ILQGS also has an interesting blog where various presentations are discussed by OTHER researchers, who often give more basic intuitive explanations of what the talk is about. Brizuela (AEI) comments on Nelson's talk, Julian Barbour (!) comments on Tim Koslowski's talk about shape dynamics, Frank Hellmann (AEI) on Jacek Puchta's about an extenstion of Spinfoam...
Check out the blog, pedagogically it complements the seminar talks and makes them more accessible.
http://ilqgs.blogspot.com/
Some future talks listed here:
http://relativity.phys.lsu.edu/ilqgs/schedulesp12.html
Note Diaz-Polo upcoming talk on Loop BH evaporation (there's a relevance to obs. testing):
http://arxiv.org/abs/1109.4239

 

[marcus]

Something I've heard a lot in talks recently is "the Loop hypothesis".

It gives a useful perspective for understanding what the various QG formulations called Loop have in common.
It is the hypothesis that you can TRUNCATE the dynamic geometry of GR to finite degrees of freedom and then recover the continuous for all practical purposes.

The Loop truncation is to consider making geometric measurements at only a FINITE SET OF POINTS. So you naturally get a graph of places where you measured some volumes and some face areas between adjacent chunk volumes. The details of what constitute possible geometric measurements are not important---angles and lengths are also allowed.

What matters is the observer can only make a finite number of measurements, and that defines the state.

I'm thinking that's what science is about:

The aim of quantitative science is to explain what we can observe and to predict thereabout .

And we never get to make more than a finite number of observations.

So a state of nature (nature's geometry) is naturally going to be truncated to a finite set of points with adjacency relations and whatever labels.
The Loop hypothesis is that this is sufficient to explain and predict what we can observe. It's minimalist. The hypothesis (a kind of gamble) is that this will prove to be sufficient to recover the continuous classical picture by taking more and more points (more elaborate networks of observation).

 

[atyy]

I think there are two different truncations.

In Rovelli's Zakopane, he talks about a truncation which is a good approximation.

The FGZ idea is that the full space can be split into nice parts and the continuum recovered exactly (not just for all practical purposes) by joining them together. The idea is that is one can do that, one just needs to quantize each part separately.

 

[marcus]

...
The FGZ idea is that the full space can be split into nice parts and the continuum recovered exactly (not just for all practical purposes) by joining them together. The idea is that is one can do that, one just needs to quantize each part separately.

Perhaps I don't understand, or I see FGZ doing something different from you. Piecewise flat decomposition with all the curvature concentrated at the joints---approximating the full range of continuously curving geometries, but not reproducing the full range exactly.

For me, what FGZ does is one further step in the process that goes back to the 1990s of finding the most mathematically convenient way to implement the Loop hypothesis*---truncating geometry to a finite number of degrees of freedom, truncating to N degrees of freedom and then letting N→∞.

There are several, many, ways this has been tried. It is all the same quest. You may wish to focus on just two initiatives: Zakopane and FGZ. But I do not wish to restrict my view that way.

Remember the Lewand--Asht measure, the holonomy-flux algebra, the Lewand--Okol--Sahl--Thiemann theorem? I see it as all part of the same journey.

I suppose that history could replace the Zako formulation and keep some features of it.

I particularly like the Hilbertspace of squareintegrable functions defined on a cartesian product of G where the Lie Group G can refer to either the rotations or the full Lorentz. And I like the gamma map from SU(2) reps to SL(2,C) reps. I hope those features are kept, but who can say?
Suppose the group G becomes somehow twistorial? It would still be like Zako, square integrable functions on G^E=#edges in a way, but it would also be different. Or suppose the hilbertspace of functions on the group manifold become not complex-valued but somethingelse valued?
I really liked Wolfgang Wieland's pirsa talk. It gave me a glimpse of where the further evolution of this "loop hypothesis" could go.

Maybe neither Zako or FQZ is final or exactly right. It would be a shock if it were
The important thing is to have something simple definite and clear--mathematically well-defined--at each step along the way. Zako served as that last year and perhaps also this year. We have to keep our eyes open for what will take shape by spring of 2013, when another Loops conference is coming due.

*I was just listening to Marc Geiller talk about the FGZ work:
http://relativity.phys.lsu.edu/ilqgs/geiller022812.pdf
http://relativity.phys.lsu.edu/ilqgs/geiller022812.wav
He calls it the "Loop assumption" instead of the Loop hypothesis, and he says concretely what he means on slide #8 early in the talk. I have the impression now that I hear many people using this idea, which has entered the shared vocabulary of the Loop community. Perhaps it was always one of the shared concepts but I didn't notice it until recently.

 

[atyy]

So are FGZ talking about what Rovelli calls the graph expansion, which he distinguishes from the vertex expansion? http://arxiv.org/abs/1102.3660 (p19).

 

[marcus]

So are FGZ talking about what Rovelli calls the graph expansion...

In a rough sense I think that's right. Very generally they are both talking about the same thing, namely truncating down to finite d.o.f. by restricting attention to a (finite) graph.

At the level of detail I think what they are talking about is different. I wouldn't use the word "expansion" in the FGZ context.

I think they have found a different way to look at the truncation which allows them to reconstruct a class of continuum states from a discrete one. So they "de-truncate" in a sense. They go back to the continuum picture without having to take a grand limit.

I think this FGZ approach is still embryonic. It might eventually augment or replace the earlier "continuum limit" part of the program. These are work-in-progress areas--parts of the program that are under development and could take new directions.

Yesterday I listened to Marc Geiller's ILQGS talk, with the slides. It's a good talk that covers the main ideas of the FGZ paper. I'd recommend it to anyone who wants to understand their approach better. I already gave the links but will do so again:
http://relativity.phys.lsu.edu/ilqgs/geiller022812.pdf
http://relativity.phys.lsu.edu/ilqgs/geiller022812.wav

 

[marcus]

A new development in QG that hasn't been reported yet or discussed here is an initiative by the Warsaw group where they develop a systematic way to enumerate all bulk spinfoams which a given spin network boundary. This is how one calculates transition amplitudes in spinfoam QG dynamics. The boundary spin network can, for instance, be thought of as representing initial and final states of geometry and the spinfoam bulk as a process transitioning from one to the other.

The Warsaw group has introduced a new OSN (operator spin network) formalism---graphs labeled by operators instead of spin numbers.

This paper just came out:
http://arxiv.org/abs/1203.1530
One vertex spin-foams with the Dipole Cosmology boundary
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 7 Mar 2012)
We find all the spin-foams contributing in the first order of the vertex expansion to the transition amplitude of the Bianchi-Rovelli-Vidotto Dipole Cosmology model. Our algorithm is general and provides spin-foams of arbitrarily given, fixed: boundary and, respectively, a number of internal vertices. We use the recently introduced Operator Spin-Network Diagrams framework.
23 pages, 30 figures

Happily enough much of the content was already presented earlier in a recorded seminar talk at ILQGS by Puchta! Having the audio and slides presentation in parallel with the paper can make it easier to understand both.
ILQGS: The Feynman diagramatics for the spin foam models
Jacek Puchta
SLIDES: http://relativity.phys.lsu.edu/ilqgs/puchta092011.pdf
AUDIO: http://relativity.phys.lsu.edu/ilqgs/puchta092011.wav
(alternative audio http://relativity.phys.lsu.edu/ilqgs/puchta092011.aif )

Nominally the recorded seminar talk was based on this paper, but there is considerable overlap with what just appeared:
http://arxiv.org/abs/1107.5185
Feynman diagrammatic approach to spin foams
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 26 Jul 2011)
"The Spin Foams for People Without the 3d/4d Imagination" could be an alternative title of our work. We derive spin foams from operator spin network diagrams} we introduce. Our diagrams are the spin network analogy of the Feynman diagrams. Their framework is compatible with the framework of Loop Quantum Gravity. For every operator spin network diagram we construct a corresponding operator spin foam. Admitting all the spin networks of LQG and all possible diagrams leads to a clearly defined large class of operator spin foams. In this way our framework provides a proposal for a class of 2-cell complexes that should be used in the spin foam theories of LQG. Within this class, our diagrams are just equivalent to the spin foams. The advantage, however, in the diagram framework is, that it is self contained, all the amplitudes can be calculated directly from the diagrams without explicit visualization of the corresponding spin foams. The spin network diagram operators and amplitudes are consistently defined on their own. Each diagram encodes all the combinatorial information. We illustrate applications of our diagrams: we introduce a diagram definition of Rovelli's surface amplitudes as well as of the canonical transition amplitudes. Importantly, our operator spin network diagrams are defined in a sufficiently general way to accommodate all the versions of the EPRL or the FK model, as well as other possible models. The diagrams are also compatible with the structure of the LQG Hamiltonian operators, what is an additional advantage. Finally, a scheme for a complete definition of a spin foam theory by declaring a set of interaction vertices emerges from the examples presented at the end of the paper.
36 pages, 23 figures

 

[marcus]

Karmerlo's original question, starting the thread, was about recent Loop developments. My take on that is based on the idea that this is a fast moving field and that at each stage there should be a clear definite testable formulation. In 2010 we got a new version of Loop that culminated in the Zakopane 2011 version ( http://arxiv.org/abs/1102.3660 ). Now I'm looking ahead to see what the 2013 formulation might be like.

This is a quiet period now when IMHO people are getting new thoughts in order.

I expect quantum relativists to construct a QG which resolves the cosmo ("bang") singularity, is testable with early universe data, and recovers a good approximation of usual GR. As long we don't have SEVERAL alternatives that do this, and therefore do not need to choose between them, I am not going to quibble about the "pedigree" or quantization ritual that was used to arrive at the theory. That comes later when we have more than one satisfactory alternative.

This is simply my (pragmatic) philosophy---other people of course assume other intellectual stances.

So thinking ahead to Loops 2013 (to be held at Perimeter Institute) what is the most important paper we should now be looking at? What has the seeds of a new formulation?
I think it is page 5 of the January 2012 paper of Bonzom and Smerlak. Merely an observant bystander's nonprofessional guess--but maybe sometimes that's OK to offer. In the excerpt that follows I used curly brackets to distinguish the moduli space {M} from the ordinary manifold M, while the authors used a special font.

==quote from page 5 of http://arxiv.org/1201.4996 ==
Relation to the loop formalism. The above method naturally gives rise to the loop quantization of BF theory. In the loop approach, one quantizes before restricting to flat gauge fields. Given an embedded, closed graph γ, cylindrical wave functions are functions of the Wilson lines along the lines of γ. For each graph there is a Hilbert space whose measure is given by the Haar measure of G on each line, ∏_e dg_e. The Hilbert spaces of two different graphs are orthogonal. The standard gauge symmetry requires invariance under G-translation on the source and end nodes of the lines.

Heuristically, the transition amplitudes in the continuum (7) suggest that they can be formulated in the loop approach by taking as boundary states cylindrical functions restricted to the moduli space {M}, the torsion still providing the measure. Assume M has two disconnected boundaries N₁,N₂, with two closed, embedded graphs γ₁, γ₂ associated with two cylindrical functions Ψ_γ1 , Ψ_γ2 . The transition is regularized by choosing a cell decomposition K of M such that γ₁,γ₂ are included into the 1-skeleton. The ungauge-fixed transition amplitude reads

⟨Ψ_γ2|Z_BF|Ψ_γ1⟩=∫∏_edg_e Ψ^∗_γ2(g_e)Ψ_γ1(g_e)∏_fδ(H_f).

As the shift symmetry does not act on Wilson lines, the process of the previous section applies. The wave-functions are evaluated on {M} because there are no fluctuations around flat connections, yielding [eqn (31)]:

⟨Ψ_γ2 |Z'_BF|Ψ_γ1⟩ = Ʃ_[φ]∈MΨ^∗_γ2([φ]) Tor[φ] Ψ_γ1([φ]).

Finally, the regulator K can be removed thanks to the topological invariance of the torsion, which makes the continuum limit result into the above formula. Let us mention an outcome of this result: the loop quantization of the BF model does not distinguish knottings of the graphs γ_1,2.

Conclusion. We have performed a topological quantization of discrete BF theory, proving its equivalence to the usual quantization in the continuum. This result solves several open problems of the field and extends previous results obtained in dimension 3 to arbitrary dimensions:
(1) transition amplitudes are finite, answering the issue of bubble divergences [11, 28];
(2) the gauge symmetries in the discrete setting exist, generalizing [11, 12], and
(3) they can be gauge-fixed to derive the loop quantization, generalizing [13];
(4) as a result, one gets a topological invariant, which proves that the classical gauge symmetries are correctly promoted to the quantum level.

The crucial steps of our quantization require to take into account cells of all dimensions in the cell complex, and not just its 2-skeleton like in the “spinfoam quantization”. A challenge for future investigations is to find a representation of (31) as a state-sum, as is done in the latter approach.

The last issue we mentioned in the introduction is the major difficulty in quantum gravity: understanding the quantum version of diffeomorphism-invariance. It is well-known that diffeomorphism-invariance in the BF model is contained within its shift symmetry [20]. Hence the substance of general relativity is to break the topological invariance while preserving diffeomorphism-invariance. Spinfoam models for quantum gravity are very much in line with this idea, as they start by quantizing BF theory and then introduce some breaking of the shift symmetry to restore the local degrees of freedom. It is also known that discrete models of gravity generically break diffeomorphism-invariance [17]. Showing that it is restored in the continuum limit (after some coarse-graining, or summing over spinfoams appropriately) is one of the main programs in the spinfoam approach. Now that the shift symmetry is correctly controlled in the discrete setting, we feel that the noose is tightening around diffeomorphisms.
==endquote==

 

[atyy]

==quote from page 5 of http://arxiv.org/abs/1201.4996 ==

[...]

The crucial steps of our quantization require to take into account cells of all dimensions in the cell complex, and not just its 2-skeleton like in the “spinfoam quantization”. A challenge for future investigations is to find a representation of (31) as a state-sum, as is done in the latter approach.

[...]

==endquote==

So they don't agree with spinfoam quantization?

 

[marcus]

My guess is that this is the new spinfoam. The old 2-skeleton approach was started back in the late 1990s by Reisenberger and Rovelli. It is over 20 years old and probably needs to be updated. Of course I could be wrong

 

[marcus]

Matteo Smerlak's PhD thesis is a useful source of background for the 6-page Bonzom-Smerlak letter.
I should give the latter's abstract--didn't do that yet.

http://arxiv.org/abs/1201.4996
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
Valentin Bonzom, Matteo Smerlak
(Submitted on 24 Jan 2012)
The spinfoam approach to quantum gravity rests on a "quantization" of BF theory using 2-complexes and group representations. We explain why, in dimension three and higher, this "spinfoam quantization" must be amended to be made consistent with the gauge symmetries of discrete BF theory. We discuss a suitable generalization, called "cellular quantization", which
(1) is finite,
(2) produces a topological invariant,
(3) matches with the properties of the continuum BF theory,
(4) corresponds to its loop quantization. These results significantly clarify the foundations - and limitations - of the spinfoam formalism, and open the path to understanding, in a discrete setting, the symmetry-breaking which reduces BF theory to gravity.
6 pages

A concise excerpt from page 1:
==quote 1201.4996==
The purpose of this letter is to argue that there is a good reason for this: when dealing with 2-complexes only, as in the spinfoam formalism, there is no shift symmetry. To identify this symmetry, one must instead resort to an extension of the spinfoam formalism including higher-dimensional cells. This realization paves the way to what we call cellular quantization. This cellular quantization solves problems 1 to 4, and sheds interesting new light on problem 5.
The letter is organized as follows. We start by reviewing the basic properties of the continuum BF theory, emphasizing its gauge symmetries and relationship to analytic torsion. We then describe the “spinfoam quantization” of BF theory, as described e.g. in Baez’s reference paper [5]. We show how to identify the gauge symmetries in a discrete setting and perform a quantization which does preserve the topological features of the continuum theory. Finally we establish that this cellular quantization corresponds to the loop canonical quantization.
==endquote==

Problems 1 through 5, mentioned in the above excerpt, are as follows:
==quote==
1. Bubble divergences. The original PRO [Ponzano-Regge-Oguri] partition functions are in general divergent. How should one regularize them?

2. Topological invariance. The PRO partition functions are formally invariant under changes of triangulations, up to divergent factors. How can one turn them into finite topological invariants?

3. Relationship to the canonical theory. The connection between the Ponzano-Regge model and loop quantum gravity in 3 dimensions was established in [13]. Can this connection be extended to 4 dimensions and higher?

4. Relationship to the continuum theory. BF theory was quantized in the continuum in [21, 22], and was showed to be related to the Ray-Singer torsion. Are the PRO models similarly related to torsion? (See [14] for a positive answer in certain three-dimensional cases.)

5. Diffeomorphism symmetry. Both the continuum BF action and the Einstein-Hilbert action are diffeomorphism-invariant. What is the fate of this symmetry in the PRO models?
==endquote==

For completeness, here is the abstract of Smerlak's thesis. It doesn't overlap in results, but shares some concepts---therefore is helpful in part simply because it is longer (over 100 pages instead of only 6) and more deliberate. Goes thru some definitions in a less condensed way.
http://arxiv.org/abs/1201.4874
Divergences in spinfoam quantum gravity
Matteo Smerlak
(Submitted on 23 Jan 2012)
In this thesis we study the flat model, the main buidling block for the spinfoam approach to quantum gravity, with an emphasis on its divergences. Besides a personal introduction to the problem of quantum gravity, the manuscript consists in two part. In the first one, we establish an exact powercounting formula for the bubble divergences of the flat model, using tools from discrete gauge theory and twisted cohomology. In the second one, we address the issue of spinfoam continuum limit, both from the lattice field theory and the group field theory perspectives. In particular, we put forward a new proof of the Borel summability of the Boulatov-Freidel-Louapre model, with an improved control over the large-spin scaling behaviour. We conclude with an outlook of the renormalization program in spinfoam quantum gravity.
113 pages. PhD thesis, introduction and conclusion in French, main text in English.Paper by Ileana Naish-Guzman and John Barrett cited on page 4 ref [14] in connection with the discrete exterior derivative on a cell complex. http://arxiv.org/abs/0803.3319
Similarly cited was [26] an earlier paper by Bonzom and Smerlak http://arxiv.org/abs/1103.3961
Additional webstuff about de Rham complex
http://en.wikipedia.org/wiki/De_Rham_cohomology
http://www.vttoth.com/CMS/pahysics-notes/43-about-the-de-rham-complex

 

[marcus]

A major international conference like the triennial Marcel Grossmann meeting can give a snapshot of "recent development".

Here is the brief statement summing up the situation from the chair of one of the LQG sessions at MG13 (Stockholm July 2012):

http://www.icra.it/MG/mg13/par_sessions_chairs_details.htm#lewandowski

Jerzy LEWANDOWSKI

Parallel Session: SQG2 - Loop Quantum Gravity, Quantum Geometry, Spin Foams

Description: Loop Quantum Gravity (LQG), a framework suited to quantize general relativity, has seen rapid progress in the last three years. The results achieved strongly suggest that the goal of finding a working and predictive quantum theory of gravity is within reach. For specific kinds of matter couplings, a way to drastically simplify the dynamics and its physical interpretation has been discovered. It gives rise to a set of examples of theories of gravity coupled to the fields in which the canonical quantization scheme can be completed. Independently, there have been important breakthroughs in the path integral formulation of the theory related to the so called Spin Foam Models. The session will review the results of canonical Loop Quantum Gravity and Spin Foam Models with the emphasis on the models admitting local degrees of freedom without the symmetry (or any other) reduction. Related approaches to quantum gravity will be also welcome. The common theme is the background independent quantization of Einstein's gravity and the occurrence of quantum geometry.​

Lewandowski heads the LQG group at Warsaw (one of a handful of leading groups: Marseille, Perimeter, PennState, Warsaw, LSU, Erlangen, AEI...)
I gather that another triennial meeting, the International Conference on General Relativity and Gravitation, will have its 2013 venue in Warsaw. Lewandowski will doubtless be the main organizer of GR20. It should have an interesting lineup in QG: background independent quantum geometry.

The next biennial Loops conference, Loops 2013, will be held at Perimeter. One way to gauge progress and follow developments is to keep an eye on the topics featured in the programmes of the main conferences as they take shape.
========================
Along the same lines, we already know the LQG talks to be presented at the April 2012 meeting of the APS (American Physical Society). I listed links and abstracts here:
https://www.physicsforums.com/showthread.php?p=3784064#post3784064
and here:
https://www.physicsforums.com/showthread.php?p=3788486#post3788486

Loop is in course of achieving parity with String, visibility-wise at major conferences. One can get an idea of which recent directions and results in Loop research are considered important by seeing what the main conference talks, especially those invited by the organizers, are about.

 

[marcus]

Another snapshot of the current definition of Loop and the problems to be worked on will be Rovelli's upcoming (23 April) talk at the Princeton Institute of Advanced Studies.
http://www.princeton.edu/physics/events/viewevent.xml?id=347

High Energy Theory Seminar - IAS - Carlo Rovelli, Aix-Marseille University, France - Loop quantum Gravity: Recent Results and Open Problems
Description: The loop approach to quantum gravity has developed considerably during the last few years, especially in its covariant ('spinfoam') version. I present the current definition of the theory and the results that have been proven. I discuss what I think is still missing towards of the goal of defining a consistent tentative quantum field theory genuinely background independent and having general relativity as classical limit.

Location: Bloomberg Lecture Hall
Date/Time: 04/23/12 at 2:30 - 3:30 pm
============================

My comment. This talk may be substantially similar to the one at Perimeter on 4 April, which I believe will subsequently be available video online. I imagine there might be an eventual write-up covering the same material.
Loop is fast moving and is reformulated from time to time. It has been having small "revolutions" on roughly a 3-5 year basis, so Rovelli's wording should be noted "present the current definition...results that have been proven...what I think is still missing..."

"Tentative" here I think means an attempt: to be tested by observation---to be put on trial in other words.
Any theory can only be tentative until its predictions are tested and either confirmed or not. Over the past few years it's been mainly up to Rovelli to give a clear precise definition of the current Loop theory, write the survey papers, list the open problems.

So these two talks will serve as a significant landmark. Will it be essentially a restatement of the February 2011 formulation (Zakopane lectures http://arxiv.org/abs/1102.3660 ) or will there be some new features? Here's the PIRSA link for the Wednesday 4 April one at Perimeter.
http://pirsa.org/12040059

 

[marcus]

The University of Vienna and Vienna Tech are holding a 5-day Quantum physics + Gravity school in early September, intended for PhD students and other young researchers wanting to get into gravity-related research.

A nice feature is how applied it is.
2 out of the 4 main lecturers are discussing applications (linked to astrophysical observation) rather than pure QG theory.

http://www.coqus.at/events/summerschool2012/

The title of the School is Quantum physics meets Gravity
Here's the poster:
http://www.coqus.at/fileadmin/user_upload/ag_quantum/Coqus/Events/CoQuS_a3.pdf

Here are the more applied, hardware-oriented topics that two of the lecture series will be about:

"Experimental gravitation and geophysics with matter-wave sensors"

"Gravitational wave detection and quantum control"
===============================

While I think of it, the journal SIGMA is publishing a special issue devoted to Loop gravity and cosmology assembled by a group of guest editors. They now have a dozen articles in final form having gone thru the peer review and editorial process. These could give some clues as to what the editors see as significant current developments in the field.
http://www.emis.de/journals/SIGMA/LQGC.html
==quote==
Papers in this Issue:

Introduction to Loop Quantum Cosmology
Kinjal Banerjee, Gianluca Calcagni and Mercedes Martín-Benito
SIGMA 8 (2012), 016, 73 pages [ abs pdf ]
Learning about Quantum Gravity with a Couple of Nodes
Enrique F. Borja, Iñaki Garay and Francesca Vidotto
SIGMA 8 (2012), 015, 44 pages [ abs pdf ]
Emergent Braided Matter of Quantum Geometry
Sundance Bilson-Thompson, Jonathan Hackett, Louis Kauffman and Yidun Wan
SIGMA 8 (2012), 014, 43 pages [ abs pdf ]
Matter in Loop Quantum Gravity
Ghanashyam Date and Golam Mortuza Hossain
SIGMA 8 (2012), 010, 26 pages [ abs pdf ]
Lessons from Toy-Models for the Dynamics of Loop Quantum Gravity
Valentin Bonzom and Alok Laddha
SIGMA 8 (2012), 009, 50 pages [ abs pdf ]
Entropy of Quantum Black Holes
Romesh K. Kaul
SIGMA 8 (2012), 005, 30 pages [ abs pdf ]
Discretisations, Constraints and Diffeomorphisms in Quantum Gravity
Benjamin Bahr, Rodolfo Gambini and Jorge Pullin
SIGMA 8 (2012), 002, 29 pages [ abs pdf ]
Numerical Techniques in Loop Quantum Cosmology
David Brizuela, Daniel Cartin and Gaurav Khanna
SIGMA 8 (2012), 001, 26 pages [ abs pdf ]
Statistical Thermodynamics of Polymer Quantum Systems
Guillermo Chacón-Acosta, Elisa Manrique, Leonardo Dagdug and Hugo A. Morales-Técotl
SIGMA 7 (2011), 110, 23 pages [ abs pdf ]
The Space of Connections as the Arena for (Quantum) Gravity
Steffen Gielen
SIGMA 7 (2011), 104, 12 pages [ abs pdf ]
Equivalent and Alternative Forms for BF Gravity with Immirzi Parameter
Merced Montesinos and Mercedes Velázquez
SIGMA 7 (2011), 103, 13 pages [ abs pdf ]
A Lorentz-Covariant Connection for Canonical Gravity
Marc Geiller, Marc Lachièze-Rey, Karim Noui and Francesco Sardelli
SIGMA 7 (2011), 083, 10 pages [ abs pdf ]
==endquote==

 

[marcus]

Judging from past performance, Rovelli can be relied on for a comprehensive insightful account of the current state of development of Loop gravity and the remaining problems to be worked out. The abstract of the talk he is to give at Princeton IAS was posted earlier.

Today I see that the abstract for the Perimeter Institute colloquium talk he is giving next Wednesday (4 April) has also been posted. It's approximately the same summary description, probably much the same talk. So if things go as expected, we'll soon have an online video of an up-to-date firsthand view of Loop with Perimeter audience Q&A.

http://pirsa.org/12040059/
Transition Amplitudes in Quantum Gravity
Speaker(s): Carlo Rovelli
Abstract: The covariant formulation of loop quantum gravity has developed strongly during the last few years. I summarize the current definition of the theory and the results that have been proven. I discuss what is missing towards of the goal of defining a consistent quantum theory whose classical limit is general relativity.
Date: 04/04/2012 - 2:00 pm

For comparison here is the announcement of the Princeton talk:
http://www.princeton.edu/physics/events/viewevent.xml?id=347
High Energy Theory Seminar - IAS - Carlo Rovelli, Aix-Marseille University, France - Loop quantum Gravity: Recent Results and Open Problems
Description: The loop approach to quantum gravity has developed considerably during the last few years, especially in its covariant ('spinfoam') version. I present the current definition of the theory and the results that have been proven. I discuss what I think is still missing towards of the goal of defining a consistent tentative quantum field theory genuinely background independent and having general relativity as classical limit.
Location: Bloomberg Lecture Hall
Date/Time: 04/23/12 at 2:30 - 3:30 pm

Rovelli will also be giving a series of lectures on QG during the first week of September at the Vienna "Quantum Physics meets Gravity" School that I mentioned in the preceding post:
http://www.coqus.at/events/summerschool2012/
=================================

The April meeting of the American Physical Society also starts next week in Atlanta. There will be several invited and contributed Loop talks. Links and abstracts are listed here:
https://www.physicsforums.com/showthread.php?p=3784064#post3784064
and here:
https://www.physicsforums.com/showthread.php?p=3788486#post3788486
Bianchi and Agullo give invited talks Monday 2 April
http://meetings.aps.org/Meeting/APR12/Event/170161
http://meetings.aps.org/Meeting/APR12/Event/170160

 

[marcus]

There will be a QG School in Beijing in August. I was interested to see who the lecturers are going to be:
http://physics.bnu.edu.cn/summerschool/en/index.php
==quote==
The 2nd BNU International Summer School on Quantum Gravity: 12-18 August 2012, Beijing
Beijing Normal University (BNU), China
The BNU International Summer School on Quantum Gravity is intended to provide a pedagogical introduction for graduate students and young post-docs to the main fields closely related to loop quantum gravity.

Topics include: Loop quantum gravity, Loop quantum cosmology, Spin foams, Group field theory, Regge calculus

Lecturers:
Abhay Ashtekar (Penn State Univ, USA)
Benjamin Bahr (Cambridge Univ, UK)
John Barrett (Univ of Nottingham, UK)
Jonathan Engle (Florida Atlantic Univ, USA)
Thomas Krajewski (Univ of Provence & CPT Marseille, France)
Jerzy Lewandowski (Univ of Warsaw, Poland)
Etera Livine (ENS de Lyon, France)
==endquote==

The University of Vienna and Vienna Tech are holding a 5-day Quantum physics + Gravity school in early September, intended for PhD students and other young researchers wanting to get into gravity-related research.
http://www.coqus.at/events/summerschool2012/
The title of the School is Quantum physics meets Gravity
Here's the poster:
http://www.coqus.at/fileadmin/user_upload/ag_quantum/Coqus/Events/CoQuS_a3.pdf
Lecturers:
· Philippe Bouyer (University of Bordeaux, Institut d'optique and CNRS, France)
· Michèle Heurs (Max Planck Institute for Gravitational Physics, Hannover, Germany)
· Ulf Leonhardt (University of St. Andrews, UK)
· Carlo Rovelli (Centre de Physique Theorique de Luminy, Marseille, France
===============================
Update on the journal SIGMA's special issue on Loop Gravity and Cosmology, being assembled by a group of guest editors. So far they have fifteen articles in final form which have passed peer review and been posted online. I was interested to see the lineup since it gives an idea of what the editors see as significant current research directions.
http://www.emis.de/journals/SIGMA/LQGC.html
Here's an updated listing of the articles. They are free online.

Colored Tensor Models - a Review
Razvan Gurau and James P. Ryan
SIGMA 8 (2012), 020, 78 pages [ abs pdf ]
Intersecting Quantum Gravity with Noncommutative Geometry - a Review
Johannes Aastrup and Jesper Møller Grimstrup
SIGMA 8 (2012), 018, 25 pages [ abs pdf ]
Relational Observables in Gravity: a Review
Johannes Tambornino
SIGMA 8 (2012), 017, 30 pages [ abs pdf ]
Introduction to Loop Quantum Cosmology
Kinjal Banerjee, Gianluca Calcagni and Mercedes Martín-Benito
SIGMA 8 (2012), 016, 73 pages [ abs pdf ]
Learning about Quantum Gravity with a Couple of Nodes
Enrique F. Borja, Iñaki Garay and Francesca Vidotto
SIGMA 8 (2012), 015, 44 pages [ abs pdf ]
Emergent Braided Matter of Quantum Geometry
Sundance Bilson-Thompson, Jonathan Hackett, Louis Kauffman and Yidun Wan
SIGMA 8 (2012), 014, 43 pages [ abs pdf ]
Matter in Loop Quantum Gravity
Ghanashyam Date and Golam Mortuza Hossain
SIGMA 8 (2012), 010, 26 pages [ abs pdf ]
Lessons from Toy-Models for the Dynamics of Loop Quantum Gravity
Valentin Bonzom and Alok Laddha
SIGMA 8 (2012), 009, 50 pages [ abs pdf ]
Entropy of Quantum Black Holes
Romesh K. Kaul
SIGMA 8 (2012), 005, 30 pages [ abs pdf ]
Discretisations, Constraints and Diffeomorphisms in Quantum Gravity
Benjamin Bahr, Rodolfo Gambini and Jorge Pullin
SIGMA 8 (2012), 002, 29 pages [ abs pdf ]
Numerical Techniques in Loop Quantum Cosmology
David Brizuela, Daniel Cartin and Gaurav Khanna
SIGMA 8 (2012), 001, 26 pages [ abs pdf ]
Statistical Thermodynamics of Polymer Quantum Systems
Guillermo Chacón-Acosta, Elisa Manrique, Leonardo Dagdug and Hugo A. Morales-Técotl
SIGMA 7 (2011), 110, 23 pages [ abs pdf ]
The Space of Connections as the Arena for (Quantum) Gravity
Steffen Gielen
SIGMA 7 (2011), 104, 12 pages [ abs pdf ]
Equivalent and Alternative Forms for BF Gravity with Immirzi Parameter
Merced Montesinos and Mercedes Velázquez
SIGMA 7 (2011), 103, 13 pages [ abs pdf ]
A Lorentz-Covariant Connection for Canonical Gravity
Marc Geiller, Marc Lachièze-Rey, Karim Noui and Francesco Sardelli
SIGMA 7 (2011), 083, 10 pages [ abs pdf ]

 

[marcus]

We can start looking ahead to the next biennial Loops conference: Loops 2013, to be held at Perimeter Institute. I'm told that planning for the conference has already started. Loop gravity is constantly evolving as a theory and a paper suggesting a new classical action sets the stage, I think, for the 2013 version of the quantum theory.

http://arxiv.org/abs/1205.0733
Discrete Symmetries in Covariant LQG
Carlo Rovelli, Edward Wilson-Ewing
(Submitted on 3 May 2012)
We study time-reversal and parity ---on the physical manifold and in internal space--- in covariant loop gravity. We consider a minor modification of the Holst action which makes it transform coherently under such transformations. The classical theory is not affected but the quantum theory is slightly different. In particular, the simplicity constraints are slightly modified and this restricts orientation flips in a spinfoam to occur only across degenerate regions, thus reducing the sources of potential divergences.
8 pages

The classical basis for the theory is the Holst action. A 4D manifold M equipped with internal Minkowski space M at each point plus a tetrad e (one-form valued in M) and a connection ω. The conventional Holst action:

S[e,ω]=∫e^IΛe^JΛ(* + 1/γ) F_{I J}

The * denotes the Hodge dual. A proposed new action S' uses the signum of det e: s = sign(det e) defined to be zero if det e = 0 and otherwise ±1.

S'[e,ω]=∫e^IΛe^JΛ(s* + 1/γ) F_{I J}

There is also a closely related alternative action S" discussed in the paper.

It looks to me as if either S' or S", suitably quantized, takes care of the semiclassical limit of the theory. See equation (43) of the paper. Or in any case represents a major step. The problem addressed was that the original EPRL looked at in the limit exhibited both spacetime and "anti-spacetime" evolution. Both time-forward and time-reversed evolution appeared. Otherwise everything was properly Regge as expected. Then there were papers by Yasha Neiman and by Jon Engle that studied this bi-directional time mixup. Rovelli and Wilson-Ewing (RWE) built on their results. So I expect this RWE paper to provide a basis for a new Loop initiative leading up to the conference about a year from now. Of course it is risky (even foolhardy) to forecast research trends. But that's what I think after reading the paper. It addresses several of the remaining problems in the theory---and it's quite interesting as well.

 

[tom.stoer]

very interesting, but many difficulties in the spin foam approach (you what I am talking about ;-) are still not addressed

 

[marcus]

(you what I am talking about ;-)

Of course ;-)
A basic premise of this thread is that there are plenty of well-defined problems to work on.

That is what this thread is fundamentally about: current progress in solving problems, essentially by gradually redefining the theory.

The story can be told, to some extent, in human terms. There are people who not only identify problems but also do something about them.

Alesci focused on a problem with the Thiemann hamiltonian and proposed a new hamiltonian (able to grow volume). He is now joining Lewandowski's Warsaw group as postdoc.
http://sites.google.com/site/grqcrumourmill/

Engle focused on a problem with largescale limit of the EPRL vertex and proposed a solution which prefigured the one in the "Discrete Symmetries" paper just mentioned. He is now faculty at Florida Atlantic.

Ryan ("Simplicity constraints and the Immirzi parameter in discrete quantum gravity" ILQGS) has accepted a tenure track position at Morelia.
http://relativity.phys.lsu.edu/ilqgs/ryan041211.pdf
http://sites.google.com/site/grqcrumourmill/

At each stage there is a rather well-defined theory, which gives focus to the program, and there are opportunities for young researchers to do things which are clearly significant.

 

[marcus]

Here's an indication of interests in the broader community of Mathematical Physics. Some of the speakers' names will be familiar to people at BtSM forum especially those who have been following developments in Group Field Theory/Loop-and-allied QG research.

==quote==
The XXIX International Colloquium on Group-Theoretical Methods in Physics
August 20-26, 2012, Chern Institute of Mathematics, Tianjin, China

Plenary Speakers (Titles and Abstracts)

Akito Arima (Musashi Gakuen, Japan)
Abhay Ashtekar (Pennsylvania State University, USA)
Murray Batchelor (Australian National University, Australia)
Edward Corrigan (University of York, UK)
Mo-Lin Ge (Nankai University, China)
Razvan Gurau (Perimeter Institute for Theoretical Physics, Canada) (Hermann Weyl Prize Awardee)
Alden Mead (University of Minnesota, USA) (Wigner Medal Awardee)
Jerzy Lewandowski (University of Warsaw, Poland)
Jun Murakami (Waseda University, Japan)
Daniel Treille (CERN, Switzerland) (Public Lecturer)
Vincent Rivasseau (University Paris-Sud XI, France)
Zhenghan Wang (Microsoft Station Q, USA)

The time allocated for each plenary talk is 60 minutes (50 min presentation + 10 min questions).
==endquote==
This time the biennial Group-Theory-in-Physics Conference will have 12 parallel sessions, of which #8 is "Loop Quantum Gravity", and #9 is "Group Field Theory for Quantum Gravity".
I'm hoping that the titles of the talks (plenary 60 minute and parallel 30 minute) will be posted soon and that we can get from them some idea of how particular foci of interest are shaping up.
http://www.nim.nankai.edu.cn/activites/conferences/hy20120820/pdf/1st-Announcement.pdf

http://www.cim.nankai.edu.cn/activites/conferences/hy20120820/index.htm