Quantum Gravity

[Martin]

Franz Hinterleitner

Canonical DSR

http://arxiv.org/gr-qc/0409087

 

this paper addresses the "soccer ball problem" of Quantum Gravity.

it may be a break through.

in any case it is a radical attempt to resolve this problem which is

a major one.

so maybe I should start a thread on the soccer ball problem, and

focus on this paper.

[Alexa]

Hi Martin,

 

I was looking for a quantum physics thread, when I foung again this one.

Do you like to have something like for Astronomy links ?

 

Alexa

[Martin]

Hi Alexa,

It would be good to have a collection of links for General Physics (including Quantum Physics, Classical Mechanics, Relativity...).

 

I started one:

http://www.scienceforums.net/forums/showthread.php?t=6176

 

It can serve as does the Reference Shelf at a usual library.

 

I don't think it would be good to use this "Quantum Gravity" thread for that.

Quantum Gravity is a specialized branch of research that is very interesting just at the moment, and comparatively new. It is hard to make it visible if it is mixed in with the main bulk of physics which includes many older, more established areas. I would like to keep the QG thread focused on QG.

 

So I propose that we gather General Physics links and assemble them in that other thread. See what you think of the idea.

[Alexa]

It would be good to have a collection of links for General Physics (including Quantum Physics, Classical Mechanics, Relativity...).

Hi Martin,

 

I propose to have them by chapters, like you've mentioned them : quantum physics, classical mechanics, etc.

 

For this one I really need you help and all the others interested on the topic. Astronomy is a hobby for me, so it was easy to gather all the infos.

 

I can try to put them together, but I have no idea of the result.

 

Alexa

[Martin]

Hi Martin' date='

 

I propose to have them by chapters, like you've mentioned them : quantum physics, classical mechanics, etc.

 

For this one I really need you help and all the others interested on the topic. Astronomy is a hobby for me, so it was easy to gather all the infos.

 

I can try to put them together, but I have no idea of the result.

 

Alexa[/quote']

 

Let's continue this conversation at the other thread (General physics links thread). I think we must be patient and wait until a bunch of links accumulates. then you will see how to organize them (if you wish to do this, if not they may remain unorganized or someone else will).

 

I would say that the best possible outcome is if the other poster, like Severian (who is a working particle physicist) and Swanson and others, would contribute some favorite links.

And each SFN poster would put his or her favorite online physics resource.

 

Then we would know what is the particular mix of information that suits the taste and convenience of this group of people. And then you (or someone else with organizing talent) might put it in order.

 

but first we must see if some of the others want to add to the collection!

 

BTW I am very happy that you are interested (at least provisionally) in this project (it is only by the greatest self-control that I am refraining from putting a smilie here)

[Martin]

There seems to be periennial confusion about the status of Quantum Gravity research. People have the impression that there are two "camps", String and Loop, squabbling over some US Government research funds, and incidentally bad-mouthing each other.

 

Actually the LQG research effort is comparatively small (although rapidly growing) and it is supported mainly outside the United States. With the exceptions of Ashtekar's Institute at Penn State, most of the research is at places like Germany, Canada, France, India, the United Kingdom, Mexico, Poland, the Netherlands, even some also Spain, Italy, the Czech Republic, Paraguay.

 

LQG researchers seem to have little active interest in criticizing String theory. On the other hand there are some physicists outside Quantum Gravity (Sheldon Glashow, Peter Woit...) who say String research is over-funded in relation to the progress being made. String research outputs on the order of a thousand or two thousand papers a year. The point is that LQG researchers typically mind their own business and leave the controversy over String to others.

 

With LQG it is more like 100 papers per year. Here are arXiv preprint counts for Loop and allied approaches over the past 10 years.

If you click on these links you will be able to see by whom the papers are written and what they are about, and the scattering of countries the people are from.

 

---these results were from some time ago---

Year 1994:

http://lanl.arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/1994/0/1

Year 1995:

http://lanl.arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/1995/0/1

Year 1996:

http://lanl.arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/1996/0/1

Year 1997:

http://lanl.arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/1997/0/1

Year 1998:

http://lanl.arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/1998/0/1

Year 1999:

http://lanl.arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/1999/0/1

Year 2000:

http://lanl.arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/2000/0/1

Year 2001:

http://lanl.arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/2001/0/1

Year 2002:

http://lanl.arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/2002/0/1

Year 2003:

http://lanl.arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/2003/0/1

Last twelve months:

http://lanl.arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/past/0/1

1994    59    
1995    81    
1996    69
1997    68
1998    59
1999    73
2000    83
2001    92
2002   113
2003   139
LTM    169

-----these are more recent, from 27 November----

 

Year 1994:

http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/1994/0/1

Year 1995:

http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/1995/0/1

Year 1996:

http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/1996/0/1

Year 1997:

http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/1997/0/1

Year 1998:

http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/1998/0/1

Year 1999:

http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/1999/0/1

Year 2000:

http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/2000/0/1

Year 2001:

http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/2001/0/1

Year 2002:

http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/2002/0/1

Year 2003:

http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/2003/0/1

Last twelve months (e.g. 27 November 2003 to 27 November 2004):

http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/past/0/1

 

1994    61    
1995    83    
1996    72
1997    70
1998    67
1999    76
2000    89
2001    98
2002   121
2003   140
LTM    180

---------

[Martin]

More quantum gravity people

 

Stephon Alexander:

http://schwinger.harvard.edu/~motl/PeoplePixWeb/StephonAlexander.JPG

 

Abhay Ashtekar at Marseille

http://perimeterinstitute.ca/images/marseille/marseille121.JPG

Abhay Ashtekar at the blackboard

http://www.phys.psu.edu/people/display/?person_id=169

 

Jerzy Lewandowski in Vienna

http://mayu.physics.duq.edu/~simo/photo9.html

and at the Marseille conference

http://perimeterinstitute.ca/images/marseille/marseille075.JPG

 

Jorge Pullin playing the bagpipes

http://www.physicscentral.com/people/people-01-12.html

 

Rafael Sorkin at Perimeter Institute (visiting QG generalist)

http://www.perimeterinstitute.ca/people/researchers/'>http://www.perimeterinstitute.ca/people/researchers/view_bio.cfm?id=221'>http://www.perimeterinstitute.ca/people/researchers/'>http://www.perimeterinstitute.ca/people/researchers/view_bio.cfm?id=221

 

Leonardo Modesto when he was a string theorist

(before he switched to QG)

http://www.strings.to.infn.it/members/modesto.html

 

Laurent Freidel 1999

http://cosmos.nirvana.phys.psu.edu/online/Html/Seminars/Spring1999/Freidel/freidel.jpg

Laurent more recently, in the longterm researchers section here:

http://www.perimeterinstitute.ca/people/researchers/'>http://www.perimeterinstitute.ca/people/researchers/

 

Miscellaneous snaps of Olaf Dreyer, Florian Girelli (scroll down the page) et al.

http://www.perimeterinstitute.ca/about/facilities/current.cfm

 

Golam Mortuza Hossain and Ghanashyam Date just posted a string of papers in LQC (bounce, inflation, effective hamiltonian):

http://www.imsc.res.in/~golam/second_page.html

 

Hermann Nicolai is a string theorist at AEI Potsdam who has taken a supportive interest in LQG research and had an impact on it:

http://www.phy.bg.ac.yu/mphys2/pictures/Lecturers%20and%20Audience/Hermann%20Nicolai.jpg

his homepage at AEI with snapshot

http://www.aei.mpg.de/cgi-bin/interface/people.cgi?key=nicolai

 

Thomas Thiemann 2001

http://www.aei.mpg.de/cgi-bin/interface/people.cgi?key=thiemann

Could be that this is Thomas Thiemann at Marseille (can't rule the possibility out)

http://perimeterinstitute.ca/images/marseille/marseille044.JPG

 

 

Hanno Sahlmann

http://www.aei.mpg.de/cgi-bin/interface/people.cgi?key=sahlmann

this could be Hanno at Marseille (just a guess)

http://perimeterinstitute.ca/images/marseille/marseille115.JPG

 

Etera Livine (in white shirt facing camera)

http://perimeterinstitute.ca/images/marseille/marseille015.JPG

 

Florian Girelli can be found in the postdoc section of PI people-list

http://www.perimeterinstitute.ca/people/researchers

[Martin]

Some links about the Kodama state (oldest first)

Edward Witten's paper

A Note On The Chern-Simons And Kodama Wavefunctions

http://arxiv.org/abs/gr-qc/0306083

 

Stephon Alexander, Justin Malecki, Lee Smolin

Quantum Gravity and Inflation

http://arxiv.org/hep-th/0309045

 

Laurent Freidel, Lee Smolin

The linearization of the Kodama state

http://arxiv.org/hep-th/0310224

 

Ichiro Oda

A Relation Between Topological Quantum Field Theory and the Kodama State

http://arxiv.org/hep-th/0311149

 

Alejandro Corichi, Jeronimo Cortez

Note on Self-Duality and the Kodama State

http://arxiv.org/hep-th/0311089

 

Justin Malecki

Inflationary Quantum Cosmology: General Framework and Exact Bianchi I Solution

http://arxiv.org/gr-qc/0407114

 

Curious story. Edward Witten, in the first paper here, said the Chern-Simon state in Yang-Mill context was not usable for various reasons, and drew the sweeping conclusion that the Kodama state, analogous to it in gravity context, would turn out to be unphysical and indeed "unnormalizable". Coming from Witten, this was widely taken at face value.

 

However various people have been finding the Kodama state normalizable in various cases and contexts, and using modified Kodama states that appear to be well-behaved. Most particularly Justin Malecki recently. So it looks as if Witten's conclusion about the Kodama state's utility or lack thereof does not apply as broadly or in every case as was thought at first.

 

 

*

[Martin]

Stanford SLAC Library has a database with a lot of this years articles accessible to keyword search.

 

 

http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+k+quantum+gravity+and+date+2004&SKIP=0

 

this gets all the papers on Quantum Gravity that appeared this year that they have cataloged so far.

 

It is what you get when you type this into the search engine box:

FIND K QUANTUM GRAVITY AND DATE 2004

 

K means "keyword", they have other modes of search too.

 

the main URL is

http://www.slac.stanford.edu/spires/hep/

 

One thing they have that is neat is a "top 100" papers list for each year, that ranks papers according to number of citations received.

http://www.slac.stanford.edu/library/topcites/

 

You can also filter the search for highly cited papers

 

For example this will get any Quantum Gravity paper which appeared in 2003 and which has been cited by 50 or more other papers:

FIND K QUANTUM GRAVITY AND TOPCITE 50+ AND DATE 2003

http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+K+QUANTUM+GRAVITY+AND+TOPCITE+50%2B+AND+DATE+2003

[Martin]

Quantum Gravity by Carlo Rovelli is scheduled to appear in November. The publisher, Cambridge University Press, has a catalog page about the book with information about how to order it and a review by Alain Connes.

 

---quote from Alain Connes---

In spite of its sociological success, string theory is still far from a solution of the problem of quantum gravity, which should be considered as wide open. The book of Carlo Rovelli provides the basis, both at the technical and the conceptual level, for research in this fundamental problem of physics. The basic issues are clearly and deeply analyzed without any dogmatic stand and with great freedom of thoughts resulting in an unvaluable opportunity to learn and think for both mathematicians and physicists.

---end quote---

 

http://titles.cambridge.org/catalogue.asp?isbn=0521837332

[Martin]

http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/index.php

 

http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/participants.php

 

Workshop on Quantum Gravity in the Americas: Status and future directions

 

Perimeter Institute, Waterloo, Ontario, Canada

October 29 - 31, 2004

 

to keep track of similar events at Perimeter Institute in future:

http://www.perimeterinstitute.ca/activities/scientific/cws/index.php

[Martin]

I was looking at the AEI publications of the division Quantum Gravity and Unified Theories in the year 2004. In a way this gives a microcosm of what is going on in QG.

 

this is just one institution, or you could say a part of one, even.

there is the Max Planck Institute for Gravity Physics, at Potsdam on the outskirts of Berlin (one of dozens of specialized MPI research centers in Germany) and a part of MPI-Potsdam

is the Albert Einstein Institute or AEI.

 

AEI is, I guess, the premier center for QG in Germany

 

there is a constant exchange of people between AEI and other places like

Ashtekar's institute at Penn State

Smolin's (Perimeter Inst.) in Canada

Rovelli's institute at Marseille

Chennai and Pune research centers in India

Various universities (e.g. Mexico City) with strong QG programs.

 

AEI has several divisions, the one of interest here being Hermann Nicolai's division

"Quantum Gravity and Unified Theories"

 

this year (2004) the output of thw whole AEI was 89 papers so far of which 47 papers came from Nicolai's division. Nicolai is a string theorist and the presence of non-string QG people seems to be a fairly recent development.

However the non-string part seems to be growing and getting increasing attention. so one can look at the 47 papers from Nicolai's division and get an idea of the shifting balance and the direction of research.

 

I'll get a link to the list of the papers but one the way I see Hermann Nicolai's overview of the division's work, a kind of introduction or mission statement on it's homepage. I have underlined a sentence referring to the Loop approach to QG, for emphasis:

 

------quote----

Max-Planck-Institut für Gravitationsphysik, * Albert-Einstein-Institute

Quantum Gravity and Unified Theories Divison

Director: H Nicolai

 

This division is concerned with the unification of general relativity and quantum mechanics into a theory of quantum gravity.

 

Despite intense efforts over the last years it is far from clear at this time what a consistent theory of quantum gravity will look like and what its main features will be. In view of these uncertainties, the best strategy appears to be one which is both diversified and "interdisciplinary". For this reason, the division aims to represent the major current approaches to quantum gravity.

 

The canonical approaches to quantum gravity emphasize the geometrical aspects and appear well suited to deal with unsolved conceptual issues of quantum gravity, such as e.g. the "problem of time" or the interpretation of the "wave function of the universe". Important new insights have been gained over the last ten years in the framework of Ashtekar's new variables and the loop formulation of quantum gravity, which is one of the group's main activities. A further research direction of the division is the study of dimensionally reduced gravity and supergravity, which give rise to integrable models of classical and (canonical) quantum gravity.

 

The requirement of mathematical consistency (in particular, finiteness of perturbation theory) and the need to incorporate the non-gravitational interactions are likely to force us to go beyond Einstein's theory. This may not only lead to a "geometrization" of the other fundamental forces (as exemplified by Kaluza Klein theories and supergravity) but to an entirely new type of theory of which Einstein's theory is only a tiny part and which can explain how space-time is dissolved at very small distances. Superstring theory is the most promising ansatz for the unification of all fundamental interactions, but an even more refined theory may be required to understand its non-perturbative features and its underlying symmetries. These issues will be a main focus of the division's efforts.

----end quote---

[Martin]

Here's a list of the 2004 publications of Nicolai's division

of "QG and Unified Theories"

 

http://www.aei.mpg.de/cgi-bin/interface/papers.cgi?division=Quantum+Gravity+and+Unified+Theories&year=2004

 

here is the tail end of the list, as a sample, I have underlined papers using the loop approach to QG, to indicate the rough size of their role. Nicolai's own papers, and many of the others, are string-related (although other lines of research are represented as well).

 

AEI-2004-060

An effective field theory description for Kaluza Klein supergravity on AdS3 x S3

H. Nicolai, H. Samtleben

Proceedings of Deserfest:, University of Michigan, Ann Arbor, April 2004, (Eds.) M.J. Duff and J.T. Liu

 

AEI-2004-065

Time dependence in Quantum Gravity

M. Bojowald, P. Singh, A. Skirzewski

gr-qc/0408094

 

AEI-2004-067

A three-loop test of the dilation operator in N=4 SYM

B. Eden, C. Jarzack, E. Sokatchev

 

AEI-2004-068

On surface states and star-subalgebras in string field theory

E. Fuchs, M. Kroyter

 

AEI-2004-069

Higher-Loop Integrability in N=4 Gauge Theory

N. Beisert

Proceedings of Strings 2004, Paris, (Eds.) G. Laval, Comptes Rendues Physique, Elsevier

hep-th/0409147

 

AEI-2004-070

Spin Chain for Quantum Strings

N. Beisert

Proceedings of RTN and EXT workshop 2004, Kolymbari, Crete, Grece, (Eds.) Dieter Lüst, Fortschritte der Physik, Wiley-VCH

hep-th/0409054

 

AEI-2004-072

Black hole evaporation: A paradigm

A. Ashtekar, M. Bojowald

 

AEI-2004-073

Free vacuum for loop quantum gravity

F. Conrady

gr-qc/0409036

 

AEI-2004-074

Less is More: Non-renormalization Theorems from Lower Dimensional Superspace

Z. Guralnik, S. Kovacs, K. Kulik

hep-th/0409091

 

AEI-2004-075

Many faces of D-branes: From flat space, via AdS to pp-waves

M. Zamaklar

 

AEI-2004-077

Stabilization of moduli by fluxes

K. Behrndt

 

AEI-2004-078

Higher spin fields from indefinite Kac-Moody algebras

S. de Buyl, A. Kleinschmidt

Proceedings of First Solvay Workshop on Higher Spin Gauge Theories, (Eds.) , Solvay Institute website, 2004, 18

[ed84c]

NO NO NO question should be how far are we from a relavistic theory of quantum mechanics

 

Me and Roger Penrose agree that we should look to change QM not General relativity

[Martin]

NO NO NO question should be how far are we from a relavistic theory of quantum mechanics

 

Me and Roger Penrose agree that we should look to change QM not General relativity

 

it is a point of view that is emerging stronger these days, or so I think.

 

Your post prompted me to google the phrase

 

"general relativistic quantum physics"

 

try it ed84c

 

and then try it again in a couple of months

[Martin]

I'm trying out the attachment feature

seeing if I can upload a snapshot of a QG researcher

this is M. Bojowald, the student of Abhay Ashtekar who

got rid of the big bang singularity. He is now at the

Albert Einstein Institute in Golm, outside Berlin.

[Martin]

another shot of M. Bojowald

from the May 2004 conference at Marseille

 

http://perimeterinstitute.ca/images/marseille/marseille017.JPG

 

M.B. is the guy in the gray T-shirt and black frames, not looking at the camera. The guy with him is Alejandro Perez

 

M.B gave an impressive talk on quantum cosmology at Penn State last year, the audio and slides are available. I will get the link.

 

To get recorded seminar talks at Penn State, Ashtekar's Gravity Physics Institute, you go here:

http://phys.psu.edu/events/

 

then if you select "Spring 2003" you get a list of the talks given that semester. somewhere on that list is

Bojowald:Quantum Cosmology: An Overview

and clicking on that gets you the slides and the audio.

download the audio first and then when you play the audio follow along with the slides.

there is also another Bojowald talk, Quantum Cosmology: Formalism

 

 

Bill Unruh is listed on page 2 of the affiliate members at PI, here:

https://perimeterinstitute.ca/people/researchers/

select "affiliate members" (his homebase is UBC Vancouver). go to page 2 and click on the name---there's a snapshot

 

he has published quantum gravity papers as far back as 30 years ago.

discovered Unruh radiation and Unruh temperature that something experiences just by accelerating----analogous to Hawking temperature and radiation at horizon of black hole---about the same time as Hawking discovered his.

Unruh also published a Loop QG paper this year.

Here are a couple of other papers

Time and the Interpretation of Canonical Quantum Gravity, Phys. Rev. D40, 2598--2614 (1989),

W.G. Unruh and R. Wald. Notes on Black Hole Evaporation, Phys. Rev. D14, 870 (1976), W.G. Unruh.

I notice that this year the topics of Time in QG and of BH Evaporation are very hot topics (top people like Ashtekar and Bojowald are giving talks about new research efforts related to them) and I see Unruh is grinning in his picture and he was researching these things in 70s and 80s.

It is great that someone like this is writing a LQG paper in 2004

[Martin]

Rodolfo Gambini

http://cgpg.gravity.psu.edu/online/Html/Seminars/Fall1998/Gambini/

 

Rodolfo at the May 2004 Marseille conference, talking to Bianca Dittrich

http://perimeterinstitute.ca/images/marseille/marseille010.JPG

 

Ganashyam Date (giving a talk at a particle physics conference in Germany summer 2003, not such a good shot)

http://www.fz-juelich.de/ikp/theorie/hpc2003/photos/speaker/shyam.jpg

[Martin]

It is getting to be time to selectively update this thread.

Three good (probably the three best online) introductions to Loop Quantum Gravity are:

 

Ashtekar Gravity and the Quantum

http://arxiv.org/gr-qc/0410054

 

Smolin An Invitation to Loop Quantum Gravity

http://arxiv.org/hep-th/0408048

 

RovelliLoop Quantum Gravity

http://cgpg.gravity.psu.edu/people/Ashtekar/articles/rovelli03.pdf

 

Of these the Rovelli is the one most aimed at general audience.

 

Smolin's is more for physicists in other fields of research who are considering switching over into LQG. Smolin has an FAQ and a list of main theoretical results so far, and specific indications of where to expect experimental tests of the theory, and a list of open problems to be worked on. So it is full of useful stuff for researchers.

 

Ashtekar's is a survey which is up-to-date, non-technical (says things in words instead of a lot of equations), but does not popularize. He summarizes briefly and gives references to other articles which cover the details.

 

These are all fairly short articles. If you like the 5-page Rovelli article and want to delve deeper, there is still a free download early draft of his 360-page book Quantum Gravity available at his website. Just google "rovelli". This is mainly textbook for grad students getting into LQG, but it has a lot of perceptive non-technical insights in the first two chapters, and at the end. So the textbook is, so to speak, sandwiched between thoughts that anyone can read and find interesting. The final version is now for sale. You can order it from Cambridge U. Press, or from Amazon.

 

Loop Quantum Cosmology (LQC) is a separate topic. Because the universe appears to be approximately homogeneous at large scales and roughly the same in all directions, the equations can be simplified. LQC is more tractible than the full LQG theory so it has gotten results and moved ahead more rapidly. the leading expert is Martin Bojowald (pronounced boyovald).

here's a recent review article by him. It is a talk he gave at the annual ICGC conference this year.

Bojowald Loop Quantum Cosmology: Recent Progress

http://arxiv.org/gr-qc/0402053

 

this is just the beginning of an update. I want to sift thru and organize the links better in this thread. Here is some more Bojowald

this one was just posted today

http://arxiv.org/gr-qc/0411101

On Loop Quantum Gravity Phenomenology and the Issue of Lorentz Invariance

Martin Bojowald, Hugo A. Morales-Tecotl, Hanno Sahlmann

16 pages,

 

"A simple model is constructed which allows to compute modified dispersion relations with effects from loop quantum gravity. Different quantization choices can be realized and their effects on the order of corrections studied explicitly. A comparison with more involved semiclassical techniques shows that there is agreement even at a quantitative level..."

 

http://arxiv.org/abs/gr-qc/0408094

Time dependence in Quantum Gravity

Martin Bojowald, Parampreet Singh, Aureliano Skirzewski

33 pages, 17 figures

 

"The intuitive classical space-time picture breaks down in quantum gravity, which makes a comparison and the development of semiclassical techniques quite complicated. By a variation of the group averaging method to solve constraints one can nevertheless introduce a classical coordinate time into the quantum theory, and use it to investigate the way a semiclassical continuous description emerges from discrete quantum evolution. Applying this technique to test effective classical equations of loop cosmology and their implications for inflation and bounces, we show that the effective semiclassical theory is in good agreement with the quantum description even at short scales."

 

a Smolin link:

the Debate between Lee Smolin and string-theorist Lenny Susskind

http://www.edge.org/3rd_culture/smolin_susskind04/smolin_susskind.html

that took place this summer (2004) under auspices of the online magazine Edge

 

evolving the geometry of the universe by Monte computer simulations----AJL (Ambjorn, Jurkiewicz, Loll)

http://arxiv.org/abs/hep-th/0404156

Emergence of a 4D World from Causal Quantum Gravity]

and the follow-up paper

http://arxiv.org/abs/hep-th/0411152

Semiclassical Universe from First Principles

 

 

 

 

 

 

Ashtekar's recent seminar talk at Penn State:

Black Hole Evaporation and Information Loss: Recent Advances

http://www.phys.psu.edu/events/index.html?event_id=934;event_type_ids=7;span=2004-08-20.2004-12-25

 

Ashtekar's list of links to online popular Loop Gravity articles

http://cgpg.gravity.psu.edu/people/Ashtekar/articles.html

 

 

Miscellaneous others

 

Thomas Thiemann

Reduced Phase Space Quantization and Dirac Observables

http://arxiv.org/abs/gr-qc/0411031

 

Bianca Dittrich

Partial and Complete Observables for Hamiltonian Constrained Systems

http://arxiv.org/abs/gr-qc/0411013

 

http://arxiv.org/abs/gr-qc/0409057

Consistent discretization and loop quantum geometry

 

Ganashyam Date and Golam Hossain

Genericity of inflation in isotropic loop quantum cosmology

http://arxiv.org/gr-qc/0407069

 

Parampreet Singh has 3 seminar talks on LQG Phenomenology

two of which are online (Fall 2004 semester at phys.psu.edu):

Phenomenological Issues in Loop Quantum Cosmology I, II

http://phys.psu.edu/events/index.html?event_id=935&event_type_ids=0&span=2004-08-20.2004-12-25'>http://phys.psu.edu/events/index.html?event_id=935&event_type_ids=0&span=2004-08-20.2004-12-25

 

http://phys.psu.edu/events/index.html?event_id=936&event_type_ids=0&span=2004-08-20.2004-12-25'>http://phys.psu.edu/events/index.html?event_id=936&event_type_ids=0&span=2004-08-20.2004-12-25

 

Jerzy Lewandowski has a recent seminar talk on BH entropy in LQG,

clearest thing on that I have seen so far:

Black Hole Entropy

http://phys.psu.edu/events/index.html?event_id=938&event_type_ids=0&span=2004-08-20.2004-12-25'>http://phys.psu.edu/events/index.html?event_id=938&event_type_ids=0&span=2004-08-20.2004-12-25

 

The Penn State QG seminars:

 

http://phys.psu.edu/events/

choose a semester from the menu.

 

E.g. in spring 2003 semester there are

Bojowald's talks Quantum Cosmology: An Overview

 

http://phys.psu.edu/events/index.html?event_id=516;event_type_ids=0;span=2002-12-26.2003-05-31

Quantum Cosmology: Formalism

http://phys.psu.edu/events/index.html?event_id=521;event_type_ids=0;span=2002-12-26.2003-05-31

[Jordan14]

I would just like to say, keep up the good work

[Martin]

I would just like to say, keep up the good work

 

hi Jordan, thanks for the encouragement. It's useful for me to have QG links collected together, thats why I do it. but it is nice to know someone else benefits too!

 

IMO the two strongest contenders in the Quantum Gravity field are currently Loop Quantum Cosmology (LQC) and Causal Dynamical Triangulations (CDT). String is pretty much a no-show because too vague: it doesnt make predictions or have a definite model that quantizes the classical theory of gravity, general relativity, in any recognizable way.

An interesting recent paper about that was From Gravitons to Gravity: Myth and Reality by Thanu Padmanabhan. Thanu says just being able to extract a graviton from string does not manage to embody gravity or successfully quantize Gen Rel. The claim that string includes gravity is a myth in other words. In any case it hardly matters, since the theory is so indeterminate with its famous inability to select among 10¹⁰⁰ possible flat ground-states (the "landscape" or "discretuum" of vacuums)

that it looks like it will not predict anything for a long time, if ever.

 

So I am focusing on the race between LQC and CDT to make testable predictions---that can be checked by astronomical observation. Also both do computer modeling of the expansion of the universe and those results have interesting similarities with the classical picture

 

Besides the LQC papers of Bojowald and a fast-growing bunch of others, the especially interesting recent papers are CDT ones which I havent yet put down here, so here goes:

 

 

 

http://arxiv.org/abs/hep-th/0411152

Semiclassical Universe from First Principles

Authors: Jan Ambjorn, Jurek Jurkiewicz, Renate Loll

15 pages, 4 figures

 

Abstract:

"Causal Dynamical Triangulations in four dimensions provide a background-independent definition of the sum over space-time geometries in nonperturbative quantum gravity. We show that the macroscopic four-dimensional world which emerges in the Euclidean sector of this theory is a bounce which satisfies a semiclassical equation. After integrating out all degrees of freedom except for a global scale factor, we obtain the ground state wave function of the universe as a function of this scale factor."

 

 

Their previous paper was:

Emergence of a 4D World from Causal Quantum Gravity

http://arxiv.org/hep-th/0404156

 

 

 

AJL do computer runs, simulating the geometry of the universe.

In the latest paper, Figure 1 shows a random universe (in one of their Monte Carlo runs) expanding and contracting---it is a computer graphic of the size parameter evolving thru time.

 

 

Here's a photo of Renate Loll with Thomas Thiemann:

http://perimeterinstitute.ca/images/marseille/marseille028.JPG

 

Here she is out for a walk with Julian Barbour and Don Marolf:

http://perimeterinstitute.ca/images/marseille/marseille103.JPG

 

I will get some more stuff to add here, about the CDT approach.

 

there are roughly 10 CDT papers, I havent actually counted. It is not a lot, but this approach has gotten considerable attention lately because of the results. here is a basic 2001 paper that the other two I mentioned refer back to:

 

http://arxiv.org/abs/hep-th/0105267

Dynamically Triangulating Lorentzian Quantum Gravity

Authors: Jan Ambjorn, Jurek Jurkiewicz, Renate Loll

[Martin]

CDT may turn out to be the most successful approach to QG that is currently available. here's why

quantum gravity is known to be non-renormalizable if constructed as a perturbation around fixed background geometry

 

the solution to this is probably to take GR seriously----GR does not like to be dependent on a fixed background----and approach quantum gravity in a non-perturbative that is to say background independent way.

 

To illustrate, here is a quote from a 2003 AJL paper:

 

----quote from page 2---

Quantum gravity in three space-time dimensions represents an interesting case in between dimensions two and four. On the one hand, it contains no propagating gravitational degrees of freedom and can be reduced classically to a finite-dimensional physical phase space, both in a metric [5] and a connection (Chern-Simons) formulation [6]. Nevertheless, the unreduced theory in terms of the metric g_mu,nu appears to be non-renormalizable when one tries to expand around a fixed background geometry, just as in four dimensions. A definition of three-dimensional quantum gravity via a “sum over geometries” therefore seems to require a genuinely non-perturbative construction, and in turn may shed light on the problem of non-renormalizability of the full, four-dimensional theory, where an explicit classical reduction is not available.

---endquote---

 

http://arxiv.org/hep-th/0307263

Renormalization of 3d quantum gravity from matrix models

J. Ambjorn (NBI, Copenhagen), J. Jurkiewicz (U. Krakow), R. Loll (Spinoza Inst. and U. Utrecht)

14 pages, 3 figures

Journal-ref: Phys.Lett. B581 (2004) 255-262

 

===============

in fact the next year they tackled 4D the same way and scored some remarkable successes---noted elsewhere. Here though is a basic point:

 

in quantum physics there is no such thing as a trajectory and therefore there can be no such thing as spacetime

 

to get from point A to point B, a particle cannot be said to follow a definite trajectory. there is no trajectory except in the imagination, only at best a finite number of measurements/interactions/observations in between.

 

likewise the gravitational field can evolve from shape A to shape B but one cannot say along which 4D cobordism. the 4D metric and the 4D manifold is the analog of the trajectory. a metric g can be given on an initial and a final submanifolds and be one possible way of getting from A to B, but you cannot say that it was the one which nature followed.

 

=========

arguably the most promising avenue to succeed in quantizing QG is to

abandon the perturbative approach, with its nonrenormalizable divergence, and take a background independent nonperturbative approach

 

and in addition (it now seems to me although this is a guess) use

a path integral

 

this means have a way of generating random spacetimes that get you from an initial to a final space-geometry and be prepared to average over those

==========

 

I am assuming that an analytical solution is not available so that montecarlo means are needed. Actually in 2D the theory has been solved analytically. In that case the montecarlo experiments agree with the analytical solution. but so far no analytical solution in 3D and 4D.

 

===========

 

the path integral approach has famous historical precedent. it is associated with Feynman, and also as applied to gravity, with Hawking.

it has just taken a long time.

the first real success in 4D has been this year.

 

in the previous post I gave the links to the two AJL papers

"Emergence" and "Semiclassical", here they are again

 

AJL (Ambjorn, Jurkiewicz, Loll)

http://arxiv.org/abs/hep-th/0404156

Emergence of a 4D World from Causal Quantum Gravity

 

http://arxiv.org/abs/hep-th/0411152

Semiclassical Universe from First Principles

[Martin]

http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v433/n7021/full/433012a_fs.html

 

the 6 january issue of Nature journal had a piece about

Bojowald, and Loop Quantum Cosmology

LQC is a derivative from LQG (loop quantum gravity)

and has been making more of a splash than the parent theory

 

simplifying assumptions are made in cosmology and so

things are more tractable, and Bojo his co-workers have been

getting results more rapidly

 

showing that there was no singularity at the Big Bang was a]

major success, and being able to describe conditions just briefly prior

to the former singularity

 

here is a listing of Bojo's papers

http://arxiv.org/find/gr-qc/1/au:+Bojowald_M/0/1/0/all/0/1

 

the one where he removed the bigbang singularity is back in 2001,

so you have to scroll back, there are currently about 42 papers in the list.

[Martin]

http://arxiv.org/gr-qc/0503020

every 6 months or year or so, he sums up the current status and recent advances in Loop Quantum Cosmology. his last survey was about one year ago.

 

(mu is the discrete internal time parameter that arises in quantum cosmology) sample exerpt from page 5:

 

" The evolution dictated by this difference equation in internal time mu does not stop at any finite value of mu. In particular, we can uniquely evolve initial values for the wave function through the classical singularity situated at mu = 0. Thus, there is no singularity where energy densities would diverge or the evolution would stop. This comes about as a consequence of the basic loop properties: the discreteness of spatial geometry leads to finite operators for the inverse volume as well as evolution in discrete internal time. Both properties enter in the demonstration of singularity free evolution. Physically, this means that around the classical singularity continuous space-time and with it the classical theory dissolve. Discrete quantum geometry, on the other hand, still makes sense and allows us to evolve to the other side of the classical singularity. "

[Johnny5]

Martin, i have a question about something on page 2 http://arxiv.org/gr-qc/0503020

 

 

2. Variables

 

A spatially isotropic space-time has the metric

[math] ds^2 = -dt^2 + \frac{a(t)^2}{(1 - kr^2)^2} dr^2 + a(t)^2r^2d\Omega^2 [/math]

 

where k can be zero or ±1 and specifies the intrinsic curvature of space, while the scale factor a(t) describes the expansion or contraction of space in time. It is subject to the Friedmann equation

 

[math] 3( (da/dt)^2 + k)a = 8 \pi G H_{matter} [/math]

 

where G is the gravitational constant and Hmatter the matter Hamiltonian (assumed here to be given only by a scalar φ and its momentum p. The matter Hamiltonian depends on the matter fields, but also on the scale factor since matter couples to geometry.

 

What does he mean when he says that "matter couples to geometry"?

 

Thanks