What is the recent development of Loop Quantum Gravity

[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

 

[marcus]

A week from today there will be two Loop talks at Perimeter, which we can expect to have available as online PIRSA video.
http://www.perimeterinstitute.ca/en/Scientific/Seminars/Seminars_Overview/?next=1338177600/

The first, at 2PM, will be the PI Colloquium talk, given by Eugenio Bianchi. Here's the Pirsa link:
http://pirsa.org/12050053
Then at 4PM there will be a QG seminar talk by Hal Haggard.
Haggard got his PhD in Loop QG from UC Berkeley last year and has been teaching courses in the Physics department at Berkeley for the past couple of semesters.
Here is the abstract and Pirsa link for his talk:

http://pirsa.org/12050084
Pentahedral Volume, Chaos, and Quantum Gravity
Speaker(s): Hal Haggard
Abstract: The space of convex polyhedra can be given a dynamical
structure. Exploiting this dynamics we have performed a Bohr-Sommerfeld
quantization of the volume of a tetrahedral grain of space, which is in
excellent agreement with loop gravity. Here we present 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. This results
in new information about cylindrical consistency in loop gravity and a couple
of surprises about polyhedra. In particular, we are interested in discovering
whether the evolution generated by this volume is chaotic or integrable as this
will impact the interpretation of the spin network basis in loop gravity.
Date: 30/05/2012 - 4:00 pm

Later this summer, Haggard will be one of the invited speakers at the "Groups 29" (Group Theoretical Methods in Physics) conference mentioned in the previous post. He then takes up a 2-year postdoc fellowship at Marseille starting in September. Part of the prior work referred to in the abstract is research he co-authored with Bianchi.