Introduction To Loop Quantum Gravity

[marcus]

Two posts back, #245, I tried to work out a brief syllabus or self-teaching course plan for a graduate student to learn LQG. The preparation included looking at some earlier pedagogical introductions to canonical (old) LQG---by Rovelli Upadhya, and by Rovelli Gaul.

Now I would add a new introductory paper to the "beginning textbook" list:
http://arXiv.org/abs/1007.0402
Introductory lectures to loop quantum gravity
Pietro Doná, Simone Speziale

This paper derives from a series of lectures given last year. It was posted July 2010.

αβγδεζηθικλμνξοπρσςτυφχψω...ΓΔΘΛΞΠΣΦΨΩ...∏∑∫∂√ ...± ÷...←↓→↑↔~≈≠≡≤≥...½...∞...(⇐⇑⇒⇓⇔∴∃ℝℤℕℂ⋅)

 

[ssivark007]

Hi Marcus, I just discovered this gem of a resource you've been compiling. Great work!

A couple of posts back, you had put up a list of places around the world where people work in LQG. I'd like to ad another Indian place to the list : Institute of Mathematical Sciences, Chennai, India. Prof. Ghanashyam Date, and a few of his colleagues there work on LQG. Right now, there are also a few PhD students working with them.

Further, right now, an LQG school is being held in IMSc(for 8 days) which I'm attending. I'm a senior year undergrad student, and this is my first exposure to the field. Here's the link to school's website : http://www.imsc.res.in/~shyam/LQGSchool/

Cheers,
Siva

 

[marcus]

Siva, that was a major omission! I know of Gananshyam Date by reputation and have read several of his papers. The Institute for Mathematical Sciences (IMSc Chennai) should have been on the list all along. Someone who knows how to do that should add it to the list at "google maps" which I was using as my source.

Thanks for letting us know about the LQG school that is currently in progress at Chennai.

... (... to go along with the Google list.)
Here is the original:http://maps.google.com/maps/ms?ie=U...985216139270436.0004843830d27f3e6c50e&t=h&z=0
...
...
India
Raman Research Institute
Permanent: Madhavan Varadarajan

IUCAA Pune
Permanent: Naresh Dadhich
...
...

IMSc Chennai
Permanent: Ganashyam Date

I can't edit post #243 but if/when I recopy it I will make the addition.

 

[ssivark007]

Here is a (partial) list. I guess this wiki page will be updated once in a while by some maintainers.

http://en.wikipedia.org/wiki/List_of_loop_quantum_gravity_researchers

 

[marcus]

Here are some QG links that have been useful to have handy. Last time I updated the list was in July 2010. You can look back a few posts to the July list to find some which for brevity's sake have been omitted here.

Possible "textbook" resources:
Rovelli Upadhya http://arxiv.org/abs/gr-qc/9806079 (easy)
Rovelli Gaul http://arxiv.org/abs//gr-qc/9910079 (just relevant parts, short spinfoam section pp 43-46)
Baez handwritten lecture notes on spin networks/foams
http://math.ucr.edu/home/baez/foam/
Doná Speziale Introductory lectures to loop quantum gravity
http://arXiv.org/abs/1007.0402
Bojowald textbook (to be released) "Canonical GR and applications"
http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521195751
Renate Loll 3-week course at Perimeter, Intro to Quantum Gravity:
http://pirsa.org/index.php?p=speaker&name=Renate_Loll
Baez on spinfoams http://arxiv.org/abs/gr-qc/9709052 (see especially pages 1-4 and 33,34)
Lewandowski et al on spinfoams http://arxiv.org/abs/0909.0939

To find out about conferences and workshops, "hyperspace" at the AEI site:
http://hyperspace.aei.mpg.de/category/Conferences/
The Nottingham QG network:
http://www.maths.nottingham.ac.uk/qg/

Some links to conferences etc.
May 23-28, 2011: Loops 11 conference in Madrid
http://www.iem.csic.es/loops11/

June 2009 PlanckScale conference video and slides:
http://www.ift.uni.wroc.pl/~rdurka/planckscale/index-video.php
Workshops at Ashtekar's Penn State IGC:
http://www.gravity.psu.edu/events/workshops.shtml
Online International LQG Seminar:
http://relativity.phys.lsu.edu/ilqgs/
November 2009 Asymptotic safety conference at Perimeter:
http://www.perimeterinstitute.ca/en/Events/Asymptotic_Safety/Asymptotic_Safety_-_30_Years_Later/
AsymSafe conference videos:
http://pirsa.org/C09025
February 2010 Oberwolfach workshop on Noncommutative Geometry and LQG ("Loops, Algebras, Spectral Triples"):
http://www.mfo.de/programme/schedule/2010/06b/OWR_2010_09.pdf
March 2010, Zakopane QG workshop (QGQG = quantum geometry/gravity):
http://www.fuw.edu.pl/~jpa/qgqg3/
July 2010 Stockholm workshop on the experimental search for QG:
http://th.physik.uni-frankfurt.de/~hossi/ESQG10/
Photos from the first Loops conference, at Marseille 2004:
http://math.ucr.edu/home/baez/marseille/

Loop papers from 2007-present (currently 500 or more papers) ranked by citation count:
(keywords "spin, foam", "field theory, group", "quantum gravity, loop space", and "quantum cosmology, loop space"):
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+K+SPIN%2CFOAM+OR+DK+FIELD+THEORY%2C+GROUP+OR+DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+QUANTUM+COSMOLOGY%2C+LOOP+SPACE+AND+DATE+%3E+2006&FORMAT=www&SEQUENCE=citecount%28d%29

Classic online sources.
Review of LQG as of May 2008:
http://relativity.livingreviews.org/Articles/lrr-2008-5/
Bianchi Rovelli Vidotto, Towards Spinfoam Cosmology:
http://arxiv.org/abs/1003.3483
Rovelli April 2010 paper, A new look at loop quantum gravity:
http://arxiv.org/abs/1004.1780
Rovelli's talk at Strings 2008 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
2003 draft of Rovelli's book online:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf
Rovelli's chapter in Oriti's book:
http://arxiv.org/abs/gr-qc/0604045
Penrose circa-1970 papers introducing spin-networks
http://math.ucr.edu/home/baez/penrose/
Baez notes on spin networks and spinfoams
http://math.ucr.edu/home/baez/foam/
Weinberg's 6 July 2009 talk, main CERN link:
http://indico.cern.ch/conferenceDisplay.py?confId=57283
Weinberg video (historical overview up to minute 58 followed by
no-frills approach to unification in the last 12 minutes):
http://cdsweb.cern.ch/record/1188567/

Recent progress in Lqg and Gft:
Livine's 17 February 2010 talk, A review of Spinfoams and Group Field Theory:
http://pirsa.org/10020079/
Related paper by Girelli Livine Oriti, 4d Deformed Special Relativity from Group Field Theories:
http://arxiv.org/abs/0903.3475
Bianchi Magliaro Perini Coherent spin-networks
http://arxiv.org/abs/0912.4054
Bianchi Magliaro Perini Spinfoams in holomorphic representation
http://arxiv.org/abs/1004.4550
Bianchi Regoli Rovelli, Face amplitude of spinfoam QG:
http://arxiv.org/abs/1005.0764
Alesci Rovelli, A regularization of the hamiltonian constraint compatible with the spinfoam dynamics:
http://arxiv.org/abs/1005.0817
Rovelli Speziale, On the geometry of LQG on a graph:
http://arxiv.org/abs/1005.2927
Ding Rovelli, Physical boundary Hilbert space and volume operator in the Lorentzian new spin-foam theory:
http://arxiv.org/abs/1006.1294
Geloun Gurau Rivasseau on LQG Spinfoam Group Field Theory
http://arxiv.org/abs/1008.0354
Lewandowski et al Gravity quantized
http://arxiv.org/abs/1009.2445
Bianchi Doná Speziale Polyhedra in LQG (quantum polyhedra as intertwiners)
http://arxiv.org/abs/1009.3402

Thermal time
Rovelli Smerlak, Thermal time and the Tolman-Ehrenfest effect: temperature and the "speed of time":
http://arxiv.org/abs/1005.2985
Connes Rovelli 1994 Thermal time paper
http://arxiv.org/abs/gr-qc/9406019
Rovelli 2009 Time essay
http://arxiv.org/abs/0903.3832

Unimodular Gravity
Smolin Unimodular loop quantum gravity and the problems of time
http://arxiv.org/abs/1008.1759
Ellis Murugan van Elst The gravitational effect of the vacuum
http://arxiv.org/abs/1008.1196
Smolin's Abhayfest talk
http://gravity.psu.edu/events/abhayfest/talks/Smolin.pdf
Chiou Geiller Unimodular Loop Quantum Cosmology
http://arxiv.org/abs/1007.0735
Smolin The quantization of unimodular gravity and the cosmological constant problem
http://arxiv.org/abs/0904.4841
Bianchi Rovelli Why all these prejudices against a constant? (contrasting viewpoint)
http://arxiv.org/abs/1002.3966

Selected longshots:
Julian Barbour Conformal superspace: configuration space of GR
http://arxiv.org/abs/1009.3559
Matilde Marcolli et al, Spin Foams and Noncommutative Geometry:
http://arxiv.org/abs/1005.1057
Kirill Krasnov Deformations of General Relativity:
http://pirsa.org/10050002/
Lisi Smolin Speziale, Unification of gravity, gauge fields, and Higgs bosons:
http://arxiv.org/abs/1004.4866
Vincent Rivasseau at the 2009 Perimeter AsymSafe conference:
http://pirsa.org/09110049/
Ted Jacobson at the 2009 Perimeter HorGrav conference:
http://pirsa.org/09110066/
Ted Jacobson Thermodynamics of Spacetime: The Einstein Equation of State:
http://arXiv.org/abs/gr-qc/9504004
Ted Jacobson Introduction to Quantum Fields in Curved Spacetime and the Hawking Effect:
http://arxiv.org/abs/gr-qc/0308048
Erik Verlinde On the Origin of Gravity and the Laws of Newton:
http://arxiv.org/abs/1001.0785
Lee Smolin, Newtonian gravity in loop quantum gravity:
http://arxiv.org/abs/1001.3668
Modesto Randono Entropic corrections to Newton's law
http://arxiv.org/abs/1003.1998
Jerzy Kowalski-Glikman, Note on gravity, entropy, and BF topological field theory:
http://arxiv.org/abs/1002.1035
Easson Frampton Smoot, Entropic Accelerating Universe:
http://arxiv.org/abs/1002.4278
Chamseddine Connes, Noncommutative Geometry as a Framework for Unification of all Fundamental Interactions including Gravity. Part I:
http://arxiv.org/abs/1004.0464
Smolin and Unger presentation at Laws of Nature workshop{ http://pirsa.org/C10001 }:
http://pirsa.org/10050053/

A list of QG researchers worldwide, mostly LQG but some other:
http://en.wikipedia.org/wiki/List_of_loop_quantum_gravity_researchers
Francesca's LQG world map:
http://maps.google.com/maps/ms?ie=U...985216139270436.0004843830d27f3e6c50e&t=h&z=0

Selected outreach links from IGC (Institute for Gravitation and the Cosmos) with a section of recent (2006-2010) popular articles:
http://gravity.psu.edu/outreach/index.shtml

αβγδεζηθικλμνξοπρσςτυφχψωΓΔΘΛΞΠΣΦΨΩ∏∑∫∂√±←↓→↑↔~≈≠≡≤≥½∞(⇐⇑⇒⇓⇔∴∃ℝℤℕℂ⋅)

 

[marcus]

Xristy helpfully assembled the links to the chapters of a new book which are available in preprint form online.
https://www.physicsforums.com/showthread.php?p=2899654#post2899654
The book is a collection of talks given at George Ellis' 70th birthday celebration, called The Foundations of Space and Time. A gathering of people from Loop, CDT, String, Cosmology communities comparing notes on basic issues and each other's approaches. Ellis co-edited the book and it is to be published by Cambridge Press.
http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=9780521114400

Some information is posted about Loops 11. It can be interesting to look down the list of plenary speakers at a major conference--it may give some idea of where the field is or where it's going. The Loops conference is held approximately biannually (once every two years) with other related conferences sometimes filling in on off-years. Here's the plenary speakers list:

Abhay Ashtekar (Penn State University, USA).
Martin Bojowald (Penn State University, USA).
Alejandro Corichi (UNAM-Morelia, Mexico).
Bianca Dittrich (Albert Einstein Institute, Germany).
Kristina Giesel (TU Munich, Germany).
Jerzy Lewandowski (Warsaw University, Poland).
Renate Loll (Utrecht University, Netherland).
Yongge Ma (Beijing Normal University, China).
Don Marolf (Univ. California-Santa Barbara, USA).
Daniele Oriti (Albert Einstein Institute, Germany).
Tomasz Pawlowski (University of New Brunswick, Canada).
Alejandro Pérez (Univ. Méditerranée, Marseille, France).
Jorge Pullin (Louisiana State University, USA).
Carlo Rovelli (Univ. Méditerranée, Marseille, France).
Hanno Sahlmann (Karslruhe University, Germany).
Parampreet Singh (Perimeter Institute, Canada).
Lee Smolin (Perimeter Institute, Canada).
Thomas Thiemann (Univ. Erlangen-Nurnberg, Germany).
Madhavan Varadarajan (Raman Research Institute, India).

The Loops 11 conference will be held 23-28 May, in Madrid:
http://www.iem.csic.es/loops11/
for comparison, here are the plenary speakers and talks of Loops 09 and Loops 07:
https://www.physicsforums.com/showthread.php?p=2296306#post2296306
http://www.matmor.unam.mx/eventos/loops07/ (links to plenary talks/abstracts)

Christine's thread about the forthcoming Hamiltonian GR textbook (applications to LQG cosmology) by Penn State's Martin Bojowald
https://www.physicsforums.com/showthread.php?t=432381

 

[marcus]

Atyy called attention to an interesting January 2010 paper by Madhavan Varadarajan
which was cited in Thiemann's latest (October 2010). This gives a good idea of what some of the plenary talks will be about at Loops 11.

Some information is posted about Loops 11... Here's the plenary speakers list:
...
...
Lee Smolin (Perimeter Institute, Canada).
Thomas Thiemann (Univ. Erlangen-Nurnberg, Germany).
Madhavan Varadarajan (Raman Research Institute, India).

The Loops 11 conference will be held 23-28 May, in Madrid:
http://www.iem.csic.es/loops11/

Rovelli's talk could draw from the October "Simple Model" paper http://arxiv.org/abs/1010.1939
Lewandowski's from his September "Gravity Quantized" http://arxiv.org/abs/1009.2445
Smolin could conceivably talk about Unimodular Gravity (and the problem of time? http://arxiv.org/abs/1008.1759 )
Thiemann's talk could draw from http://arxiv.org/abs/1010.2426
Varadarajan's from http://arxiv.org/abs/1001.3505

LQG-RELATED TEXTBOOKS etc. scheduled for release:

Christine's thread about Bojowald's Hamiltonian GR textbook (applications to LQG cosmology)
https://www.physicsforums.com/showthread.php?t=432381
Cambridge Press page about Canonical Gravity and Applications
http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521195751

Amazon page about Once Before Time, Bojowald's forthcoming Loop cosmology popularization
https://www.amazon.com/dp/0307272850/?tag=pfamazon01-20

World Scientific press page about Montani's textbook Primordial Cosmology:
http://www.worldscibooks.com/physics/7235.html

Cambridge Press page about Foundations of Space and Time (Ellisfest collection)
http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=9780521114400

Chennai basic LQG/LQC Lectures http://arxiv.org/pdf/1004.2952
(Ganashyam Date gave a series of beginning lectures on LQG which explain certain things most clearly--only 72 pages, not a textbook but could be used as supplemental reading.)

Xristy's links to draft chapters of the Ellisfest book:
https://www.physicsforums.com/showthread.php?p=2899654#post2899654

 

[marcus]

Eugenio Bianchi will give a talk soon at Perimeter, about Loop Quantum Gravity, and it will go on the PIRSA online lecture archive
http://pirsa.org/10110052/
9 November

 

[marcus]

A newcomer was asking about math prep for LQG--here are some online notes on group representation theory---if you know better online notes please say. There is quite a variety here to pick and choose from:

http://www.math.columbia.edu/~woit/RepThy/

 

[marcus]

A one-year masters (MSc) program is being set up by John Barrett and colleagues at Nottingham. The first year of operation will start September 2011. We may be able to pick up some information about their curriculum, lecture notes, textbooks---get some clues from this.
http://johnwbarrett.wordpress.com/

==quote==
New MSc course
Here at Nottingham we are starting an MSc in Gravity, Particles and Fields. This is very specifically aimed at students interested in getting into relativity and particle physics research, in areas such as quantum gravity, cosmology, quantum information, etc.

The first run starts in September 2011, so we are open for applications now.
==endquote==

http://pgstudy.nottingham.ac.uk/postgraduate-courses/gravity-particles-and-fields-masters-msc_1163.aspx
==quote==
Gravity, Particles and Fields Masters (MSc)
Duration: 1 year full-time

The course provides an introduction to the physical principles and mathematical techniques of current research in general relativity, quantum gravity, particle physics, quantum field theory, quantum information theory, cosmology and the early universe.
The programme of study includes a taught component of closely-related modules in this popular area of mathematical physics. The course also includes a substantial project that will allow students to develop their interest and expertise in a specific topic at the frontier of current research, and develop their skills in writing a full scientific report.
The course will provide training in advanced methods in mathematics and physics which have applications in a wide variety of scientific careers and provide students with enhanced employability compared with undergraduate Bachelors degrees. In particular, it will provide training appropriate for students preparing to study for a PhD in the research areas listed above. For those currently in employment, the course will provide a route back to academic study.
Key facts
• The course is taught jointly by the School of Mathematical Sciences and the School of Physics and Astronomy.
• Dissertation topics are chosen from amongst active research themes of the Particle Theory group, the Quantum Gravity group, and the Quantum Information group
• In addition to the lectures on the course, there are several related series of research-level seminars to which Masters students are welcomed.
• The University of Nottingham is ranked in the top 1 per cent of all universities worldwide.
...
==endquote==

 

[QGravity]

Does LHC Can test Loop quantum gravity or other approaches to QG?

 

[marcus]

Does LHC Can test Loop quantum gravity or other approaches to QG?

I haven't thought much about it. It seems to me that in this period of history new physics is often going to be revealed by the huge river of data coming from the sky---astrophysical observations by new types of instruments (many in space but also on ground.)

The importance of LHC may have been exaggerated relative to other less expensive instruments. I really don't know, it's hard to judge. But I'll mention a few things to think about besides the LHC. Some might be relevant to testing LQG and others not.

Things to be investigated include: dark energy or cosmological constant, dark matter, Gammaray Bursts, high-energy cosmic rays, early universe neutrinos, polarization and other features of Microwave Backgound, gravitational wave imprint in CMB, signals from black hole formation (unusual stellar explosions). Largescale structure of universe.

Instruments: interferometers including spaceborn, synthetic aperture telescopes, atmospheric Cherekov gammaray telescopes, CMB-mapping spacecraft beyond COBE and WMAP, X-ray astronomy using spacecraft , gravitational wave detectors, neutrino detectors like IceCube in Antarctica, Cosmic Ray detector arrays like the Auger, gammaray astronomy in space as with FermiLAT or combining orbital and ground as with SWIFT.

This is a quick impromptu list bound to leave stuff out. I'd guess that LQG testing comes in here somewhere with CMB early universe astronomy, and maybe somehow with some of the ways of gathering information.

 

[marcus]

In a sense cosmology is the "collider" for quantum gravity and unification. The main arena for testing models is going to be the early universe. It's is where the energy is, and the phenomena we have to make sense of.
So when Weinberg talked to Strings 2010 (about his "Plan B" in case string although great mathematics proved physically disappointing) he focused on applying his Plan B idea to cosmology. Its where everything has to go eventually, not just hobble on to the next particle accelerator.

So we have to watch developments in quantum cosmology and related areas. Here is a unique resource. Penrose June 2009 talk at the Institute for Gravitation and Cosmology:

The title was "Fashion, Faith, and Fantasy in the New Science of the Universe"

1.
2.
3. http://www.youtube.com/watch?v=4BtYn2Wde4o&NR=1
4. http://www.youtube.com/watch?v=zVTCaaP5Y0s&feature=related
5. http://www.youtube.com/watch?v=bQNUwHnQzvA&feature=related
6. http://www.youtube.com/watch?v=DvfRYzRgqWE&feature=related
7. http://www.youtube.com/watch?v=afsd3_PJais&feature=related
8. http://www.youtube.com/watch?v=UMi_QB0vWC0&feature=related
9. http://www.youtube.com/watch?v=tiRy07yweA0&feature=related
10. http://www.youtube.com/watch?v=5jXj1TwiFys&feature=related

http://gravity.psu.edu/events/abhayfest/
=======================================

Weinberg on Asymptotic Safe gravity/cosmology, at Strings 2010:
https://mediamatrix.tamu.edu/streams/327756/PHYS_Strings_2010_3-18-10C

A recent Asyptotic Safe gravity paper that I think will prove important (sometimes computational power enables progress as much as a new idea, it's happened before in other fields.)

http://arxiv.org/abs/1012.3081
The Universal RG Machine
Dario Benedetti, Kai Groh, Pedro F. Machado, Frank Saueressig
38 pages
(Submitted on 14 Dec 2010)
"Functional Renormalization Group Equations constitute a powerful tool to encode the perturbative and non-perturbative properties of a physical system. We present an algorithm to systematically compute the expansion...
... In a first illustrative example, we re-derive the gravitational beta-functions of the Einstein-Hilbert truncation, demonstrating their background-independence. As an additional result, the heat-kernel coefficients for transverse vectors and transverse-traceless symmetric matrices are computed to second order in the curvature."

Here is Roberto Percacci's website index page for AsymSafe gravity.
http://www.percacci.it/roberto/physics/as/index.html
Thanks to Atyy for pointing out the new picture (Benedetti, Machado, Rahmede collaborating at the blackboard)
Percacci has a FAQ for AsymSafe, and useful links to stuff.
Someone was asking if Pedro is from Brazil or Portugal--here's his current page:
http://www.gen.cam.ac.uk/research/AMA/members/Pedro.html

 

[marcus]

The previous post mentioned some Asymptotic Safe gravity stuff (from Weinberg, Percacci, Benedetti et al...). On the same subject, one of the major figures in AS gravity, Martin Reuter, just came out with a paper formulating AS in a way that looks compatible with LQG. The Holst action, which LQG uses, and its Immirzi parameter. This could serve as a bridge between the two QG approaches.
http://arxiv.org/abs/1012.4280

Another Loop textbook coming out.
http://www.springer.com/physics/the...+computational+physics/book/978-1-4419-8275-9

This will be in the Springer Lecture Series, planned release date March 2011.
Quantum Cosmology, by Martin Bojowald
approx. 250 pages

This might serve as a companion to another textbook which Cambridge Press is bringing out
http://www.cambridge.org/gb/knowledge/isbn/item5692826/?site_locale=en_GB
Canonical Gravity and Applications: Cosmology, Black Holes, and Quantum Gravity, by the same author
approx. 310 pages

The UK Amazon says it is temporarily out of stock:
http://www.amazon.com/dp/0521195756/?tag=pfamazon01-20
US Amazon says you can pre-order, available January 2011.

It's interesting to see QG and QC take shape as academic subjects. When I started watching the field in 2003 there were, as far as I know, no textbooks. At least not for quantum *cosmology*, which (because we have so much data on the early universe) seems to be the key to the whole business.

 

[marcus]

John Baez *Week 280* has a valuable discussion of the 2009 Corfu QG school and workshop:
http://math.ucr.edu/home/baez/week280.html

Also interesting was the lively discussion at the ⁿCategory Cafe which followed.
http://golem.ph.utexas.edu/category/2009/09/this_weeks_finds_in_mathematic_41.html
(MTd2 took active part, exotic smooth structures came up, and Torsten A-M who has posted some at PF Beyond also got into it.)

 

[piareround]

So I while back I asked about the relationship between quantum foam, Quantum Loop Gravity theory, string theory, and how many dimensions each theory requires in this link https://www.physicsforums.com/showthread.php?t=432962". As you may note nismaratwork helped me to understand how string theory would smear out quantum foam. He also helped remind me that String Theory require multiple dimensions but quantum foam requires only four.

However, I as I was reading your thread I realized that I assumed two things:
First, quantum loop gravity only requires 4 dimensional Minkowski Space
Second, that quantum foam actually plays a role in quantum loop gravity.

So I was hoping you could help me with the following questions:

1. Are there any limits, such as a minimum or maximum, on the number of dimensions that quantum loop gravity, independent of string theory, would require?
2. Is their a relationship between quantum foam and quantum loop gravity? If so, what role exactly does quantum foam play in quantum loop gravity theory? If not, does quantum loop gravity, replace quantum foam with something analogous to quantum foam purpose in solution the Klein-Gordon Equation(*)?

 

[marcus]

So I while back I asked about the relationship between quantum foam, Quantum Loop Gravity theory, string theory, and how many dimensions each theory requires in this link https://www.physicsforums.com/showthread.php?t=432962". As you may note nismaratwork helped me to understand how string theory would smear out quantum foam. He also helped remind me that String Theory require multiple dimensions but quantum foam requires only four.

However, I as I was reading your thread I realized that I assumed two things:
First, quantum loop gravity only requires 4 dimensional Minkowski Space
Second, that quantum foam actually plays a role in quantum loop gravity.

So I was hoping you could help me with the following questions:

1. Are there any limits, such as a minimum or maximum, on the number of dimensions that quantum loop gravity, independent of string theory, would require?
2. Is their a relationship between quantum foam and quantum loop gravity? If so, what role exactly does quantum foam play in quantum loop gravity theory? If not, does quantum loop gravity, replace quantum foam with something analogous to quantum foam purpose in solution the Klein-Gordon Equation(*)?

Let's start a separate thread, for discussion/explanation. I try to use this thread just to provide selected links to useful source material for people following current LQG research.
It would get unwieldy to include discussion and explanation in addition to what it's doing already.

I will start a thread for you, with your questions about LQG.

 

[marcus]

This year (first two weeks March 2011) the best ever QG school will be held at the ski resort Zakopane. It is aimed at postdocs and advanced PhD students ready to enter QG research. The school is definitive of the field, so we can learn some things from examining the schedule.

Of the lectures on LQG, those I have colored bright blue will probably be more from the manifold-free perspective----no spacetime continuum manifold. That will be about half, I guess. Those I have colored dark or navy blue will probably use the older development which starts with a 4D manifold. Talks listed in black are on other types of QG, or neutral as regards this change in LQG.

So one thing we can learn by looking at the school line-up is how this reconstitution or reformulation of LQG is progressing. Another thing to gauge is how much the Loop people are looking towards Group Field Theory, Noncommutative Field Theory, and Noncommutative Geometry for additional inspiration. I think it has to do with a sense that LQG is ready to take on matter---to rebuild QFT on a quantum geometrical basis---and they are on the lookout for ideas.

In any case here is the schedule color-coded as I described earlier:

Mar 1
A. Ashtekar 2h — Opening lecture
C. Rovelli 2h — Spin foams
K. Giesel 2h — Loop quantum gravity
H. Sahlmann 2h — Loop quantum gravity

Mar 2
K. Giesel 2h — Loop quantum gravity
H. Sahlmann 2h — Loop quantum gravity
C. Rovelli 2h — Spin foams

Mar 3
K. Giesel 1h — Loop quantum gravity
H. Sahlmann 1h — Loop quantum gravity
J. Brunnemann 2h — Volume operator
C. Rovelli 2h — Spin foams

Mar 4 Break (Friday)

Mar 5
C.Rovelli 2h — Spin foams
W. Kamiński 2h — EPRL map: SO(4) and SO(1,3), integrability
P. Singh 2h — Loop quantum cosmology

Mar 6
S. Hollands 2h — Exact QFT in curved backgrounds
C. Rovelli 2h — Spin foams
P. Singh 2h — Loop quantum cosmology

Mar 7
S. Hollands 2h — Exact QFT in curved backgrounds
S. Speziale 2h— LQG and twisted geometries
C. Perini 1h — Graviton propagator
E. Bianchi 1h — Friedmann equation from the EPRL vertex amplitude

Mar 8
T. Krajewski 2h — Group field theories
S. Hollands 2h — Exact QFT in curved backgrounds
J.B. Geloun/V. Rivasseau 2h — EPRL GFT

Mar 9 Break (Wednesday)

Mar 10
H. Steinacker 2h — Non-commutative geometry and matrix models
J. Barrett 2h — Large j limit of spin foam amplitudes
T. Krajewski 2h — Group field theories

Mar 11
H. Steinacker 2h — Non-commutative geometry and matrix models
H. Grosse 2h — Renormalisation of the Grosse-Wulkenhaar model
M. Burić 2h — Renormalisation of the Grosse-Wulkenhaar model

Mar 12
H. Steinacker 2h — Non-commutative geometry and matrix models
K. Noui 2h — SL_q(2,C)
W. Fairbairn 2h — SL_q(2,C) EPRLK model

Mar 13
B. Dittrich 2h — Diffeomorphisms, renormalisation and perfect action in discrete theories
J. Jurkiewicz 2h — Causal dynamical triangulations
C. Meusburger 2h — 2+1 gravity

Offhand it seems like the school lineup is about evenly balanced in respect to manifoldy/manifoldless or however you want to imagine the shifting reconstitution going on.
http://www.fuw.edu.pl/~kostecki/school3/

 

[marcus]

Short history of LQG (courtesy Inspire database)

Stanford-SLAC has a new physics database called Inspire. It improves on Spires, which has been in use for over a decade. Inspire is still in beta and being worked-on.

Here, in effect, is a history of Loop Gravity from 2005 onwards obtained simply by finding each year's papers and listing them ranked by cite count. The cites reflect the interest of the research community, so you can see what were considered (roughly speaking) the most interesting/valuable Loop papers of each year. These leading papers influence the direction that research in the field moves. You will see the focus and makeup of the top papers change over time.
(To learn more about any paper click on the corresponding "detailed record" link.)

The number of papers which appeared each year are given in parenthesis.

2005: (42)
http://inspirebeta.net/search?ln=en...2y=2005&sf=&so=a&rm=citation&rg=25&sc=0&of=hb

2006: (77)
http://inspirebeta.net/search?ln=en...2y=2006&sf=&so=a&rm=citation&rg=25&sc=0&of=hb

2007: (120)
http://inspirebeta.net/search?ln=en...2y=2007&sf=&so=a&rm=citation&rg=25&sc=0&of=hb

2008: (142)
http://inspirebeta.net/search?ln=en...2y=2008&sf=&so=a&rm=citation&rg=25&sc=0&of=hb

2009: (145)
http://inspirebeta.net/search?ln=en...2y=2009&sf=&so=a&rm=citation&rg=25&sc=0&of=hb

2010: (153)
http://inspirebeta.net/search?ln=en...2y=2010&sf=&so=a&rm=citation&rg=25&sc=0&of=hb

The keywords used are "quantum gravity: loop space", "quantum cosmology: loop space", "spin: foam".

 

[marcus]

I will try to assemble here links which can help assess the current status of LQG and LQC (loop gravity and loop cosmology)---as well as provide an introduction to the modern formulation of loop gravity using combinatorial spinfoam dynamics. This spinfoam formulation is sometimes referred to as the "full theory" as contrasted with loop cosmology which has been developed as a symmetry reduced version: made simpler by assuming some degree of uniformity in the universe. There are several points to be made about current developments, for which corresponding source links will be given.

Quantum cosmology has become observational. Here are articles mostly by early universe phenomenologists about ways of testing LQC against CMB data. Since LQC derives from LQG, this turns out to be the most accessible route to testing LQG.
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+POWER+SPECTRUM+or+dk+cosmic+background+radiation%29+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

Loop cosmology connects with the full (spinfoam) theory:
http://arxiv.org/abs/1003.3483
Towards Spinfoam Cosmology

The distinctive prediction of a bounce carries over:
http://arxiv.org/abs/1010.1258
Big Bounce in Dipole Cosmology

LQC bounce naturally initiates a period of superinflation even without the usual scalar ("inflaton") field. If the usual scalar field is present as well, the bounce ensures ample inflation without any further adjustments.
http://arxiv.org/abs/1103.2475
Probability of Inflation in Loop Quantum Cosmology
This is a kind of observational confirmation because it shows that LQC makes observed features of the universe associated with inflation more likely.

A definitive presentation of LQG, with some math prerequisites:
http://arxiv.org/abs/1102.3660
Lectures on Loop Gravity

A definitive presentation of LQC, as of mid-2010:
http://arxiv.org/abs/1005.5491.
The Big Bang and the Quantum

A comparatively non-technical presentation of LQG, including its historical development and evidence suggesting the theory recovers classical GR.
http://arxiv.org/abs/1012.4707
Loop quantum gravity: the first twenty five years

http://arxiv.org/abs/1101.4049
Cosmological constant in spinfoam cosmology

http://arxiv.org/abs/1101.3264
Spinfoam Fermions: PCT Symmetry, Dirac Determinant, and Correlation Functions

http://arxiv.org/abs/1012.1739
Lorentz covariance of loop quantum gravity

Formation of an inclusive research community of Quantum Gravity/Geometry/Cosmology is being helped by European Science Foundation "QG" events such as this year's conference at the Zurich ETH:
http://www.conferences.itp.phys.ethz.ch/doku.php?id=qg11:start

Quantum Theory and Gravitation
Topics to be covered include:
General quantum theory, relativistic quantum theory, emergence of space(-time)
General quantum field theory, including deformations of QFTs
QFT on curved and NC space-times
Canonical quantum gravity and supergravity
Regge calculus
String theory and M-theory
Loop gravity, spin foam
Quantum cosmology

International Advisory Board
John Barrett
Harald Grosse
Hermann Nicolai
Carlo Rovelli
Roger Picken

 

[mehul ahir]

sir my question is that is it literally impossible to prdict the volume f the previous universe or it can be pradicted with accurate theorues?please tell me sir

 

[marcus]

Mehul Ahir, sir, that is a helpful question because it reminds us of how little we know. There are several different approaches to Quantum Cosmology, and on the whole they are young unformed theories. We do not know which, if any, is to be trusted.

Loop cosmology is only just beginning to be compared with real observational data. You can see in the recent Ashtekar-Sloan paper how they confront the theory with WMAP data. http://arxiv.org/abs/1103.2475 So far there are some pretty nice agreements with data but further testing could prove the theory wrong!

And even if the theory survives a number of tests and begins to be tentatively trusted, it still says very little definite about the contracting phase of the universe, before the bounce. Even if we believed the LQC theory we could still say almost nothing about the spatial volume. Only that distances were contracting in the same sense that they are now expanding.

We do not know the total spatial volume the universe has even today. We only can estimate how far away the most distant material is that we can see.

The great thing, as I see it, is that scientists are doing their job. They are supposed to construct simple TESTABLE theories that fit the data. And then their job is to derive predictions from theory that can be checked by observation---and if those predictions are not observed then the theory is disproved and one must construct a better one. The good news is not that Loop Cosmology is right (we don't know that) but that it is testable. It replaces the former singularity with a particular kind of bounce that has measurable consequences to look for in the cosmic microwave background (CMB, the ancient light).

 

[marcus]

The next biannual Loops conference, Loops 2011, will be held in Madrid the last week of May.
To date about 150 people have registered to partciipate:
http://www.iem.csic.es/loops11/

Strings 2011 will be held one month later, at the end of June, in Uppsala, Sweden.
http://www-conference.slu.se/strings2011/
The registered participants include Stephen Hawking, Juan Maldacena, John Schwarz, Michael Green, and Edward Witten:
https://www.akademikonferens.se/list.jsf?conf=strings2011

 

[marcus]

Loop cosmology linking up with early universe phenomenology is a recent development that has changed the QG picture somewhat. Here are 29 early U. phenom. papers that appeared 2009 or later:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+primordial%2C+fluctuation+OR+DK+INFLATION+OR+DK+COSMIC+BACKGROUND+RADIATION%29+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

The Spires search is:
IND (DK QUANTUM GRAVITY, LOOP SPACE OR DK QUANTUM COSMOLOGY, LOOP SPACE) AND (DK PRIMORDIAL, FLUCTUATION OR DK INFLATION OR DK COSMIC BACKGROUND RADIATION) AND DATE > 2008
====================
The next biannual Loops conference, Loops 2011, will be held in Madrid the last week of May.
We can get an idea of where in the US you can study Loop gravity/cosmology by looking at the list of 170 registered participants.
Only 25 are from US institutions---the US lags Europe in QG.
http://www.iem.csic.es/loops11/
Here's where the US participants come from

PSU (Penn State) 13
LSU (Louisiana) 4
UC Berkeley 2
Hamilton 1
FAU (Florida Atlantic, Boca Raton) 1
FSU (Florida State, Talahassee) 1
Utah State 1
Dartmouth College 1
Le Moyne College 1

Chris Duston, the guy coming from FSU, is doing his PhD in quantum gravity/cosmology under Matilde Marcolli (Caltech). So Caltech has a kind of spectral presence in this listing as well.
==================================
Strings 2011 will be held one month later, at the end of June, in Uppsala, Sweden.
http://www-conference.slu.se/strings2011/
https://www.akademikonferens.se/list.jsf?conf=strings2011
The three public lectures (by Stephen Hawking, Brian Greene, Andrei Linde) will feature the stringy Landscape and the Multiverse.
===================================

In between these two, in mid June, there's the Zurich conference
http://www.conferences.itp.phys.ethz.ch/doku.php?id=qg11:start
Quantum Theory and Gravitation
87 registered participants so far, ecumenical (brings together prominent people from various different approaches to QG) interesting lineup of speakers. 30 from some 5 or 6 different lines of research.

 

[marcus]

Here is a page about the new one-year MSc program at Nottingham, in Gravity, Particles and Fields
http://pgstudy.nottingham.ac.uk/postgraduate-courses/gravity-particles-and-fields-masters-msc_1163.aspx

==quote==
The course provides an introduction to the physical principles and mathematical techniques of current research in general relativity, quantum gravity, particle physics, quantum field theory, quantum information theory, cosmology and the early universe.
The programme of study includes a taught component of closely-related modules in this popular area of mathematical physics. The course also includes a substantial project that will allow students to develop their interest and expertise in a specific topic at the frontier of current research, and develop their skills in writing a full scientific report.
The course will provide training in advanced methods in mathematics and physics which have applications in a wide variety of scientific careers and provide students with enhanced employability compared with undergraduate Bachelors degrees. In particular, it will provide training appropriate for students preparing to study for a PhD in the research areas listed above. For those currently in employment, the course will provide a route back to academic study.
Entry requirements: At least a second class honours (2:2) BSc degree (or equivalent from other countries) in Physics, Mathematical Physics or Mathematics, or joint degrees containing substantial elements of physics or mathematics.
Other requirements: Previous knowledge of mechanics, quantum mechanics, special relativity and methods of mathematical physics (all as taught typically at BSc level 2) is required.
IELTS: 6.0 (with no less than 5.0 in any element)
TOEFL paper based: 550 (with at least 4.0 on the TWE)
TOEFL IBT: 79 (with no less than 17 in any element)
==endquote==

The main person responsible for the new program is John Barrett. His research is in several QG areas including Loop and NC-geometry. He's also director of ESF-QG the European Science Foundation setup for funding quantum gravity and quantum geometry.
Here's Barrett's Nottingham faculty page:
http://www.nottingham.ac.uk/mathematics/people/john.barrett
and personal homepage
http://johnwbarrett.wordpress.com/
http://johnwbarrett.wordpress.com/talks/
Exemplifying directions in the European QG community, here's the website of a conference that Barrett's ESF-QG agency is sponsoring this summer in Zurich.
http://www.conferences.itp.phys.ethz.ch/doku.php?id=qg11:start
It brings together Loop, NCG, String, SUGRA, QFT on Curved space---speakers chosen who the organizers consider able to talk to each other and exchange ideas. Amazing conference, first of its kind. Barrett is one of the organizers, with Carlo Rovelli, Hermann Nicolai* and some other top people.

*director of the QG+unification wing at MPI Potsdam
=================
I have some more stuff related to the Loop academic terrain and entry to the field in this other thread:
https://www.physicsforums.com/showthread.php?p=3229102#post3229102
which was started by Barak (about doing one's PhD in Loop).
==================
Incidental information about the annual Strings conference (late June, Uppsala).
http://www-conference.slu.se/strings2011/
Preliminary list of confirmed speakers (sorted by type):

OPENING TALK
David Gross (KITP, Santa Barbara) opening talk

REVIEWS and other talks
Henriette Elvang (University of Michigan) review talk
Michael Green (Cambridge University)
Juan Maldacena (IAS, Princeton)
Liam McAllister (Cornell University) review talk
Greg Moore (Rutgers University) review talk
Subir Sachdev (Harvard University) review talk
Nathan Seiberg (IAS, Princeton) review talk
Ashoke Sen (Harish-Chandra Research Institute, Allahabad)
Samson Shatashvili (Trinity College, Dublin and IHES)
Gerard `t Hooft (University of Utrecht)
Erik Verlinde (University of Amsterdam)
Edward Witten (IAS, Princeton)
Fabio Zwirner (University of Padua) review talk

SUMMARY TALK AT END
Jeff Harvey (University of Chicago) summary talk

PUBLIC LECTURES (inflation, landscape of multiverses...)
Brian Greene (Columbia University)
Andrei Linde (Stanford University)
Stephen Hawking (TBC)

 

[marcus]

Thanks to Barak for this item from Oxford University Press catalogue:
http://ukcatalogue.oup.com/product/academic/physics/9780199590759.do

A First Course in Loop Quantum Gravity
Rodolfo Gambini and Jorge Pullin
280 pages | 18 b/w line figures | 246x171mm
to be released: August 2011
TOC
1: Why quantize gravity
2: Special relativity and electromagnetism
3: Some elements of general relativity
4: Hamiltonian mechanics including constraints and fields
5: Yang-Mills theories
6: Quantum mechanics and elements of quantum field theories
7: General relativity in terms of Ashtekar's variables
8: Loop representation for general relativity
9: An application: loop quantum cosmology
10: Further developments
11: Open issues and controversies

Publisher's decription:
"First book on the subject at the undergraduate level
Quick introduction to many major topics in fundamental theoretical physics suitable for undergraduate courses
High interest currently in quantum gravity among physicists worldwide
Controversial subject which needs good, impartial introduction from respected scientists
This book provides an accessible introduction to loop quantum gravity and some of its applications, at a level suitable for undergraduate students and others with only a minimal knowledge of college level physics. In particular it is not assumed that the reader is familiar with general relativity and only minimally familiar with quantum mechanics and Hamiltonian mechanics. Most chapters end with problems that elaborate on the text, and aid learning. Applications such as loop quantum cosmology, black hole entropy and spin foams are briefly covered. The text is ideally suited for an undergraduate course in the senior year of a physics major. It can also be used to introduce undergraduates to general relativity and quantum field theory as part of a 'special topics' type of course.
Readership: Undergraduates and beginning graduate physics students, lecturers in physics."

This forthcoming undergrad Loop gravity text came up in Barak's thread here:
https://www.physicsforums.com/showthread.php?p=3226419#post3226419
https://www.physicsforums.com/showthread.php?p=3232530#post3232530

 

[marcus]

First undergraduate textbook could appear this year. Scheduled for September 2011. Oxford U Press:
http://ukcatalogue.oup.com/product/academic/physics/9780199590759.do

EDIT: Atyy points out that Bojowald has already published a textbook in LOOP COSMOLOGY


and also one in the CANONICAL APPROACH TO GENERAL RELATIVITY.

Neither is explicitly an undergrad text in LQG, but the former treats the application of LQG to cosmo which is pretty close.
And the latter although mainly it is not about QG it has a chapter at the end, I recall.

======================
The question is: should we call the Oxford Press book the "First undergrad LQG text"?

I think yes, because for one thing that is what OUP calls it:
==quote==
Publisher's decription:

• First book on the subject at the undergraduate level
• Quick introduction to many major topics in fundamental theoretical physics suitable for undergraduate courses
• High interest currently in quantum gravity among physicists worldwide

==endquote==

And for another thing Bojo book about Hamiltonian formulation of GR is not directly aimed at LQG. It does not start out talking QG. It puts it in as a last chapter.

The Gambini Pullin is explicitly aimed. It starts out with chapter 1 "Why quantized gravity?"

And it is TITLED as a textbook for LQG, not just as a GR text with apps.

 

[atyy]

First undergraduate textbook could appear this year. Scheduled for September 2011. Oxford U Press:
http://ukcatalogue.oup.com/product/academic/physics/9780199590759.do

Not Bojowald's, or is that not undergarduate? https://www.physicsforums.com/showthread.php?t=432381

 

[marcus]

First undergraduate textbook could appear this year. Scheduled for September 2011. Oxford U Press:
http://ukcatalogue.oup.com/product/academic/physics/9780199590759.do

Not Bojowald's, or is that not undergarduate? https://www.physicsforums.com/showthread.php?t=432381

Atyy, good question! I edited so as to respond in the appropriate place, right after the OUP publisher's blurb. I think in fact that the Gambini IS the first explicitly LQG undergrad text!
The Bojo text is on Canonical GR, with several applications. It could be used as a text but doesn't have the clear focus on LQG.
Oxford press says Gambini is the first LQG text for undergrads. And they are right. If you look at the title and TOC of the Bojo books you will see what I mean.

New page, so I will make the update post here for easy reference:
It hasn't been done for over a month so it's probably time to redo the regularly updated links post to give an introductory overview of the Loop program.
==================================

BRIEF OVERVIEW OF THE LOOP QG PROGRAM

Loop Gravity is a small growing field. It has had regular biannual conferences only since 2005 (Loops '05 was held at Potsdam.) Attendance at the biannual (Loops '11 Madrid) is now up to about 170.

Because centers of LQG research are spreading rather rapidly, jobs and postdoc positions seem currently to be ample compared to the number of new PhDs. It's hard to predict the future, but at least for now job prospects seem pretty good. Most new PhDs are out of Marseille,Penn State, AEI-Potsdam, and they all get placed. There are starting to be more PhDs now out of other places: Nottingham, Erlangen, Lyon, Cambridge, London...hard to give an exhaustive list.

First undergraduate textbook could appear this year. Scheduled for September 2011. Oxford U Press:
http://ukcatalogue.oup.com/product/academic/physics/9780199590759.do

Newly established Masters program aimed at Loop and other QG research (among other paths) starting at Nottingham in 2011:
http://pgstudy.nottingham.ac.uk/postgraduate-courses/gravity-particles-and-fields-masters-msc_1163.aspx

Survey for non-specialists, historical development and indications the theory recovers classical GR.
http://arxiv.org/abs/1012.4707
Loop quantum gravity: the first twenty five years

Definitive presentation of Loop gravity, with some math prerequisites:
http://arxiv.org/abs/1102.3660
Lectures on Loop Gravity

Loop cosmology, as of mid-2010:
http://arxiv.org/abs/1005.5491.
The Big Bang and the Quantum

Since Loop cosmology derives from LQG, this turns out to be the most accessible route to testing LQG. Here are 32 or more articles mostly by early universe phenomenologists about ways of testing LQC against CMB data,
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+PRIMORDIAL%2C+FLUCTUATION+OR+DK+INFLATION+OR+DK+COSMIC+BACKGROUND+RADIATION%29+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29
which have appeared since 2009.

Some other papers of possible interest:

http://arxiv.org/abs/1101.3264
Spinfoam Fermions: PCT Symmetry, Dirac Determinant, and Correlation Functions

http://arxiv.org/abs/1012.1739
Lorentz covariance of loop quantum gravity

The European Science Foundation "QG" network is encouraging development of a broad-base research community by events such as this year's QG 2011 conference in Zurich:
http://www.conferences.itp.phys.ethz.ch/doku.php?id=qg11:start

Topics to be covered include:
General quantum theory, relativistic quantum theory, emergence of space(-time)
General quantum field theory, including deformations of QFTs
QFT on curved and NC space-times
Canonical quantum gravity and supergravity
Regge calculus
String theory and M-theory
Loop gravity, spin foam
Quantum cosmology
===============================

The biannual Loops conference starts 23 May: Loops 2011 in Madrid.
http://www.iem.csic.es/loops11/
Click on "scientific program" in the menu on the left to see the list of talks. Abstracts of most talks are now available, just click on the title.

Many of the talks are about early universe pheno---the prospects for testing/constraining the theory.
Emerging challenges to the current formulation of Loop gravity are also represented at the conference and include "Shape dynamics" (see the session devoted to that on 23 May) and "Relative locality" (see talks by Laurent Freidel and Lee Smolin).

One of the signs of growth is the number of "parallel session" talks scheduled for Loops 2011, last time I checked there were 99 such. Younger researchers and newcomers to the field normally give talks in these parallel sessions.
=========================

Other signs of growth in the field:

• the number of institutions now hosting Loop gravity (+ related phenomenology) research, and where one can do a Loop PhD
• citations in the relevant categories (gr-qc, quantum cosmology, e.g. a growing share of "top 10 or top 30" papers)
• simply the increased number of research papers coming out. Approximately doubled since 2006.

Loop gravity papers, by year 
2005    41
2006    81
2007   121
2008   142
2009   141
2010   154

Loop cosmology papers, by year 
2006    21
2007    39
2008    46
2009    45
2010    57

The current Loop cosmo formulation (Ashtekar et al) only appeared in 2006 so I start with that year.
Here are the relevant Spires searches, and links, if anyone wishes to see how the listing looks.

FIND DK SPIN,FOAM OR QUANTUM GRAVITY, LOOP SPACE OR QUANTUM COSMOLOGY, LOOP SPACE AND DATE = 2005
FIND DK QUANTUM COSMOLOGY, LOOP SPACE AND DATE = 2006
2005:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+DK+SPIN%2CFOAM+OR+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+QUANTUM+COSMOLOGY%2C+LOOP+SPACE+AND+DATE+%3D+2005+&FORMAT=WWW&SEQUENCE=citecount%28d%29
2006:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+DK+SPIN%2CFOAM+OR+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+QUANTUM+COSMOLOGY%2C+LOOP+SPACE+AND+DATE+%3D+2006+&FORMAT=WWW&SEQUENCE=citecount%28d%29
2007:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+DK+SPIN%2CFOAM+OR+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+QUANTUM+COSMOLOGY%2C+LOOP+SPACE+AND+DATE+%3D+2007+&FORMAT=WWW&SEQUENCE=citecount%28d%29
2008:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+DK+SPIN%2CFOAM+OR+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+QUANTUM+COSMOLOGY%2C+LOOP+SPACE+AND+DATE+%3D+2008+&FORMAT=WWW&SEQUENCE=citecount%28d%29
2009:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+DK+SPIN%2CFOAM+OR+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+QUANTUM+COSMOLOGY%2C+LOOP+SPACE+AND+DATE+%3D+2009+&FORMAT=WWW&SEQUENCE=citecount%28d%29
2010:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+DK+SPIN%2CFOAM+OR+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+QUANTUM+COSMOLOGY%2C+LOOP+SPACE+AND+DATE+%3D+2010+&FORMAT=WWW&SEQUENCE=citecount%28d%29

 

[marcus]

A new PF member asked "what exactly is LQG?" and requested a non-mathematical (effectively high school level, I think) answer. Tom Stoer supplied the following which then got some editorial attention from Ben Crowell and Redbelly (who suggested copying it here.) I think it's an unbiased informative account:

==quote tom.stoer post==

What exactly is LQG?

It's difficult w/o math.

The problem with quantum gravity is that naive mechanisms to quantize gravity (which have been applied successfully to other fields) fail for gravity. That means that something fundamental has to be changed for quantum gravity.

There are different approaches to solve these problems, e.g.
a) string theory
b) asymptotic safety
c) loop quantum gravity (LQG)

I don't want to comment on a) and b) here.

Essentially LQG does the following: it introduces new variables which replace the (in GR) well-known metric that describes spacetime + curvature. This is pure math, so I don't want to go into details here, but what happens is that these new variables are rather close to fields that we know from gauge theory like QED and QCD. Indeed in a certain sense gravity looks rather similar to QCD, but there is one additional property of gravity that allows one to apply a second mathematical trick which essentially replaces the fundamental fields with something like "fluxes through surfaces" or "fluxes along circles". These surfaces and circles are embedded into spacetime.

The next step is again rather technical and it becomes possible due to so-called diffeomorphism invariance: one can get rid of the the embedding of circles and surfaces into spacetime. Instead one replaces these entities with a so-called spin network, i.e. a graph with nodes and links between nodes where each link and each node carries some numbers which represent abstract entities from which certain properies of spacetime can be reconstructed. You can think about spacetime as made of cells (I will soon tell you that you can't :-); each cell has a certain volume carried by a node; each cell has certain surfaces and the link between different nodes (sitting inside these cells) carry the areas of the surfcaes.

The problem with this picture is that one might think about these cells as sitting in spacetime - but this is fundamentally wrong: this picture is only due to the construction, but basically there is no spacetime anymore; all there is are nodes and links (and certain numbers attributed to nodes and links). Spacetime is no longer fundamental but becomes an entity emerging from the more fundamental graphs with their nodes and links. The graphs are called spin networks b/c the numbers they are carrying have properties well-known from spins. But this is a mathematical property only, it does not means that there are real spinning objects.

Compare this emerging spacetime to a water surface of a lake. We know that it consists of atoms, and as soon as we get this picture it is clear that there is no water between the atoms; the surface is only an emerging phenomenon, the true fundamental objects are the atoms. In the same sense the spin networks are the fundametal entities from which spacetime, surfaces etc. and their properties like volume, area, curvature etc. can be constructed. Dynamics of spacetime (which was curvature, gravitational waves etc. in GR) is replaced by dynamics of spin networks: within a given graph new nodes with new links can appear (there are mathematical rules, but I don't want to go into detail here).

The last puzzle I have for you is the fact that such a spin network is not a mechanical object which "is" spacetime. Instead quantized spacetime is a superposition of (infinitly many) spin networks. This is well-known in quantum mechanics; there is no reason why an atom should be in a certain state; we can achieve that via preparation or measurement, but in principle a single atom can be in an arbitrary complex quantum state which is a superposition of "an atom sitting here, an atom moving in a certain direction over there, an atom moving in this or that direction, ...".

So classical spacetime is recovered by two averaging process: first there seems to be a regime were this superposition of spin networks is peaked around a single classical spacetime, i.e. where one network dominates the superposition of infinitely many spin networks; second from this single spin network one can reconstruct spacetime in the same sense as one can reconstruct the water surface from the individual atoms. But there may be different regimes (e.g. in black holes or closed to the big bang) where is classical picture and this averaging does no longer work. It may be that in these regimes all there is are spin networks w/o any classical property like smooth spacetime, areas, volume etc. It's like looking at a single atom: there is no water surface anymore.

Eventually this is why one started with this stuff: the classical picture of spacetime seems to become inconsistent when one tries to quantize it, i.e. when one defines these superpositins etc. These inconsistencies do not bother us as long as we talk about spacetime here, in the solar system etc. But they become a pain in the a... when we talk about spacetime near a singularity like a black hole or like the big bang. In order to understand these new non-classical regimes of spacetime a fundamentally new picture is required. This is what LQG (and other approaches) are aiming for: construct a new mathematical model from which well-known classical spacetime (like in GR) can be reconstrcuted, but which does not break down in certain regimes but remains well-defined and consistent.

He must so to speak throw away the ladder, after he has climbed up on it - Ludwig Wittgenstein
=====endquote=====

Original post:
https://www.physicsforums.com/showthread.php?p=3319185#post3319185

 

[marcus]

The Loops 2011 conference in Madrid last month had 165 participants. The program of talks is posted at this site:
http://www.iem.csic.es/loops11/
The QG11 conference in Zurich starts 14 June. 120 participants have registered so far.
http://www.conferences.itp.phys.ethz.ch/doku.php?id=qg11:programme

 

[marcus]

I'm trying out this Spires search to keep track of Loop early universe phenomenology papers (studying ways to test Loop cosmology.)
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+inflation+or+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+AND+DATE%3E2008&FORMAT=www&SEQUENCE=citecount%28d%29

It is a reworking of a search I was using earlier--what gets typed into the search terms box is:

FIND (DK LOOP SPACE AND (QUANTUM GRAVITY OR QUANTUM COSMOLOGY) ) AND (GRAVITATIONAL RADIATION OR PRIMORDIAL OR INFLATION OR POWER SPECTRUM OR COSMIC BACKGROUND RADIATION) AND DATE>2008

Just now when I tried it, the search came up with 42 items--papers that appeared 2009 or later.

The proposal of ways to test LQG---in particular the effects of the bounce at the start of expansion (which is a robust feature)---is an important development, I think. Another development is represented by this year's Quantum Theory and Gravitation conference ("QG11") which brings several QG communities together for comparison, sharing ideas, and discussion.
It is a mixed QG conference. We will see if the idea catches on. I have color-coded the list of talks to give an idea of the mix.

Causal Dynamical Triangulations(CDT)
Spectral Action (Connes Noncommutative Geometry)
LQG (incl. Group Field Theory)
Asymptotic Safety
Foundations-general considerations
String
======
Ambjorn: CDT, a quantum theory of geometry
Arnlind: Poisson Algebraic Geometry and Matrix Regularizations
Ashtekar: Quantum Cosmology and the Very Early Universe
Bachas: The problem of localization of gravity
Baez: Higher gauge theory, division algebras and superstrings
Barrett: State sum models and the spectral action
Beisert: Symmetries and Integrability for Scattering Amplitudes in N=4 Super Yang-Mills Theory
Blau: String Theory as a Theory of Quantum Gravity: a Status Report
Bodendorfer: Towards Loop Quantum Supergravity
Bossard: Toward the consistency of N=8 supergravity as a quantum field theory
Chamseddine: The Spectral Action
Compere: The translation anomaly of asymptotically flat spacetimes
Craps: Cosmological singularities in string theory
de Goursac: Renormalizability of noncommutative quantum field theories
Dixon: Ultraviolet behavior of quantum (super)gravity through four loops
Elvang: Symmetry constraints on the UV behavior of N=8 supergravity
Freidel: The principle of relative locality
Giulini: Very basic issues concerning quantum mechanics and gravitation
Hollands: Quantum field theory correlators on manifolds at very large and very short distances
Hoppe: Fundamental Structures of M-brane Theory
Jacobson: How general is the generalized second law?
Lechner: Covariant and local deformations of quantum field theories
Lewandowski: Canonical LQG: soluble models and other advances
Litim: Renormalisation group and the Planck scale
Loll: Nonperturbative highlights on quantum gravity from CDT
Longo: Boundary Quantum Field Theory and Conformal Field Theory
Morton: Extended Field Theories and Higher Gauge Theories
Mukhanov: Massive Gravity
Nicolai: Infinite-dimensional symmetries and the Wheeler-DeWitt equation
Oriti: Group field theory: a brief survey of recent developments
Reiterer: A class of gauges for the Einstein equations
Reuter: Einstein-Cartan Theory and Asymptotic Safety
Rovelli: Loop quantum gravity: the covariant dynamics
Shaposhnikov: Scale-invariant alternatives to general relativity
Speziale: Spin networks as twisted geometries
Steinacker: Matrix models, noncommutative gauge theory and emergent geometry
Wulkenhaar: Ward identities in matrix models arising from noncommutative geometry

I I'm not sure what Nicolai's talk is going to be about but just as a guess classify it "foundations-general considerations"--a potentially important topic suggested on the front page by the organizers.
http://www.conferences.itp.phys.ethz.ch/doku.php?id=qg11:programme

 

[marcus]

Currently the best review of LQG is this video talk:
http://loops11.iem.csic.es/loops11/...n-problems&catid=35:plenary-lectures&Itemid=1

The talk was given 25 May 2011 in Madrid, at the biannual Loops conference.

For a printed journal-style review try http://arxiv.org/abs/1012.4707 which was posted in December 2010.

There is also the LQG tutorial http://arxiv.org/abs/1102.3660

 

[Peter McKenna]

Marcus,

About Causal Dynamic Triangulation, there still hasn't been much published out in layman's land. In the above discussion of LQG those terms almost describe an engineering model using an element system (something like a hydraulic model using links and nodes). Does this suggest that LQG acts like a secondary field much like eddy currents within an electromagnetic field (solved using a matrice)?

When looking at spin states 2D respresentation seem like "Fractals" as you described previously. How does CDT model or resolve loop gravity?

 

[marcus]

Marcus,

About Causal Dynamic Triangulation, there still hasn't been much published out in layman's land... How does CDT model or resolve loop gravity?

Have you seen the SciAm article on CDT? It's well-written as an introduction for wide audience, and it has links to further reading, some of which is pretty accessible. I keep the URL to the SciAm article in my signature (check the end of this post). It is the "signal lake" link.
http://www.signallake.com/innovation/SelfOrganizingQuantumJul08.pdf

In the above discussion of LQG those terms almost describe an engineering model using an element system (something like a hydraulic model using links and nodes). Does this suggest that LQG acts like a secondary field much like eddy currents within an electromagnetic field (solved using a matrice)?

When looking at spin states 2D respresentation seem like "Fractals" as you described previously...

I'm not sure I can respond adequately. Maybe one of us will start a separate thread specifically to discuss these ideas. This thread does not have much discussion--it is mainly used to provide quick access to selected source material on Lqg.