Introduction To Loop Quantum Gravity

marcus · Jan 2, 2009

Some helpful links, for staying informed
http://www.gr19.com/index.php
http://grg.maths.qmul.ac.uk/hyperspace/conference/08Dec.8.html
http://www.icra.it/MG/mg12/en/parallel_session.htm#
http://www.icra.it/MG/mg12/en/
http://grg.maths.qmul.ac.uk/hyperspace/conference/
http://strings2009.roma2.infn.it/home.html

Rovelli's 2008 LQG overview:
http://relativity.livingreviews.org/Articles/lrr-2008-5/
His 2008 LQG survey talk to the Strings '08 conference, video
http://cdsweb.cern.ch/record/1121957?ln=en

Page for Yongge Ma, a principal organizer of the August 2009 Loops 09 conference.
http://physics.bnu.edu.cn/application/research/gravity/LQG/eng/research.html

Most cited quantum cosmology papers, 2006 and later:
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+cosmology+AND+DATE+%3E+2005&FORMAT=www&SEQUENCE=citecount%28d%29

marcus · Jan 19, 2009

Preliminary list of speakers for the XXV Max Born
(Warsaw 29 June-3 July)

J. Ambjorn (NBI, Copenhagen)
G. Amelino-Camelia (La Sapienza, Rome)
M. Arzano (PI, Waterloo)
A.P. Balachandran (Syracuse Univ.)
J. Barrett (Univ. Nottingham)
E. Bergshoeff (Univ. Groeningen)
M. Bojowald (Penn State)
S. Carlip (UC Davis)
M. Chaichian (Univ. Helsinki)
A. Chamseddine (Univ. Beirut)
L. Dabrowski (SISSA)
S. Doplicher (La Sapienza, Rome)
O. Dreyer (MIT)
M. Duff (Imperial Coll. London)
J. Figueroa-O'Farrill (Univ. Edinburgh)
L. Freidel (PI, Waterloo)
G. Gibbons (DAMTP Cambridge)
S. Giddings (UC Santa Barbara)
T.R. Govindarayan (Chennai)
A. Grillo (Gran Sasso)
J. Jurkiewicz (Jagiellonian Univ.)
F. Lizzi (Univ. Naples)
R. Loll (Univ. Utrecht)
J. Lukierski (Univ. Wroclaw)
J. Magueijo (Imperial Coll. London)
F. Markopoulou (PI, Waterloo)
D. Marolf (UC Santa Barbara)
N. Mavromatos (King's Coll. London)
C. Meusburger (Univ. Nottingham)
L. Modesto (PI, Waterloo)
H. Nicolai (AEI, Golm)
D. Oriti (PI, Waterloo)
K. Peeters (Univ. Utrecht)
G. Piacitelli (SISSA)
M. Pospelov (Victoria Univ & PI)
C. Rovelli (Marseille, CPT)
B. Schroers (Heriot-Watt Univ. Edinburgh)
B. Sidharth (Udine Univ.)
K. Skenderis (Univ. Amsterdam)
L. Smolin (PI, Waterloo)
R. Szabo (Univ. Edinburgh)
T. Thiemann (AEI, Golm)
A. Tureanu (Univ. Helsinki)
S. Vaidya (Indian Inst. Sci. Bangalore)
Ch. Young (Univ. Durham)

The five major themes of the Symposium will be:

Loops and spin foams
Strings, AdS/CFT, and quantum gravity
Field theoretical perspective on Planck scale
Noncommutative spacetimes
Quantum gravity phenomenology

Organizers:

Jerzy Kowalski-Glikman (University of Wroclaw)
Krzysztof Meissner (University of Warsaw)

The Warsaw symposium is interesting in part because of the preliminary mix of speakers. Here's a partial list of some you might recognize from their association with nonperturbative background independent QG (Loop and related, Causal Dynamical Triangulations, Noncommutative Geometry, QG Phenom)

J. Ambjorn (NBI, Copenhagen)
G. Amelino-Camelia (La Sapienza, Rome)
J. Barrett (Univ. Nottingham)
M. Bojowald (Penn State)
S. Carlip (UC Davis)
A. Chamseddine (Univ. Beirut)
O. Dreyer (MIT)
L. Freidel (PI, Waterloo)
J. Jurkiewicz (Jagiellonian Univ.)
R. Loll (Univ. Utrecht)
J. Lukierski (Univ. Wroclaw)
J. Magueijo (Imperial Coll. London)
F. Markopoulou (PI, Waterloo)
L. Modesto (PI, Waterloo)
D. Oriti (PI, Waterloo)
C. Rovelli (Marseille, CPT)
L. Smolin (PI, Waterloo)
T. Thiemann (AEI, Golm)

marcus · Jan 19, 2009

Updated helpful links, for staying informed

General
http://www.maths.nottingham.ac.uk/qg/
http://grg.maths.qmul.ac.uk/hyperspace/conference/

Rome 22-26 June
http://strings2009.roma2.infn.it/home.html

Warsaw (XXV Max Born) 29 June-3 July
http://www.ift.uni.wroc.pl/~planckscale/
http://www.ift.uni.wroc.pl/~planckscale/index.html?page=speakers
http://www.fuw.edu.pl/~meissner/home.html

Paris (MG12) 12-18 July
http://www.icra.it/MG/mg12/en/
http://www.icra.it/MG/mg12/en/parallel_session.htm#

Beijing (Loops09) 2-8 August
http://grg.maths.qmul.ac.uk/hyperspace/conference/08Dec.8.html

Mexico City (GR19)
http://www.gr19.com/index.php Rovelli's 2008 LQG overview:
http://relativity.livingreviews.org/Articles/lrr-2008-5/
2008 LQG survey talk to the Strings '08 conference, video and slides
http://cdsweb.cern.ch/record/1121957?ln=en
http://indico.cern.ch/getFile.py/access?contribId=30&resId=0&materialId=slides&confId=21917
2008 LQG survey talk to Loops '08, general, slides, audio
http://www.maths.nottingham.ac.uk/qg/wiki/index.php/QGsquared-slides
http://www.maths.nottingham.ac.uk/qg/wiki/images/3/3f/RovelliCarlo1214510081.pdf Most cited Loop QG papers, 2006 and later:
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+GRAVITY+AND+DK+LOOP+SPACE+AND+DATE+%3E+2005&FORMAT=www&SEQUENCE=citecount%28d%29
Most cited (all types of) quantum cosmology papers, 2006 and later:
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+cosmology+AND+DATE+%3E+2005&FORMAT=www&SEQUENCE=citecount%28d%29

marcus · Feb 4, 2009

A list of plenary speakers for MG12 (Paris 12-16 July) has been posted:
http://www.icra.it/MG/mg12/en/invited_speakers.htm

Speakers include Alain Connes, Laurent Freidel, Herbert Hamber

Hamber is known for discrete nonperturbative QG work, Regge-related and lattice QG methods. Here is a recent review paper of his:
http://arxiv.org/abs/0901.0964

Speakers list has been posted for
Black Holes and Loop Quantum Gravity
Valencia, Spain 26-28 March, 2009.
Abhay Ashtekar
Martin Bojowald *
Alejandro Corichi
Bianca Dittrich
Jonathan Engle
Amit Ghosh
Viqar Husain
Jerzy Lewandowski *
Jorge Pullin
Hanno Sahlmann
Madhavan Varadarajan
* To be confirmed
http://www.uv.es/bhlqg/speakers.htm
http://www.uv.es/bhlqg/

Topics for the BH and LQG workshop:
- Black hole entropy in LQG
- Spin foam approach to black holes
- Singularity resolution and information loss
- Prospects for a detailed description of the Hawking radiation
- Comparison between results from LQG and other approaches

marcus · Feb 13, 2009

Another useful link for keeping informed:
http://www.maths.nottingham.ac.uk/qg/Meetings.html
The names of the main speakers have been posted for the Second Quantum Geometry and Quantum Gravity School.
In addition there will be invited seminars by a number of others (not yet listed.)
Two PF members were at the first QGQG school, and reported on it. It was at Zakopane (ski resort in Poland.)
This time the school will be at Corfu,13-20 September.
http://www.maths.nottingham.ac.uk/qg/CorfuSS.html

Each of the main speakers will give a series of lectures during the course of the week (a minicourse):

Abhay Ashtekar Loop Quantum Gravity

John Baez Categorification in Fundamental Physics

John Barrett Spin networks and quantum gravity

Vincent Rivasseau Renormalization in Fundamental Physics

Carlo Rovelli (*) Covariant loop quantum gravity and its low-energy limit

Projected course descriptions are at the website. I quote here just one--Rovelli's:
"I present a new look on Loop Quantum Gravity, aimed at giving a better grasp on its dynamics and its low-energy limit. Following the highly successful model of QCD, general relativity is quantized by discretizing it on a finite lattice, quantizing, and then studying the continuous limit of expectation values. The quantization can be completed, and two remarkable theorems follow.

The first gives the equivalence with the kinematics of canonical Loop Quantum Gravity.
This amounts to an independent re-derivation of all well known Loop Quantum gravity kinematical results.

The second [gives] the equivalence of the theory with the Feynman expansion of an auxiliary field theory. Observable quantities in the discretized theory can be identified with general relativity n-point functions in appropriate regimes. The continuous limit turns out to be subtly different from that of QCD, for reasons that can be traced to the general covariance of the theory.

I discuss this limit, the scaling properties of the theory, and I pose the problem of a renormalization group analysis of its large distance behavior."

(my highlighting etc.)

marcus · Feb 28, 2009

"Top 20" snapshot: themes of recent LQG papers

Here's an updated "top 20" snapshot of the field, the 20 most-cited papers from the past 2 years (date > 2006).

The list makes clear which have been the predominant themes of research recently: the new vertex, establishing a new covariant version of LQG consistent with the older canonical version, checking that the low energy limit comes out right (graviton propagator), and as always cosmology (LQC).

There is one new theme that did not show up until paper number 23 in the list. The bridge between LQG and LQC--which may lead to cosmological tests of LQG. Although it didn't make the top 20 this time, I will append that paper.

1) The Loop-quantum-gravity vertex-amplitude.
Jonathan Engle, Roberto Pereira, Carlo Rovelli (Marseille, CPT) . May 2007. 6pp.
Published in Phys.Rev.Lett.99:161301,2007.
e-Print: arXiv:0705.2388 [gr-qc]
Cited 49 times

2) An Introduction to Loop Quantum Gravity Through Cosmology.
Abhay Ashtekar (Penn State U. & Santa Barbara, KITP) . 20pp.
Published in Nuovo Cim.122B:135-155,2007.
e-Print: gr-qc/0702030
Cited 46 times

3) A New Spin Foam Model for 4d Gravity.
Laurent Freidel (Perimeter Inst. Theor. Phys.) , Kirill Krasnov (Perimeter Inst. Theor. Phys. & Nottingham U.) . 40pp.
Published in Class.Quant.Grav.25:125018,2008.
e-Print: arXiv:0708.1595 [gr-qc]
Cited 46 times

4) Flipped spinfoam vertex and loop gravity.
Jonathan Engle, Roberto Pereira, Carlo Rovelli (Marseille, CPT) . 37pp.
Published in Nucl.Phys.B798:251-290,2008.
e-Print: arXiv:0708.1236 [gr-qc]
Cited 38 times

5) The Complete LQG propagator. I. Difficulties with the Barrett-Crane vertex.
Emanuele Alesci (Rome U. & Marseille, CPT) , Carlo Rovelli (Marseille, CPT) . 31pp.
Published in Phys.Rev.D76:104012,2007.
e-Print: arXiv:0708.0883 [gr-qc]
Cited 36 times

6) LQG vertex with finite Immirzi parameter.
Jonathan Engle (Marseille, CPT) , Etera Livine (Lyon, Ecole Normale Superieure) , Roberto Pereira, Carlo Rovelli (Marseille, CPT) . 8pp.
Published in Nucl.Phys.B799:136-149,2008.
e-Print: arXiv:0711.0146 [gr-qc]
Cited 33 times

7) Singularities and Quantum Gravity.
Martin Bojowald (Penn State U. & Santa Barbara, KITP) . 41pp.
Lectures given at 12th Brazilian School of Cosmology and Gravitation (XII BSCG), Rio de Janeiro, Brazil, 10-23 Sep 2006.
Published in AIP Conf.Proc.910:294-333,2007. Also in *Rio de Janeiro 2006, Cosmology and gravitation* 294-333
e-Print: gr-qc/0702144
Cited 20 times

8) The Perturbative Regge-calculus regime of loop quantum gravity.
Eugenio Bianchi (Pisa, Scuola Normale Superiore & INFN, Pisa) , Leonardo Modesto (Bologna U. & INFN, Bologna & Marseille, CPT) . 43pp.
Published in Nucl.Phys.B796:581-621,2008.
e-Print: arXiv:0709.2051 [gr-qc]
Cited 20 times

9) Loop quantum gravity corrections to gravitational wave dispersion.
Martin Bojowald, Golam Mortuza Hossain (Penn State U.) . 27pp.
Published in Phys.Rev.D77:023508,2008.
e-Print: arXiv:0709.2365 [gr-qc]
Cited 20 times

10) Lorentzian LQG vertex amplitude.
Roberto Pereira (Marseille, CPT) . 9pp.
Published in Class.Quant.Grav.25:085013,2008.
e-Print: arXiv:0710.5043 [gr-qc]
Cited 20 times

11) Spin foam model from canonical quantization.
Sergei Alexandrov (Montpellier U., Astropart. Group) . 25pp.
Published in Phys.Rev.D77:024009,2008.
e-Print: arXiv:0705.3892 [gr-qc]
Cited 17 times

12) The Complete LQG propagator. II. Asymptotic behavior of the vertex.
Emanuele Alesci (Rome III U. & Marseille, CPT) , Carlo Rovelli (Marseille, CPT) . 16pp.
Published in Phys.Rev.D77:044024,2008.
e-Print: arXiv:0711.1284 [gr-qc]
Cited 17 times

13) The Dark Side of a Patchwork Universe.
Martin Bojowald (Penn State U.) . 24pp.
Published in Gen.Rel.Grav.40:639-660,2008.
e-Print: arXiv:0705.4398 [gr-qc]
Cited 15 times

14) Inflationary universe in loop quantum cosmology.
Xin Zhang (Beijing, KITPC) , Yi Ling (Nanchang U.) . 25pp.
Published in JCAP 0708:012,2007.
e-Print: arXiv:0705.2656 [gr-qc]
Cited 14 times

15) Quantum Gravity Phenomenology.
Giovanni Amelino-Camelia . 82pp.
e-Print: arXiv:0806.0339 [gr-qc]
Cited 14 times

16) Path integral representation of spin foam models of 4d gravity.
Florian Conrady, Laurent Freidel (Perimeter Inst. Theor. Phys.) . 29pp.
Published in Class.Quant.Grav.25:245010,2008.
e-Print: arXiv:0806.4640 [gr-qc]
Cited 14 times

17) Loop quantum cosmology.
Martin Bojowald (Penn State U. & Potsdam, Max Planck Inst.) . 131pp.
Published in Living Rev.Rel.11:4,2008.
Cited 14 times

18) Loop quantization of spherically symmetric midi-superspaces.
Miguel Campiglia, Rodolfo Gambini (Republica U., Montevideo) , Jorge Pullin (Louisiana State U.) . 18pp.
Published in Class.Quant.Grav.24:3649-3672,2007.
e-Print: gr-qc/0703135
Cited 13 times

19) Loop Quantum Gravity: Four Recent Advances and a Dozen Frequently Asked Questions.
Abhay Ashtekar (Penn State U.) . 21pp.
To appear in the proceedings of 11th Marcel Grossmann Meeting on Recent Developments in Theoretical and Experimental General Relativity, Gravitation, and Relativistic Field Theories, Berlin, Germany, 23-29 Jul 2006.
e-Print: arXiv:0705.2222 [gr-qc]
Cited 13 times

20) Linearized dynamics from the 4-simplex Regge action.
Bianca Dittrich, Laurent Freidel, Simone Speziale (Perimeter Inst. Theor. Phys.) . 25pp.
Published in Phys.Rev.D76:104020,2007.
e-Print: arXiv:0707.4513 [gr-qc]
Cited 13 times

=========================================
The following represents an important new direction in research which I expect to become increasingly important in the next couple of years. I'll tack it on here even though it did not make the top 20:

23) Relating loop quantum cosmology to loop quantum gravity: Symmetric sectors and embeddings.
Jonathan Engle (Marseille, CPT & Provence U. & Marseille U., Luminy) . 20pp.
Published in Class.Quant.Grav.24:5777-5802,2007.
e-Print: gr-qc/0701132
Cited 12 times

marcus · Feb 28, 2009

Featured speakers at upcoming meetings, workshops etc.

A certain amount can be told about how things are going in a field by noting who the invited plenary speakers are to be at upcoming meetings. They tend to be people who have things to report that everybody wants to hear about. At the time it struck me as remarkable that the last GR meeting, GR17 Sydney 400-500 participants, had Renate Loll and Laurent Freidel as plenary speakers. Now the upcoming MG12 Paris, another biggie, will have Alain Connes and Laurent Freidel. There may be a general waking up to nonstring quantum gravity on the part of cosmologists, astrophysicists, GR-theorists, GR and gravity experimentalists, etc. I'll list some upcoming meetings and workshops in chrono order, along with a few of those giving talks.

Black Holes and Loop Quantum Gravity (Valencia 26-28 March)
http://www.uv.es/bhlqg/
Ashtekar, Bojowald, Corichi, Dittrich, Engle,..., Sahlmann...
Topic list is interesting, two of those included are:
- Spin foam approach to black holes
- Singularity resolution and information loss

Strings 2009 (Rome 22-26 June)
http://strings2009.roma2.infn.it/home.html
Last year they had Rovelli give a plenary talk on LQG. Let's see if they get anyone from the outside this year.

XXV Max Born (Wroclaw 29 June-3 July)
http://www.ift.uni.wroc.pl/~planckscale/
John Barrett, Laurent Freidel, Renate Loll,...many other outstanding speakers...

MG12 (Paris 12-16 July)
http://www.icra.it/MG/mg12/en/invited_speakers.htm
Alain Connes, Laurent Freidel

Loops 2009 (Beijing 2-8 August)
http://grg.maths.qmul.ac.uk/hyperspace/conference/08Dec.8.html
no speakers list posted so far

QG School 2 (Corfu 13-20 September)
http://www.maths.nottingham.ac.uk/qg/CorfuSS.html
Ashtekar, Baez, Barrett, Rivasseau, Rovelli

GR 19 (Mexico City 5-9 July 2010)
http://www.gr19.com/index.php
Rovelli
Helpful links, for staying informed
http://www.maths.nottingham.ac.uk/qg/
http://grg.maths.qmul.ac.uk/hyperspace/conference/
http://grg.maths.qmul.ac.uk/hyperspace/conference/latest.html

marcus · Mar 2, 2009

Loops 2009 has posted speakers now
http://www.loops09.org/asp/xtgl/zzview.asp?id=10
==quote==
The incomplete list of plenary speakers：

A. Ashtekar (Penn state)
J. Barrett (Nottingham)
M. Bojowald (Penn state)
A. Corichi (Morelia)
B. Dittrich (Utrecht)
L. Freidel (Perimeter)
H.Y. Guo (ITP, CAS)
J. Lewandowski (Warsaw)
R. Maartens (Portsmounth)
J. Pullin (Louisiana)
V. Rivasseau (Paris-Sud XI)
C. Rovelli (Marseille)
H. Sahlmann (Karlsruhe)
P. Singh (Perimeter)
L. Smolin (to be confirmed, Perimeter)
S. Speziale (Marseille)
T. Thiemann (AEI)
M. Varadarajan (Raman Inst)
==endquote==

There is a now a Loops 2009 website. Here's how they list the topics

==quote==
* Loop quantum gravity
* Loop quantum cosmology and other symmetric models
* Spin foams and their low energy ramifications
* Fundamental questions of quantum gravity
* Black holes and Cosmology related to quantum gravity
* Phenomenology of quantum gravity.
==endquote==

I believe this will be the first time that Bianca Dittrich will be a featured plenary speaker at a major international conference.
Maybe also a first for Simone Speziale, I don't recall for sure.

etera · Mar 5, 2009

Hey Marcus,
sorry for the self-advertisement... but each time people talk about the nex spinfoam models by Rovelli and al or Freidel and al, they keep forgetting these two papers where we introduced the coherent state techniques used for the semi-classical analysis of these models... and which could make it hopefully in the top 20 of the last couple of years...

A New spinfoam vertex for quantum gravity.
Etera R. Livine (Lyon, Ecole Normale Superieure) , Simone Speziale (Perimeter Inst. Theor. Phys.) . PI-QG-45, May 2007. 25pp.
Published in Phys.Rev.D76:084028,2007.
e-Print: arXiv:0705.0674 [gr-qc]
Cited 43 times

Consistently Solving the Simplicity Constraints for Spinfoam Quantum Gravity.
Etera R. Livine (Lyon, Ecole Normale Superieure) , Simone Speziale (Perimeter Inst. Theor. Phys.) . Aug 2007. 6pp.
Published in Europhys.Lett.81:50004,2008.
e-Print: arXiv:0708.1915 [gr-qc]
Cited 37 times

and there's even another one:
Numerical evidence of regularized correlations in spin foam gravity.
J.Daniel Christensen (Western Ontario U.) , Etera R. Livine (Lyon, Ecole Normale Superieure) , Simone Speziale (Perimeter Inst. Theor. Phys.) . PI-QG-57, Oct 2007. 5pp.
Published in Phys.Lett.B670:403-406,2009.
e-Print: arXiv:0710.0617 [gr-qc]
Cited 15 times

~etera

marcus said:

Here's an updated "top 20" snapshot of the field, the 20 most-cited papers from the past 2 years (date > 2006).

The list makes clear which have been the predominant themes of research recently: the new vertex, establishing a new covariant version of LQG consistent with the older canonical version, checking that the low energy limit comes out right (graviton propagator), and as always cosmology (LQC).

There is one new theme that did not show up until paper number 23 in the list. The bridge between LQG and LQC--which may lead to cosmological tests of LQG. Although it didn't make the top 20 this time, I will append that paper.

1) The Loop-quantum-gravity vertex-amplitude.
Jonathan Engle, Roberto Pereira, Carlo Rovelli (Marseille, CPT) . May 2007. 6pp.
Published in Phys.Rev.Lett.99:161301,2007.
e-Print: arXiv:0705.2388 [gr-qc]
Cited 49 times

2) An Introduction to Loop Quantum Gravity Through Cosmology.
Abhay Ashtekar (Penn State U. & Santa Barbara, KITP) . 20pp.
Published in Nuovo Cim.122B:135-155,2007.
e-Print: gr-qc/0702030
Cited 46 times

3) A New Spin Foam Model for 4d Gravity.
Laurent Freidel (Perimeter Inst. Theor. Phys.) , Kirill Krasnov (Perimeter Inst. Theor. Phys. & Nottingham U.) . 40pp.
Published in Class.Quant.Grav.25:125018,2008.
e-Print: arXiv:0708.1595 [gr-qc]
Cited 46 times

4) Flipped spinfoam vertex and loop gravity.
Jonathan Engle, Roberto Pereira, Carlo Rovelli (Marseille, CPT) . 37pp.
Published in Nucl.Phys.B798:251-290,2008.
e-Print: arXiv:0708.1236 [gr-qc]
Cited 38 times

5) The Complete LQG propagator. I. Difficulties with the Barrett-Crane vertex.
Emanuele Alesci (Rome U. & Marseille, CPT) , Carlo Rovelli (Marseille, CPT) . 31pp.
Published in Phys.Rev.D76:104012,2007.
e-Print: arXiv:0708.0883 [gr-qc]
Cited 36 times

6) LQG vertex with finite Immirzi parameter.
Jonathan Engle (Marseille, CPT) , Etera Livine (Lyon, Ecole Normale Superieure) , Roberto Pereira, Carlo Rovelli (Marseille, CPT) . 8pp.
Published in Nucl.Phys.B799:136-149,2008.
e-Print: arXiv:0711.0146 [gr-qc]
Cited 33 times

marcus · Mar 5, 2009

Etera, thanks for pointing that out. The omissions were certainly not intentional!

If you have some tips about using Spires, please let me know. I gather they have no Boolean OR. So if I want keyword "loop space" or keyword "spin, foam" I need to do two searches and merge the lists manually.

I just realized that when I search with keywords "quantum gravity AND loop space" it does not come up with some papers tagged "spin, foam" like the ones you mentioned.

So now we need a new routine for getting the LQG top 20.
First we do this "spin, foam" search:
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND++DK+SPIN%2C+FOAM+AND+DATE+%3E+2006&FORMAT=www&SEQUENCE=citecount%28d%29

Then we do a "quantum gravity" and "loop space" search:
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=dk+quantum+gravity+and+dk+loop+space+and+date+%3E+2006&FORMAT=www&SEQUENCE=citecount%28d%29

Then we merge the lists by hand:

1) The Loop-quantum-gravity vertex-amplitude.
Jonathan Engle, Roberto Pereira, Carlo Rovelli (Marseille, CPT) . May 2007. 6pp.
Published in Phys.Rev.Lett.99:161301,2007.
e-Print: arXiv:0705.2388 [gr-qc]
Cited 49 times

2) An Introduction to Loop Quantum Gravity Through Cosmology.
Abhay Ashtekar (Penn State U. & Santa Barbara, KITP) . IGPG-07-01-02, NSF-KITP-07-05, Feb 2007. 20pp.
Published in Nuovo Cim.122B:135-155,2007.
e-Print: gr-qc/0702030
Cited 47 times

3) A New Spin Foam Model for 4d Gravity.
Laurent Freidel (Perimeter Inst. Theor. Phys.) , Kirill Krasnov (Perimeter Inst. Theor. Phys. & Nottingham U.) . Aug 2007. 40pp.
Published in Class.Quant.Grav.25:125018,2008.
e-Print: arXiv:0708.1595 [gr-qc]
Cited 47 times

4) A New spinfoam vertex for quantum gravity.
Etera R. Livine (Lyon, Ecole Normale Superieure) , Simone Speziale (Perimeter Inst. Theor. Phys.) . PI-QG-45, May 2007. 25pp.
Published in Phys.Rev.D76:084028,2007.
e-Print: arXiv:0705.0674 [gr-qc]
Cited 43 times

5) Flipped spinfoam vertex and loop gravity.
Jonathan Engle, Roberto Pereira, Carlo Rovelli (Marseille, CPT) . Aug 2007. 37pp.
Published in Nucl.Phys.B798:251-290,2008.
e-Print: arXiv:0708.1236 [gr-qc]
Cited 39 times

6) Consistently Solving the Simplicity Constraints for Spinfoam Quantum Gravity.
Etera R. Livine (Lyon, Ecole Normale Superieure) , Simone Speziale (Perimeter Inst. Theor. Phys.) . Aug 2007. 6pp.
Published in Europhys.Lett.81:50004,2008.
e-Print: arXiv:0708.1915 [gr-qc]
Cited 37 times

7) The Complete LQG propagator. I. Difficulties with the Barrett-Crane vertex.
Emanuele Alesci (Rome U. & Marseille, CPT) , Carlo Rovelli (Marseille, CPT) . Aug 2007. 31pp.
Published in Phys.Rev.D76:104012,2007.
e-Print: arXiv:0708.0883 [gr-qc]
Cited 36 times

8) LQG vertex with finite Immirzi parameter.
Jonathan Engle (Marseille, CPT) , Etera Livine (Lyon, Ecole Normale Superieure) , Roberto Pereira, Carlo Rovelli (Marseille, CPT) . Nov 2007. 8pp.
Published in Nucl.Phys.B799:136-149,2008.
e-Print: arXiv:0711.0146 [gr-qc]
Cited 34 times

9) The Perturbative Regge-calculus regime of loop quantum gravity.
Eugenio Bianchi (Pisa, Scuola Normale Superiore & INFN, Pisa) , Leonardo Modesto (Bologna U. & INFN, Bologna & Marseille, CPT) . Sep 2007. 43pp.
Published in Nucl.Phys.B796:581-621,2008.
e-Print: arXiv:0709.2051 [gr-qc]
Cited 20 times

10) Singularities and Quantum Gravity.
Martin Bojowald (Penn State U. & Santa Barbara, KITP) . IGPG-07-2-4, Feb 2007. 41pp.
Lectures given at 12th Brazilian School of Cosmology and Gravitation (XII BSCG), Rio de Janeiro, Brazil, 10-23 Sep 2006.
Published in AIP Conf.Proc.910:294-333,2007. Also in *Rio de Janeiro 2006, Cosmology and gravitation* 294-333
e-Print: gr-qc/0702144
Cited 20 times

11) The Perturbative Regge-calculus regime of loop quantum gravity.
Eugenio Bianchi (Pisa, Scuola Normale Superiore & INFN, Pisa) , Leonardo Modesto (Bologna U. & INFN, Bologna & Marseille, CPT) . Sep 2007. 43pp.
Published in Nucl.Phys.B796:581-621,2008.
e-Print: arXiv:0709.2051 [gr-qc]
Cited 20 times

12) Loop quantum gravity corrections to gravitational wave dispersion.
Martin Bojowald, Golam Mortuza Hossain (Penn State U.) . IGC-07-9-1, Sep 2007. 27pp.
Published in Phys.Rev.D77:023508,2008.
e-Print: arXiv:0709.2365 [gr-qc]
Cited 20 times

13) Lorentzian LQG vertex amplitude.
Roberto Pereira (Marseille, CPT) . Oct 2007. 9pp.
Published in Class.Quant.Grav.25:085013,2008.
e-Print: arXiv:0710.5043 [gr-qc]
Cited 20 times

14) Spin foam model from canonical quantization.
Sergei Alexandrov (Montpellier U., Astropart. Group) . PTA-07-24, May 2007. 25pp.
Published in Phys.Rev.D77:024009,2008.
e-Print: arXiv:0705.3892 [gr-qc]
Cited 17 times

15) The Complete LQG propagator. II. Asymptotic behavior of the vertex.
Emanuele Alesci (Rome III U. & Marseille, CPT) , Carlo Rovelli (Marseille, CPT) . Nov 2007. 16pp.
Published in Phys.Rev.D77:044024,2008.
e-Print: arXiv:0711.1284 [gr-qc]
Cited 17 times

16) The Dark Side of a Patchwork Universe.
Martin Bojowald (Penn State U.) . IGPG-07-5-5, May 2007. 24pp.
Published in Gen.Rel.Grav.40:639-660,2008.
e-Print: arXiv:0705.4398 [gr-qc]
Cited 15 times

17) Path integral representation of spin foam models of 4d gravity.
Florian Conrady, Laurent Freidel (Perimeter Inst. Theor. Phys.) . PI-QG-84, Jun 2008. 29pp.
Published in Class.Quant.Grav.25:245010,2008.
e-Print: arXiv:0806.4640 [gr-qc]
Cited 15 times

18) Inflationary universe in loop quantum cosmology.
Xin Zhang (Beijing, KITPC) , Yi Ling (Nanchang U.) . May 2007. 25pp.
Published in JCAP 0708:012,2007.
e-Print: arXiv:0705.2656 [gr-qc]
Cited 14 times

19) Quantum Gravity Phenomenology.
Giovanni Amelino-Camelia . Jun 2008. 82pp. Temporary entry
e-Print: arXiv:0806.0339 [gr-qc]
Cited 14 times

20) Loop quantum cosmology.
Martin Bojowald (Penn State U. & Potsdam, Max Planck Inst.) . 2008. 131pp.
Published in Living Rev.Rel.11:4,2008.
Cited 14 times

21) Loop quantization of spherically symmetric midi-superspaces.
Miguel Campiglia, Rodolfo Gambini (Republica U., Montevideo) , Jorge Pullin (Louisiana State U.) . LSU-REL-032707, NSF-KITP-07-28, Mar 2007. 18pp.
Published in Class.Quant.Grav.24:3649-3672,2007.
e-Print: gr-qc/0703135
Cited 13 times

22) Loop Quantum Gravity: Four Recent Advances and a Dozen Frequently Asked Questions.
Abhay Ashtekar (Penn State U.) . IGPG-07-5-3, May 2007. 21pp.
To appear in the proceedings of 11th Marcel Grossmann Meeting on Recent Developments in Theoretical and Experimental General Relativity, Gravitation, and Relativistic Field Theories, Berlin, Germany, 23-29 Jul 2006.
e-Print: arXiv:0705.2222 [gr-qc]
Cited 13 times

23) Harmonic cosmology: How much can we know about a universe before the big bang?
Martin Bojowald (Penn State U., Astron. Astrophys.) . Oct 2007. 16pp.
Published in Proc.Roy.Soc.Lond.A464:2135-2150,2008.
e-Print: arXiv:0710.4919 [gr-qc]
Cited 13 times

24) Linearized dynamics from the 4-simplex Regge action.
Bianca Dittrich, Laurent Freidel, Simone Speziale (Perimeter Inst. Theor. Phys.) . PI-QG-48, Aug 2007. 25pp.
Published in Phys.Rev.D76:104020,2007.
e-Print: arXiv:0707.4513 [gr-qc]
Cited 13 times

marcus · Mar 17, 2009

Periodically checking to see how the major conference/workshop programs take shape, which are the invited talks, the topics of the parallel sessions and who is chairing them, helps to get an idea of what directions of research are currently attracting interest. Here is an updated list of links

Black Holes and Loop Quantum Gravity (Valencia 26-28 March)
http://www.uv.es/bhlqg/
Speakers include Ashtekar, Bojowald, Corichi, Dittrich, Engle,..., Sahlmann...

XXV Max Born (Symposium on the Planck Scale; Wroclaw 29 June-3 July)
http://www.ift.uni.wroc.pl/~planckscale/
Many speakers including John Barrett, Laurent Freidel, Renate Loll,...

MG12 (Paris 12-16 July)
http://www.icra.it/MG/mg12/en/invited_speakers.htm
Alain Connes, Laurent Freidel among others

Loops 2009 Preparatory (Beijing 26 July-1 August)
http://www.loops09.org/School/School-en.htm
introductory week of lectures prior to conference

Loops 2009 (Beijing 2-8 August)
Now has a website and a list of speakers.
http://www.loops09.org/asp/xtgl/zzview.asp?id=10

QG School 2 (Corfu 13-20 September)
http://www.maths.nottingham.ac.uk/qg/CorfuSS.html
Ashtekar, Baez, Barrett, Rivasseau, Rovelli

GR 19 (Mexico City 5-9 July 2010)
http://www.gr19.com/index.php
Rovelli among others

The topics list for the March BH+LQG workshop is
- Black hole entropy in LQG
- Spin foam approach to black holes
- Singularity resolution and information loss
- Prospects for a detailed description of the Hawking radiation
- Comparison between results from LQG and other approaches

Themes listed for the June Max Born symposium are
Loops and spin foams
Strings, AdS/CFT, and quantum gravity
Field theoretical perspective on Planck scale
Noncommutative spacetimes
Quantum gravity phenomenology

The listed topics for the August Loops '09 conference are
* Loop quantum gravity
* Loop quantum cosmology and other symmetric models
* Spin foams and their low energy ramifications
* Fundamental questions of quantum gravity
* Black holes and Cosmology related to quantum gravity
* Phenomenology of quantum gravity.

Other links to check.
http://www.maths.nottingham.ac.uk/qg/
http://grg.maths.qmul.ac.uk/hyperspace/conference/
http://grg.maths.qmul.ac.uk/hyperspace/conference/latest.html

marcus · Mar 17, 2009

It will be interesting to see how Yong-ge Ma and Thomas Thiemann structure the week-long school for students and postdocs interested in getting into LQG research, that will precede the August Loops 09 conference. Let's see who they got to give lectures.

It's an intensive cram course to get young researchers up to speed so they can benefit as much as possible from the main Loops 09 conference that takes place the following week.
http://www.loops09.org/School/School-en.htm

Topics and lecturers will be:

Loop quantum cosmology, Martin Bojowald (Penn State, USA)
Group field theory, Daniele Oriti (AEI, Germany)
Spin foams, Carlo Rovelli (Univ of Mediterranee, France)
Loop quantum gravity, Thomas Thiemann (AEI, Germany)
Regge calculus, Ruth Williams (Cambridge Univ, UK)

BNU (Beijing Normal U) is emerging as a center for LQG research in China. Hosting the conference at BNU will help to establish it in this position. I see Dah-wei Chiou is there now. He was a postdoc 2006-2008 in Ashtekar's group at Penn State. Most of his papers were then about the LQG black hole, confirming a bounce at the singularity in various cases. Chiou is helping to organize the pre-conference school.

ccdantas · Mar 18, 2009

From the Loops'09 site main title: "Quanturn Granvity" [sic] -- typos or something I don't get? (like the "turn" of a "grand" theory?

)

marcus · Mar 18, 2009

ccdantas said:

From the Loops'09 site main title: "Quanturn Granvity" [sic] -- typos or something I don't get? (like the "turn" of a "grand" theory? )

Typos obviously, Christine.

Actually it looks to me as if Beijing Normal University has acquired the services of a professional website designer (one with a highly unreliable grasp of the roman alphabet).

Either that or one of Yongge Ma's departmental secretaries is not comfortable with English text. For there are many typos. Many. Or were the last time I looked.

However I thought the website graphic design was very pretty along traditional Chinese lines.

That complex knot in the tasseled red silken cord works graphically as a kind of ideogram for Loop gravity, I think.

MTd2 · Mar 27, 2009

There is a press release for Dario Benedetti and some comments of him about a new article published on PRL.

“Fractal Properties of Quantum Spacetime.” Physical Review Letters 102, 111303 (2009).

http://www.physorg.com/news157203574.html

marcus · Apr 3, 2009

The abstracts for most of the plenary talks planned for MG12 in Paris are available.
http://www.icra.it/MG/mg12/en/invited_speakers_details.htm#freidel

Laurent Freidel
Talk: Spin Foam models: models of quantum dynamical space time

Abstract: In this talk I will give an overview of spin foam models which describe the dynamics of quantum gravity in a background independent context. I will focus especially and the recent developments which concerns the construction of these models in 4 dimensional gravity and present some of the key results obtained in this context like the construction of the model, the proof of the semi-classical limit and the relationship with loop quantum gravity and SU(2) spin network states.

Alain Connes is also one of the invited speakers, but the abstract of his talk is not posted.

Another invited QG talk is:
Herbert W. Hamber
Talk: Ultraviolet Divergences and Scale-Dependent Coupling in Quantum Gravity

Abstract: I will discuss how non-perturbative approaches to the problem of ultraviolet divergences in quantum gravity, similar in spirit and methods to what is done in the modern renormalization group treatment of non-abelian gauge theories, point to a possible weak scale dependence of Newton's constant at very large distances. I will then discuss ways by which such a scale dependence could in principle be verified observationally.

ensabah6 · Apr 3, 2009

marcus said:

The abstracts for most of the plenary talks planned for MG12 in Paris are available.
http://www.icra.it/MG/mg12/en/invited_speakers_details.htm#freidel

Laurent Freidel
Talk: Spin Foam models: models of quantum dynamical space time

Abstract: In this talk I will give an overview of spin foam models which describe the dynamics of quantum gravity in a background independent context. I will focus especially and the recent developments which concerns the construction of these models in 4 dimensional gravity and present some of the key results obtained in this context like the construction of the model, the proof of the semi-classical limit and the relationship with loop quantum gravity and SU(2) spin network states.

How would SF prove it has the right semiclassical limit -- via graviton and perturbation theory or some other mechanism

marcus · Apr 3, 2009

Anyone interested in forefront of QG research, LQG in particular, will probably have noticed that a bunch of spin foam graviton papers appeared since 2007, actually goes back to 2006 if I remember right.
Folks may not have seen Perini's recent seminar talk on that, though. Just this past February. The title of the talk is
Graviton propagator from EPRL spinfoam model
More information is here, for instance:
https://www.physicsforums.com/showpost.php?p=2145438&postcount=10

marcus · Apr 9, 2009

Some of the talks planned for the XXV Max Born (Symposium on the Planck Scale)
have been listed. Notice the wide range of approaches:
asymptotic safety/renormalization group
supergravity
loop/spinfoam
group field theories
string theories
non-commutative geometry
etc.

A.P. Balachandran Moyal versus Voros
A. Ballesteros On noncommutative velocity spaces
D. Benedetti Asymptotic safety and higher-derivative gravity
E. Bergshoeff (Super-)gravity in Three Dimensions
M. Bojowald Quantum geometry and quantum dynamics at the Planck scale
B. Dittrich Diffeomorphism invariance in quantum gravity models
S. Doplicher Quantum Spacetime and Noncommutative Geometry
M. Duff Black holes, qubits and octonions
J. Figueroa-O'Farrill 3-algebras and M2-branes
F. Girelli Group field theories: from spinfoam to non-commutative geometries
H. Gomes Asymptotic analysis of the EPRL four-simplex amplitude
R. Helling Allowing Spontaneous Breaking of Diffeomorphism Invariance
B. Koch Quantum gravity or quantum due to gravity?
D. Litim Quantum Gravity and the Renormalisation Group
F. Lizzi Twisting and star products
J. Magueijo Quantum gravity and cosmological fluctuations
N. Mavromatos High Energy Gamma Ray Astrophysics and Quantum-Gravity/String Theory
M. Montesinos n-dimensional non-Abelian BF theory as interacting (n-2)-forms
G. Piacitelli Models of Quantum Spacetime, and Approaches to Covariance
W. Piechocki Energy Scale of the Big Bounce
J. Pullin Spherically symmetric quantum scalar field in a quantum space time in loop quantum gravity the vacuum and the cosmological constant
Y. Sasai The Cutkosky rule of three dimensional noncommutative field theory in Lie algebraic noncommutative spacetime
D. Terno Dynamics and entanglement in spherically symmetric quantum gravity
S. Vaidya Unruh Effect in Noncommutative Spacetime
S. Weinfurtner The effective Planck-scale in emergent gravity

The Symposium will be held at the Polish city of Breslau (now Wroclaw) and will go on for one week at the beginning of July. (June 29 - July 3)

Breslau (or Wroclaw) looks to be only about 150 miles from Prague, and about the same distance to Dresden. Not much farther than that from Berlin and from Vienna as well.

marcus · May 5, 2009

The main QG session at the annual meeting of the American Physical Society (APS) was
http://meetings.aps.org/Meeting/APR09/sessionindex2/?SessionEventID=103077.
Session G5: Developments in Quantum Gravity
Chair: Jorge Pullin, Louisana State University
Sunday, May 3, 2009
8:30AM - 9:06AM
G5.00001: Einstein Prize Talk: The Quantum Origin of Our Classical Universe
Invited Speaker: James Hartle

Sunday, May 3, 2009
9:06AM - 9:42AM
G5.00002: Spin Foam models: models of quantum space time
Invited Speaker: Laurent Freidel

Sunday, May 3, 2009
9:42AM - 10:18AM
G5.00003: The Formulation of Quantum Field Theory in Curved Spacetime
Invited Speaker: Robert Wald

These were 36 minute invited talks. James Hartle and Robert Wald are prominent senior figures. Laurent Freidel made faculty in 2006 if I remember. He is also giving an invited talk at the Marcel Grossmann (MG12) meeting in Paris this summer.

Here is Freidel's abstract:
Spin Foam models: models of quantum space time
Laurent Freidel (Perimeter Institute)

"I will give an overview of spin foam models which describe the dynamics of quantum gravity in a background independent context. I will focus especially and the recent developments which concerns the construction of these models in 4 dimensional gravity and present some of the key results obtained in this context like the proof of the semi-classical limit and the relationship with loop quantum gravity and SU(2) spin network states."

http://meetings.aps.org/Meeting/APR09/Event/103078

There were also some 12 minute talks in various other sessions, mixed in with other topics. For instance, in session D11 there was a talk by Raj Konnu on Causal Dynamical Triangulations (CDT). In session T14 there was a 12 minute talk that had something to do with string theory--Kezerashvili et al.
=======================
't Hooft's chapter in Oriti's book is here:
http://www.phys.uu.nl/~thooft/gthpub/QuantumGrav_06.pdf.
Rovelli's chapter in Oriti's book is here:
http://arxiv.org/abs/gr-qc/0604045
The amazon page:
http://www.amazon.com/dp/0521860458/?tag=pfamazon01-20
The Cambridge U. Press page:
http://cambridge.org/uk/catalogue/catalogue.asp?isbn=9780521860451
The CUP page is set up for extensive browsing in the book.
=======================

I notice that Laurent Freidel, as invited speaker at both the Denver APS and the Paris Marcel Grossmann meeting, if you judge by the abstract, is giving essentially the same talk. (Spin foam, semiclassical limit, compatibility with earlier LQG) This must be a quantum gravity talk that a lot of physicists want to hear. At the APS meeting they have him bracketed by Robert Wald and James Hartle (the only other invited speakers in the QG session). I think it is another sign that we are passing a kind of landmark this year.

marcus · May 12, 2009

I'm seeing more conferences where there is a mix of different research lines. For example:
http://www.mth.uct.ac.za/~jeff/Quantum_Gravity/About.html
The statement of purpose for this week-long conference in August 2009 South Africa (winter, cool) is very clear about that. Time to get the different parties together.

Same theme surfaced with several other 2009 workshops, schools etc.
Like the June school for Nonperturbative Gravity and QCD at Zakopane.
http://th-www.if.uj.edu.pl/school/2009/

And the Conference on Planck Scale
http://www.ift.uni.wroc.pl/~planckscale/
Ten new speakers have been added since the last time I listed the talks---some 35 titles so far.

Program of talks for Abhay Ashtekar's 60th birthday party, the "Abhayfest"
http://igc.psu.edu/events/abhayfest/program.shtml

marcus · May 17, 2009

One of our PF members has compiled some lists of selected LQG resources, with links.

http://mushfiq.net/loop-quantum-gravity/

I think this could be helpful to a newcomer to the subject. And also for someone who has been reading the research literature already for some time---maybe you will see something you missed.

The site also lists select resource links for other topics besides quantum gravity.

SuperStringboy · May 18, 2009

marcus said:

One of our PF members has compiled some lists of selected LQG resources, with links.

http://mushfiq.net/loop-quantum-gravity/

I think this could be helpful to a newcomer to the subject. And also for someone who has been reading the research literature already for some time---maybe you will see something you missed.

The site also lists select resource links for other topics besides quantum gravity.

Here is a correction. This page is shifted to http://cosmicposts.wordpress.com/loop-quantum-gravity/

marcus · May 23, 2009

The Abhayfest* takes place in just a few days from now, June 4-6, at Penn State.
celebrating Abhay Ashtekar's 60th birthday.
http://igc.psu.edu/events/abhayfest/program.shtml
Here is a sampling of some of the speakers and talks:

Jim Hartle TBA
Gary Horowitz TBA
Steve Fairhurst "Searching for Gravitational Waves from Coalescing Binary Systems"
Alex Corichi "Black Hole Entropy"
Rodolfo Gambini The Issue of Time in Generally Covariant Theories and Quantum Gravity
Carlo Rovelli "What is a Particle"
Robert Wald "The Formulation of Quantum Field Theory in Curved Spacetime"
Lee Smolin "Unimodular Gravity and the Cosmological Constant Problem"
Thomas Thiemann TBA
Laurent Freidel "Quantum Geometry from Spin Foam"
Jerzy Lewandowski TBA
John Stachel TBA
Roger Penrose "Conformal Cyclic Cosmology: Its Present Status"

Here is a partial list of upcoming conferences, workshops, schools, in case anyone wants to check who and what are on the programs. This helps get an idea of current directions in research and areas of interest.

XXV Max Born (Symposium on the Planck Scale; Wroclaw 29 June-3 July)
http://www.ift.uni.wroc.pl/~planckscale/

MG12 (Paris 12-16 July)
http://www.icra.it/MG/mg12/en/invited_speakers.htm

Loops 2009 Prep (Beijing 26 July-1 August)
http://www.loops09.org/School/School-en.htm

Loops 2009 (Beijing 2-8 August)
http://www.loops09.org/asp/xtgl/zzview.asp?id=10

QG School 2 (Corfu 13-20 September)
http://www.maths.nottingham.ac.uk/qg/CorfuSS.html

GR 19 (Mexico City 5-9 July 2010)
http://www.gr19.com/index.php

Links to check for new announcements.
http://www.maths.nottingham.ac.uk/qg/
http://grg.maths.qmul.ac.uk/hyperspace/conference/
http://grg.maths.qmul.ac.uk/hyperspace/conference/latest.html

*BTW at the Abhayfest, coming up in a few days, Roger Penrose will also be giving a public lecture Fashion Faith and Fantasy: How big is infinity?

This will presumably have some of the same themes as the talk he gave in Berkeley in 2006, video available free from the Math Science Research Institute.
http://www.msri.org/communications/vmath/special_productions/
This was an integrated sociology&physics critique called
Fashion, Faith, and Fantasy in Modern Physical Theory
(raises foundational issues re string thinking, inflation cosmology and other possible physics fads, unscientific beliefs, and comments on sociology of intellectual fads in physics)
http://www.msri.org/communications/vmath/VMathVideosSpecial/VideoSpecialInfo/3005/show_video

marcus · May 28, 2009

The video and slides of Carlo Rovelli's talk at Strings 2008 provides a good introductory overview of LQG.
Here are the links:
Video:
http://cdsweb.cern.ch/record/1121957?ln=en
Slides:
http://indico.cern.ch/getFile.py/access?contribId=30&resId=0&materialId=slides&confId=21917

An early draft of Rovelli's book online free for anyone who doesn't have the published version:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf

Another good overview of the whole field of quantum geometry/gravity, Rovelli's chapter in Oriti's book:
http://arxiv.org/abs/gr-qc/0604045

marcus · May 31, 2009

Today (May 31) starts another school at Zakopane. Here are some sample lectures and seminar titles. The first four are Causal Dynamical Triangulations (CDT) talks. The other two speakers we know in connection with their work in Asymptotic Safe quantum gravity.

Jan Ambjorn (NBI) - ambjorn at nbi.dk
Quantum Gravity: The Self-Organizing Universe

Renate Loll (Utrecht) - R.Loll at uu.nl
CDT and the Quest for Observables

Andrzej Goerlich (Kraków/UJ) atg at th.if.uj.edu.pl
Geometry of the Universe in Causal Dynamical Triangulations

Willem Westra (Iceland Uni.) willemwestra at hotmail.com
Towards Solvable Matter Models in Causal Quantum Gravity

Roberto Percacci (Trieste) - percacci at sissa.it
A Particle Physicist's Approach to Gravity

Daniel Litim (Sussex) - litim at mail.cern.ch
Non-perturbative Gravitation and the Renormalisation Group

Together Ambjorn and Loll are scheduled to give three 1-hour talks.
Percacci and Litim each give a series of three 1-hour talks.

http://th-www.if.uj.edu.pl/school/2009/lectures.html

The title of the school is "Non-perturbative gravity and QCD". Besides the quantum gravity lectures there are many other speakers.

marcus · Jun 2, 2009

A central idea in quantum gravity is illustrated by Rovelli's parable of the whale.
In the published version of Quantum Gravity, this is on page 9.
In the free online draft it is in section 1.1.3 on page 7 of this PDF:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf
===========

This wide audience article by Smolin in PhysicsWorld (June 2, 2009) could turn out to be influential. It is part of the development of Unimodular Relativity (UR) in conjuctions with evolutionary cosmology (the conjectured evolutionary basis for the laws of physics).
It's a very readable article, called The Unique Universe.
http://physicsworld.com/cws/article/print/39306

It's a pity there is a time conflict between the Zakopane school (May 31-June 6) and the Abhayfest (June 4-6).
http://th-www.if.uj.edu.pl/school/2009/lectures.html
http://igc.psu.edu/events/abhayfest/program.shtml
Friday June 5 is the big day of the Abhayfest, with talks followed by a banquet in the evening. Friday's talks include:
9:30 AM - 10:15 AM Carlo Rovelli What is a Particle?
10:15 AM - 11:00 AM Robert Wald The Formulation of Quantum Field Theory in Curved Spacetime
2:00 PM - 2:30 PM Laurent Freidel Quantum Geometry from Spin Foam
2:30 PM - 3:00 PM Jerzy Lewandowski Spin Foams from Quantum Geometry
3:00 PM - 3:30 PM Lee Smolin Unimodular Gravity and the Cosmological Constant Problem
5:00 PM - 5:30 PM Rodolfo Gambini The Issue of Time in Generally Covariant Theories and Quantum Gravity

Quantum Geometry is the name used for LQG within the research community. It is how Ashtekar refers to it. That makes sense because General Relativity is a theory of dynamic geometry (not forces or gravitons) so a quantized version of GR is a theory of quantum geometry---and LQG is the leading approach. Anyway, for whatever reason that is how Ashtekar and his friends have been calling LQG (which after all is a misnomer since the theory is not about loops). I see that Freidel and Lewandowski have acknowledged that in the titles of their talks, which are about the relation of LQG to Spin Foam. Looks to me like a very nice 60th birthday party indeed!

marcus · Jun 13, 2009

Rovelli recently gave a thumbnail sketch of LQG. Around 10 slides illustrated by drawings. You can look at the original slides and listen to the audio here:
http://relativity.phys.lsu.edu/ilqgs/panel050509.pdf
http://relativity.phys.lsu.edu/ilqgs/panel050509.mp3

It is a 3-way discussion and Rovelli is the second in line, so when you open the slides PDF you must scroll down about 10 slides first to come to Rovelli's section, likewise with the Audio mp3 you have to fast forward to about one third of the way, and just get the middle third of the recording. He will tell you in the audio which slide, when to switch slides and so forth. The other presentations, by Ashtekar and Freidel, are also good. But here I am focusing just on this very short 20 minute sequence by Rovelli.

==quote Rovelli's slides, text only==
Some questions

1. Is quantum space made out of loops and spin-networks or tetrahedra and 4-simplices?

2. Is ﬂat space formed by many small tetrahedra with low-spin, or by few large tetrahedra with with high-spin?

3. Is low-energy physics given by quantum gravity on a single 4-simplex? Or by an inﬁnite triangulation limit?

4. How do we study the the continuum limit from Planck scale discretness to the macroscopic continuum?

5. A spinfoam model is like a new version of quantum Regge calculus. So, why it should work better than quantum Regge calculus?

Claim: these questions are ill posed.

***
1. Is quantum space made out of loops and spin-networks or tetrahedra and 4-simplices?

The meaningful question in quantum theory is not how something is, but how it responds to a measurement.

There is no space “between” quanta of space, and it makes no sense to ask what is the geometry between one quantum and another, or inside a quantum, or what is the “geometry of quantum”. It is like asking for the “shape of a photon”. Or “What do I measure if I measure the energy in the space occupied by half a photon?”

***
2. Is ﬂat space formed by few tetrahedra with high spin, or by many tetrahedra (or loops) with low spin?
How many particles are there in the Fock vacuum?
How many particles are invoved in a two-particle interaction?

Quantum theory gives the probability for measurement outcomes: it does not describe “what is between measurements”.

“In between pictures” are just descriptions of the ways I decide to do calculations. They are diﬀerent for diﬀerent measurements, and at diﬀerent orders in perturbation theory.

[my comment. Rov sometimes like Feyn has flashes of unusual common sense. they illuminate.]

***
The good question, I think, is:
What can we compute that makes sense?
and
How can we compute it?

***
The problem of quantum gravity is–in a sense–two problems:
1. What is the right (background independent) theory?
2. How do extract physics from a background independent QFT?

Diﬃculties:
1. In standard QFT distances and time intervals gives locations in spacetime where the ﬁeld is measured. In quantum gravity distances and time interval are quantum measurements of the gravitational ﬁeld.
2. I think it is interesting to compute scattering amplitudes. These depend on a background: they describe interactions of excitations in a ﬂat space context. How do we tell a background independent theory that there is a background?

***
The only solution I know:
1. Boundary formalism
2. Vertex expansion
3. Large spin expansion

***
1. Boundary formalism
– Scattering amplitudes depend on the measured geometry around the scattering region.
– It is the boundary state that tells the theory about the background.
– Not diﬀerent than in standard QFT:
W (x, x′) = ⟨0|φ(x)φ(x′)|0⟩ (1)

= ⟨0| e^iHt φ(⃗x)e^−iHt e^iHt′ φ(⃗x′ )e^−iHt′|0⟩ (2)

= ⟨φ(⃗x)0_t | e^{−iH(t′−t)}|φ(⃗x′)0_t′ ⟩. (3)

W (x, x′) = ⟨e^{−iH(t′−t)} | φ(⃗x) φ(⃗x′) |0_t ⊗ 0_t′ ⟩_{H_in ⊗H_out} (4)

In quantum gravity →
W (x, y, Ψ_boundary ) = ⟨W | φ(⃗x′) φ(⃗x′) |Ψ_boundary ⟩_{H_boundary}
This quantity is a 4d diﬀeomorpism invariant and well-deﬁned. It reduces to standard 2-point function in the ﬂat space theory. Locations of x and x′ are well deﬁned with respect to the boundary state.

***
2. Vertex expansion

1. There is no way in physics you can compute without a suitable approximation scheme.

2. QFT expansion = truncation to a ﬁnite number of degrees of freedom.

QED: a ﬁnite order in perturbation theory has a ﬁnite number of particles.

Lattice QCD: ﬁnite lattice with # of cells determined by (size of the phenomenon L )/(minimal relevant wavelength λ).

3. Is there a truncation to a ﬁnite number of d.of f. in gravity, which is physically good in some regimes?

4. Yes! Truncate GR to a ﬁnite triangulation of spacetime (vertex expansion). # of simplices determined by (size of the phenomenon L)/(minimal relevant wavelength λ).

5. It is background independent, in the same sense in which Regge calculus is.

6. Where is it good? Many instances: Cosmology! Long wavelength at ﬁxed distance. Large distance expansion of the propagator...

7. Precise characterization of the regime of validity (on the boundary state)
require us to compute higher orders and compare.

***
2. Large spin limit

• In quantum gravity there is a built in scale, l_Planck . Unlikely quantum Regge calculus.

• The vertex expansion is not a large distance L ≪ l_Planck approximation.

• At ﬁxed order in the vertex expansion, large distance with respect to the Planck scale means high spins; because Area ∼ j .

• High spins = high quantum numbers = semiclassical limit. Therefore the theory must go to GR (truncated on a ﬁxed lattice) for high spins, at ﬁxed triangulation.

***
Summary
1. “Loopy, polymer, triangulated” spaces are helps for intuition, not descriptions of reality. No incompatibility between them.

2. In quantum gravity, ﬂat space is neither many small Planck scale things not few big large-spin 4 simplices. It is a process with a transition amplitudes. We can represent it with diﬀerent pictures, according to the measurements we are considering, the calculation scheme, and the approximation scheme.

3. We must compute diﬀ-invariant amplitudes, including when dealing with excitations over a ﬂat space. The only way of doing so that I know is to code the background into the boundary space. (Boundary formalism.)

4. We need an approximation scheme. For scattering amplitudes, we can truncate degrees of freedom to a ﬁnite number, very much like is done in computing in QED and QCD. (Vertex expansion.)

5. Regime of validity of the vertex expansion: processes whose size L is not much larger than the minimal relevant wavelength λ. Includes the large distance behavior of the scattering amplitudes in coordinate space.

6. At given ratio λ/L, the Large-spin Limit captures processes at scales larger than the Planck length. It gives the semiclassical limit.

→ This does not mean that ﬂat space is “made out of large 4-simplices”!
→ It means that we describe measurements performed at scales larger that the
Planck scale, at low order.

MTd2 · Jun 13, 2009

Marcus, the links did not appear.

marcus · Jun 14, 2009

Thanks for catching that! I fixed the trouble.

marcus · Jul 7, 2009

Earlier in this thread I gave a link to the Strings 2009 website. The main annual string conference. Need to check out string theorizing from time to time for perspective and contrast.
http://strings2009.roma2.infn.it/cgi-bin/roma_program.pl.cgi
Recall that Strings 2008 was noteworthy for several reasons: Witten was not there (he has not been doing much string since around 2006), Susskind didn't give a talk (the organizers excluded all talk about Landscape and Multiverse and Anthropic principle) and Carlo Rovelli was invited to give a talk about Loop Quantum Gravity which got a lively response from the audience. Questions spilled over into the coffee break period after the talk. Excellent survey. Video is online.

Strings 2009 had Witten give a public lecture and he chose not to talk about string, he talked about the particle physics frontier, future prospects, instead. Again Landscape Multiverse talks were excluded from the conference itself. That stuff is in bad odor. No Susskind. None of the KKLT folks that gave us the 10⁵⁰⁰string vacua. Petr Horava talked, about his nonstring QG and he cited results by Ambjorn, Loll, Jurkiewicz from another type of non-string QG called causal dynamical triangulations. His slides #22 and #23 headline Loll's CDT results.
http://strings2009.roma2.infn.it/talks/Horava_Strings09.pdf
He cites them, on #22 and on the next slide he gives several of their numbers, verbatim. Great. A sign of opening to the outside. Velvet revo.

The most interesting set of slides, for me, were those of the Opening Lecture, by David Gross.
http://strings2009.roma2.infn.it/talks/Gross_Strings09.pdf
He seems to be the official string overview-giver, at each major conference.
He gave the concluding Summary Talk at both Strings 2007 (madrid) and Strings 2008 (geneva). So we listen up.
Here are some excerpts from Gross' slides.

"4. What is the nature of string perturbation theory?

Our present understanding of string theory has
been restricted to perturbative treatments. Does
this perturbation theory converge? Most likely it
does not. In that case when does it give a reliable
asympototic expansion of physical quantities? How
can one go beyond perturbation theory and what is
the nature of nonperturbative string dynamics?
This question is particularly difficult since we
currently lack a useful nonperturbative formulation
of the theory. "

"5. String Phenomenology?

Here there are many questions that can all be
summarized by asking whether one can construct a
totally realistic four-dimensional model which is
consistent with string theory and agrees with
observation?
Great progress, but still not constructed."

"8. Is There a Measurable, Qualitatively Distinctive
Prediction of String Theory?

... It would be nice to predict
a phenomenon, which would be accessible at
observable energies and is uniquely
characteristic of string theory."

"How many more string revolutions
are required?

What is the fundamental
formulation of string theory?

Quantum Space of all 2-d field theories
Second Quantized Functionals of loops (SFT)
M-theory . . .

Is string theory a framework, not a theory?
What is missing?"

marcus · Jul 11, 2009

The video and slides of Carlo Rovelli's talk at Strings 2008 provides a good introductory overview of LQG, so I keep the links handy.
Here are the links:
Video:
http://cdsweb.cern.ch/record/1121957?ln=en
Slides:
http://indico.cern.ch/getFile.py/access?contribId=30&resId=0&materialId=slides&confId=21917

An early draft of Rovelli's book online free for anyone who doesn't have the published version:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf

Another good overview of the whole field of quantum geometry/gravity, Rovelli's chapter in Oriti's book:
http://arxiv.org/abs/gr-qc/0604045

Another prime source link we should have handy is Steven Weinberg's talk at Cern, 7 July 2009. It helps define a new mainstream direction (he cites papers by many people whose work we have discussed here at PF: Ambjorn, Loll, Smolin, Litim, Percacci (with Codello Rahmede and others) Reuter, Bonanno, Saueressig (with Dario Benedetti and others). The talk will bear study so here is the main Cern link for the talk:
http://indico.cern.ch/conferenceDisplay.py?confId=57283
Here is the video:
http://cdsweb.cern.ch/record/1188567/
The part of the talk where he references a lot of papers, including new work in AsymSafe QG and in CausDynTr QG is in the last 10 or 15 minutes, fast forward to 58 minutes.
He also mentioned a conference on AsymSafe QG at Perimeter to be held December 2009, which should be quite interesting.

Another 2009 event which can help define the New Mainstream for us is the Ellis-fest at Cape Town in August.

http://www.mth.uct.ac.za/~jeff/Quantum_Gravity/About.html
In the list of invited speakers I think it is the string, or former-string, people we should focus on and try to understand their being chosen. How do they fit in? Why were they invited instead of some other string researchers? I have highlighted some names of interest, people who (whether they are string theorists or something else) don't fit into the expected Loop-and-allied pattern:
Jan Ambjorn (Niels Bohr Insititute)
Cliff Burgess (Perimeter)
Steve Carlip (UC Davis)
Brian Greene (Columbia)
Joe Henson (Perimeter)
Renate Loll (Neils Bohr Institute)
Hermann Nicolai (Albert Einstein Institute)
Daniele Oriti (Albert Einstein Institute)
Thanu Padmanabhan (IUCAA)
Roger Penrose (Oxford)
Malcolm Perry (DAMTP)
Carlo Rovelli (Marseille)
Lee Smolin (Perimeter)
Kelly Stelle (Imperial)
Rafael Sorkin (Perimeter)
Thomas Thiemann (Albert Einstein Institute)

marcus · Jul 12, 2009

The upcoming AsymSafe QG conference that Weinberg mentioned in his 7 July Cern talk is actually in early November (not December):
http://www.perimeterinstitute.ca/en/Events/Asymptotic_Safety/Asymptotic_Safety_-_30_Years_Later/

One way we tell the current status of Loop and allied QG research is by watching the programs at the major international conferences. The 12th Marcel Grossmann MG12 has just begun in Paris (12-18 July) and the two Loop-type QG sessions are Thursday and Friday.

Invited speakers Laurent Freidel and Alain Connes will speak on Tuesday at a session chaired by Abhay Ashtekar. Laurent's abstract is:
Spin Foam models: models of quantum dynamical space time

"In this talk I will give an overview of spin foam models which describe the dynamics of quantum gravity in a background independent context. I will focus especially and the recent developments which concerns the construction of these models in 4 dimensional gravity and present some of the key results obtained in this context like the construction of the model, the proof of the semi-classical limit and the relationship with loop quantum gravity and SU(2) spin network states."

Thursday session:

Ashtekar, Abhay Recent Developments in Loop Quantum Cosmology

Bojowald, Martin Perturbations in Loop Quantum Cosmology

Wilson-Ewing, Edward Loop quantum cosmology of Bianchi I space-times

Pawlowski, Tomasz Concept of evolution in Loop Quantum Cosmology on the example of a vacuum Bianchi I model

Mena Marugan, Guillermo Big Bounce in inhomogeneous cosmologies

Pullin, Jorge Spherically symmetric quantum scalar field in a quantum space time in loop quantum gravity: the vacuum and the cosmological constant

Puchta, Jacek Quantum corrections in quantum spacetime

Marciano, Antonino Stepping out of Homogeneity in Loop Quantum Cosmology, II Part: introducing inhomgeneities into a Bianchi IX model in LQG

Battisti, Marco Valerio Big-bounce in a Snyder-deformed quantum cosmology

Tsobanjan, Artur Effective Constraints for Quantum Systems

Barbero G., J. Fernando Generating functions for black holes in loop quantum gravity

Villaseñor, Eduardo J S Flux-area operator and black hole entropy in loop quantum gravity

Perez, Alejandro The Theta parameter in loop quantum gravity: Effects on quantum geometry and black hole entropy

F. Borja, Enrique Computing black hole entropy in LQG from a conformal field theory perspective

Yu, Hoi-Lai Black hole as elementary particle in superspace

The Bianchi cosmo models are not necessarily isotropic. Some of these papers are generalizing Loop cosmo to more complex realistic cases ( without the usual strong assumptions of uniformity, so more like the full theory.)

marcus · Jul 12, 2009

The Friday QG session:

Dowdall, Richard Asymptotic analysis of recent 4d spinfoam models

Alesci, Emanuele LQG propagator

Mikovic, Aleksandar Spin foam perturbation theory for quantum gravity

Conrady, Florian Quantum geometry from phase space reduction

Bianchi, Eugenio Spin networks and simplicial geometries

Bahr, Benjamin Perfect actions in Regge calculus

Lewandowski, Jerzy Extention of the recent SF modles to all the states of LQG

Görlich, Andrzej Quantum de Sitter Universe in 4D Causal Dynamical Triangulations

Velhinho, José Groups of flux-like transformations in Loop Quantum Gravity

Sahlmann, Hanno Chern-Simons theory, Stokes' Theorem, and generalized connections

Kazmierczak, Marcin The Poincaré Gauge Theory of Gravity and the Immirzi Parameter

Cianfrani, Francesco On the Removal of Time-Gauge in Loop Quantum Gravity, with and without Matter

Montesinos, Merced Topological field theories in n-dimensional spacetimes and Cartan's equations

Morrison, Ian Group Averaging for quantum fields in de Sitter

Velazquez , Mercedes Husain-Kuchar model as a constrained BF theory

Christodoulakis, Theodosios Towards Canonical Quantum Gravity for Geometries Admitting Maximally Symmetric Two-dimensional Surfaces

2009 is a pivotal year for Loop and allied QG. You can see here that considerable progress has been made on establishing the largescale limit. (See talks by Laurent Freidel for spinfoam, and Andy Goerlich, for CDT, plus various others listed here like Alesci's LQG graviton propagator talk.)

You can also see a trend I mentioned some 5 or 6 months ago---in Loop Cosmology they are getting out of homogeneity. That means to gradually reduce the simplifying assumptions in cosmology and bridge the gap between Loop Cosmology and the full LQG theory (which has no assumptions of homogeneity and isotropy.) This means introducing new degrees of freedom and shifting to more complex models. Several talks are about this.

It is important because testing of QG effects has begun in cosmology. The situation is still in disorder but everyone realizes that QG models will be tested thru astro observations, so there is a big need to connect the full LQG theory with Loop cosmology so that empirical tests of cosmo models will become phenomenologically relevant to the full theory.

Right now we have a disconnect. LQG by itself does not predict any kind of Lorentz violation. But various versions of DSR do! So far no one has been able to get LQG to imply any type of DSR.

=======================
EDIT Atyy That is such a kind thought. Do I wish! For various reasons I do not travel much. I would so like to be in Paris right now, right around Bastille day (14 July) and hear the talks of Laurent Freidel and Alain Connes and the others, go on a riverboat, watch the fireworks.
Even just thinking about it is fun.

atyy · Jul 13, 2009

marcus said:

Laurent's abstract is:
Spin Foam models: models of quantum dynamical space time

"In this talk I will give an overview of spin foam models which describe the dynamics of quantum gravity in a background independent context. I will focus especially and the recent developments which concerns the construction of these models in 4 dimensional gravity and present some of the key results obtained in this context like the construction of the model, the proof of the semi-classical limit and the relationship with loop quantum gravity and SU(2) spin network states."

Are you attending?

Introduction To Loop Quantum Gravity

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