Introduction To Loop Quantum Gravity

Cinquero · Jun 20, 2005

marcus said:

YOU CAN GET FROM ONE SET OF WEIGHTS TO THE OTHER SET OF WEIGHTS by the simple expedient of changing the eye into a minus sign, or the minus into an eye. this is called the WICK ROTATION, in honor of Joe Wick born in Torino around 1906.

Is there a general theorem available for that purpose? I believe to remember that the mathematics behind that is usually quite non-obvious.

selfAdjoint · Jun 20, 2005

It's a procedure in complex variables, called analytic continuation.

Cinquero · Jun 20, 2005

selfAdjoint said:

It's a procedure in complex variables, called analytic continuation.

So Wick rotation = analytic continuation?

selfAdjoint · Jun 20, 2005

Cinquero said:

So Wick rotation = analytic continuation?

You have the integrals defined on the real axis, corresponding to Minkowski space, but they don't converge there, because they have factors like [tex]e^{ut}[/tex], which is unbounded as t goes to infinity. They are howeve analytic in the half plane above the real axis, and by continuation therefore on the imaginary axis, which correspond to [tex]i\tau = t[/tex], or euclidean four space. Then the integrals convege because the factors now read [tex]e^{ui\tau}[/tex] which is bounded for all tau. Then after you evaluate the integrals (they mostly reduce to a gaussian quadrature) you can rotate back.

CarlB · Jul 25, 2005

selfAdjoint said:

You have the integrals defined on the real axis, corresponding to Minkowski space, but they don't converge there, because they have factors like [tex]e^{ut}[/tex], which is unbounded as t goes to infinity. They are howeve analytic in the half plane above the real axis, and by continuation therefore on the imaginary axis, which correspond to [tex]i\tau = t[/tex], or euclidean four space. Then the integrals convege because the factors now read [tex]e^{ui\tau}[/tex] which is bounded for all tau. Then after you evaluate the integrals (they mostly reduce to a gaussian quadrature) you can rotate back.

You've got that exactly reversed. It's the imaginary exponentials that fail to converge and are converted by Wick rotation. Another way of saying the same thing is that a Wick rotation takes a QFT (which has a [tex]i\hbar[/tex] in the exponential) to a statistical mechanics (which has a [tex]-k/T[/tex] in the exponential). Your note is essentially saying that [tex]+k/T[/tex] diverges and this is true, but the solution in a Wick rotation is to rotate in the opposite direction. That way you end up with exponentials that converge. This all reminds me of the method of "steepest descent"[sp] that is used in Schroedinger's equation.

But that's not why I was reading the thread.

Stephen Hawking's latest paper uses "Euclidean Quantum Gravity":
http://arxiv.org/PS_cache/hep-th/pdf/0507/0507171.pdf

Does EQG have anything to do with LQG? My field is elementary particles, not gravitation. Sorry for the laziness. Hawking references a book I don't have immediate access to.

Carl

marcus · Jul 25, 2005

CarlB said:

...
Stephen Hawking's latest paper uses "Euclidean Quantum Gravity":
http://arxiv.org/PS_cache/hep-th/pdf/0507/0507171.pdf

Does EQG have anything to do with LQG? My field is elementary particles, not gravitation. Sorry for the laziness. Hawking references a book I don't have immediate access to.

Carl

"Euclidean QG" developed by hawking and friends in 1980s was a path integral AFAIK
and so it would be closer akin to Renate Loll Lorentzian path integral by CDT method ("causal dynamical triangulations") that we hear a lot about these days

Hawking never got Euclidean path integral to work, but he uses it to think with. It sounds a bit eccentric for him to call it the "only sane way to do nonperturbative QG"
the Lorentzian path integral people (Loll et al) have an equally nonperturbative approach that they are getting results with, including confirming a conjecture or two of hawking. No way is Loll's approach not sane. It is at least as sane as the Euclidean version.

I need to get you some online links. there is a 1998 survey of QG methods by rovelli which describes hawking Euclid. path integral. More recent online stuff do not discuss hawking's method very much because it is long obsolete except for him and one or two proteges. But I will get the link to the 1998 survey

Yes, here:
http://arxiv.org/abs/gr-qc/9803024
Strings, loops and others: a critical survey of the present approaches to quantum gravity
Carlo Rovelli
Plenary lecture on quantum gravity at the GR15 conference, Pune, India

"I review the present theoretical attempts to understand the quantum properties of spacetime. In particular, I illustrate the main achievements and the main difficulties in: string theory, loop quantum gravity, discrete quantum gravity (Regge calculus, dynamical triangulations and simplicial models), Euclidean quantum gravity, perturbative quantum gravity, quantum field theory on curved spacetime, noncommutative geometry, null surfaces, topological quantum field theories and spin foam models. I also briefly review several recent advances in understanding black hole entropy and attempt a critical discussion of our present understanding of quantum spacetime."

marcus · Jul 25, 2005

http://arxiv.org/abs/gr-qc/9803024
Strings, loops and others: a critical survey of the present approaches to quantum gravity
Carlo Rovelli

Section B. "Old hopes (becoming) approximate theories"

---quote Rovelli---
B. Old hopes -> approximate theories

1. Euclidean quantum gravity

Euclidean quantum gravity is the approach based on a formal sum over Euclidean geometries [[my comment: HERE ROVELLI GIVES THE PATH INTEGRAL, BUT I CAN'T COPY IT EASILY, it is labelled equation (6)]] As far as I understand, Hawking and his close collaborators do not anymore view this approach as an attempt to directly define a fundamental theory. The integral is badly ill defined, and does not lead to any known viable perturbation expansion. However, the main ideas of this approach are still alive in several ways. First, Hawking’s picture of quantum gravity as a sum over spacetimes continues to provide a powerful intuitive reference point for most of the research related to quantum gravity. Indeed, many approaches can be sees as attempts to replace the ill defined and non-renormalizable formal integral (6) with a well defined expression. The dynamical triangulation approach (Section IVA) and the spin foam approach (Section VC2) are examples of attempts to realize Hawking’s intuition. Influence of Euclidean quantum gravity can also be found in the Atiyah axioms for TQFT (Section VC1). Second, this approach can be used as an approximate Second, this approach can be used as an approximate method for describing certain regimes of nonperturbative quantum spacetime physics, even if the fundamental dynamics is given by a more complete theory. In this spirit, Hawking and collaborators have continued the investigation of phenomena such as, for instance, pair creation of black holes in a background de Sitter spacetime. Hawking and Bousso, for example, have recently studied the evaporation and “anti-evaporation” of Schwarzschild-de Sitter black holes [61]...
---end quote---

Equation (6) here looks very much like Loll's dynamical triangulations path integral. but they start with exp(iS) where S is the Regge form of Einst action.
Loll et al do a Wick rotation to get a euclidean version which gets used in the computer calculations.
This equation (6) is still very much like what Loll CDT starts with, but instead of a metric [g] there is a TRIANGULATION T. so they are summing over all triangulations of a particular kind. Otherwise it looks formally the same.

However there is a practical difference in that Loll et al can actually calculate. they do the sum (using montecarlo method) and get results.
some of these results have born out hawking conjectures, so they cite him a lot.

but his particular type of (euclidean) path integral i don't think any significant effort is being made to use it.

to compare hawking EQG with current CDT path integral, have a look at the first 2 or 3 pages of these two papers
http://arxiv.org/hep-th/0105267
http://arxiv.org/hep-th/0505154

you will see how close the CDT path integral is to Hawking's euclidean one.

marcus · Aug 4, 2005

in the past couple of pages of this thread we have been responding to questions from CarlB and cinquero and it may be time to regroup. I decided earlier that unless there is some reason not to do so we ought to make this thread serve as an introduction NOT ONLY to narrowly defined canonical LQG but to the main approaches to NONPERTURBATIVE QUANTUM GRAVITY.

That includes canonical LQG but also spin foams, and other path integral approaches like CDT. selfAdjoint, at one point, proposed the term "Background Independent Quantum Gravity" for the general field. Renate Loll seems to favor "Nonperturbative QG". The organizers of the Loop 05 conference use the collective modifier
"background independent/nonperturbative"
And Lee Smolin has started to say "relational".

But I think "nonperturbative" is going to win out as the mainest of mainstream term. As sideline observers we can't reform language, just have to go with the prevailing talk.

I think one of the ambient ideas in the Loop 05 conference is that if you can forge a concept "NQG" and impress on people's minds the idea that there is research in "nonperturbative quantum gravity" then maybe a few more universities will establish professorships in NQG or faculty positions of some kind in NQG. It will be perceived as a lack not to have some research in nonperturbative QG being conducted in the physics department.

It also means recommending each other's graduate students. if it is a field then there is more solidarity than if it is just a bunch of splinter group research lines.

Hermann Nicolai definitely would like some professorships in German universities that are echo or counterpart to his lines of reseach at AEI, he has talked about that in Die Zeit interview. And AEI is hosting Loop 05.

so it is time to assemble into a research field with an identifying label which is not String, and to get it recognized that a physics department has an embarrassing GAP if it doesn't have some research under way in Nonper Quavity.

marcus · Aug 4, 2005

Let's recap the introduction to the triangulations approach---Loll CDT.
Here is a reading list from earlier in this thread
https://www.physicsforums.com/showthread.php?p=585294#post585294

Here's a short popularization by Loll, at her website, written for general audience
http://www.phys.uu.nl/~loll/Web/research/research.html

This PF thread has more stuff like that
https://www.physicsforums.com/showthread.php?t=77639&page=1&pp=15

========================
To give an idea of where the field is at the moment, I am simply going to quote, in its entirety, the first paragraph of each of Loll's three most recent papers. These papers are dated May, June, July 2005. The first paragraph of a research paper often gives a bit of an overview or some perspective on the field. This is a fastmoving field and this will be one way to keep up with where things are at the moment. We have no more recent survey available.

http://arxiv.org/hep-th/0505154
Reconstructing the Universe
http://arxiv.org/gr-qc/0506035
Counting a black hole
http://arxiv.org/hep-th/0507012
Taming the cosmological constant...topology change

Very encouraging progress has been made recently in constructing spacetime dynamically from a nonperturbative gravitational path integral, by studying the continuum limit of causal dynamical triangulations [1, 2, 3, 4]. The quantum geometries generated in this way exhibit semiclassical properties at sufficiently large scales: they are four-dimensional [5, 6] and the large-scale dynamics of their spatial volume is described by an effective cosmological minisuperspace action [7]. Their short-distance behaviour is highly nonclassical, including a smooth dynamical reduction of the spectral dimension from four to two [8] and evidence of fractality [6].

Despite recent progress [1, 2], little is known about the ultimate configuration space of quantum gravity on which its nonperturbative dynamics takes place. This makes it difficult to decide which (auxiliary) configuration space to choose as starting point for a quantization. In the context of a path integral quantization of gravity, the relevant question is which class of geometries one should be integrating over in the first place. Setting aside the formidable difficulties in “doing the integral”, there is a subtle balance between including too many geometries – such that the integral will simply fail to exist (nonperturbatively) in any meaningful way, even after renormalization – and including too few geometries, with the danger of not capturing a physically relevant part of the configuration space.

Nonperturbative quantum gravity can be defined as the quest for uncovering the true dynamical degrees of freedom of spacetime geometry at the very shortest scales. Because of the enormous quantum fluctuations predicted by the uncertainty relations, geometry near the Planck scale will be extremely rugged and nonclassical. Although different approaches to quantizing gravity do not agree on the precise nature of these fundamental excitations, or on how they can be determined, most of the popular formulations agree that they are neither the smooth metrics... (or equivalent classical field variables) of general relativity nor straightforward quantum analogues thereof. In such scenarios, one expects the metric to re-emerge as an appropriate description of spacetime geometry only at larger scales.

I'll try to interpret some---as time permits. but hopefully this is already fairly clear and doesn't need much explication

marcus · Aug 16, 2005

I had better keep a list of links to the prediction polls that folks at PF have so that when the time comes to look we can easily find the thread with the predictions

Background independence talks at Strings 06
https://www.physicsforums.com/showthread.php?t=85207
(when the programme of talks is posted, check to see who was right)

August-September hits on Smolin latest
https://www.physicsforums.com/showthread.php?t=83578
(in late September 2005, start checking
http://citebase.eprints.org/cgi-bin/citations?id=oai:arXiv.org:hep-th/0507235
to see if they are counting and registering downloads of "The case for background independence")

String Forecast Poll
https://www.physicsforums.com/showthread.php?t=81739
(around March 2006 check SLAC/Stanford for the 2005 HEP Topcites. This year around March 2005 they brought out the 2004 Topcites as usual. But they have not yet done the full job with Michael Peskin's review, which is worrisome. the list to check is whatever is analogous to this
http://www.slac.stanford.edu/library/topcites/2004/annual.shtml )

Will Loll etc. achieve sum over topologies in 4D?
https://www.physicsforums.com/showthread.php?t=81626
(this prediction poll has no definite declared cut-off date, which was an oversight. we will have to use reasonableness and see whether, in a reasonable time, Loll et al manage to extend the results on topology change to higher dimensions)

marcus · Sep 12, 2005

A major chronological bibliography for LQG
Over a thousand papers (arranged by date) often with arxiv numbers making online access easy
Over forty books and PhD dissertations.
Plus miscellaneous other useful sources of information.

http://www.arxiv.org/abs/gr-qc/0509039
Bibliography of Publications related to Classical Self-dual variables and Loop Quantum Gravity

Alejandro Corichi, Alberto Hauser
45 pages
"This bibliography attempts to give a comprehensive overview of all the literature related to what is known as the Ashtekar-Sen connection and the Rovelli-Smolin loop variables, from which the program currently known as Loop Quantum Gravity emerged..."

Corichi gives some guidance as to his own judgement of what are good introductions, primers, surveys, mathematical treatments.

======================================
Dan Christensen's SpinFoam website at U Western Ontario
is another resource for people wanting to get acquainted with LQG and related QG

http://jdc.math.uwo.ca/spin-foams/

he has links to things sorted out by topic, and level and different users' needs and purposes, and he has some links to some Greg Egan JAVA applets. Seeing how he organizes things gives you a practical overview of QG from his perspective.

Dan says he has room for some more grad students and postdocs in his QG/computation program. It looks like anybody who might want to study QG (or massive parallel computation applied to QG) should probably check this out.

----------------------------
EDIT TO REPLY TO CINQUERO
Hi Cinquero, since i can still edit this I will reply this way and save making a new post. Please go to Dan Christensen site. He has many links in an organized convenient form. If there is anything that you need a further PDF link for, tell me what it is is and I will try to find it. I am not certain I understand your request for links to PDF----was it links to things found at Dan's UWO page or for something else?

Cinquero · Sep 13, 2005

Thx!

But could someone please add hyperlinks to the PDF output? :-)))

marcus · Sep 23, 2005

Hi Cinquero, I responded to your post #82 by editing the previous post. Hope you saw the note.
At the moment just need a place to stash the links to the audio of a two-part Bojowald talk given last Friday and concluded today at Penn State. He is talking about the LQG model Black Hole.

the audio of the first part is here
http://www.phys.psu.edu/events/index.html?event_id=1255;event_type_ids=0;span=2005-08-20.2005-12-25
Loop Quantum Cosmology of the Kantowski-Sachs Model
Gravity Theory Seminar by Martin Bojowald from Albert Einstein Institute (Germany)
Friday at 11:00 AM in 318 Osmond (9/16/2005)

and the second part (which was today) is here
http://www.phys.psu.edu/events/index.html?event_id=1256&event_type_ids=0&span=
Spherically Symmetric Quantum Geometry
Gravity Theory Seminar by Martin Bojowald from Albert Einstein Institute
Friday at 11:00 AM in 318 Osmond (9/23/2005)

at the same page there was also this audio
http://www.phys.psu.edu/events/index.html?event_id=1268;event_type_ids=0;span=
Generalizing Quantum Mechanics for Quantum Gravity
IGPG Seminar by James Hartle from University of California, Santa Barbara
Monday at 3:00 PM in 318 Osmond (9/19/2005)

and this audio as well
http://www.phys.psu.edu/events/index.html?event_id=1260;event_type_ids=0;span=2005-08-20.2005-12-25
Quantum Nature of the Big-Bang: Numerical Issues
Gravity Theory Seminar by Thomas Pawlowski & Parampreet Singh
Friday at 11:00 AM in 318 Osmond (9/9/2005)

Ashtekar has announced that he has a paper, written with Thomas Pawlowski & Parampreet Singh, to appear about this topic: LQG picture of the big bang.
Several of these seminar talks relate to the Ashtekar Bojowald collaboration about LQG of big bang and black hole, see for example their recent paper
http://www.arxiv.org/gr-qc/0509075
Quantum geometry and the Schwarzschild singularity

Cinquero · Sep 24, 2005

Actually, mys request for hyperlinks was in regard to:

"Bibliography of Publications related to Classical Self-dual variables and Loop Quantum Gravity"

:)

marcus · Sep 24, 2005

Cinquero said:

Actually, mys request for hyperlinks was in regard to:

"Bibliography of Publications related to Classical Self-dual variables and Loop Quantum Gravity"

:)

Ah!, I see what you mean. Corichi writes the URLs out for online sources, so one could paste them in and get to them, but in the PDF version these URLs do not automatically function as hyperlinks, as they might if he had provided an HTML version. I understand you may be joking, but it wouldn't be a bad idea for Corichi to make an up-to-date selective HTML bibliography of online quantum gravity sources.

If you want to encourage him to do this you could email him. Be sure to mention PF. He--or else a good friend of his--has often visited us, I believe, and supplied helpful information.

marcus · Oct 6, 2005

This is an update of post #56 which was about a book edited by Abhay Ashtekar scheduled to be published this year by World Scientific. here is the publisher's webpage

http://www.worldscibooks.com/physics/5876.html

A Hundred Years of Relativity.

Several chapters of this book are already online as preprints:

Martin Bojowald
http://arxiv.org/abs/gr-qc/0505057
Elements of Loop Quantum Cosmology

Larry Ford
http://arxiv.org/abs/gr-qc/0504096

Rodolfo Gambini and Jorge Pullin
http://arxiv.org/abs/gr-qc/0505023
Discrete space-time

Hermann Nicolai
http://www.arxiv.org/abs/gr-qc/0506031
Gravitational Billiards, Dualities and Hidden Symmetries

Thanu Padmanabhan
http://arxiv.org/abs/gr-qc/0503107
Understanding Our Universe: Current Status and Open Issues

Alan Rendall
http://arxiv.org/abs/gr-qc/0503112

Clifford Will
http://arxiv.org/abs/gr-qc/0504086
Was Einstein Right? Testing Relativity at the Centenary
========
other stuff:
Ashtekar there are several useful surveys, such as
http://arxiv.org/abs/gr-qc/0410054
Gravity and the Quantum
http://arxiv.org/abs/gr-qc/0404018
Background Independent Quantum Gravity: A Status Report

marcus · Nov 1, 2005

Agons, moments of truth.
for some reason I keep thinking back to the times Smolin spoke up at the Toronto string panel discussion, and each time immediately afterwards he was put down by you know who.

https://www.physicsforums.com/showthread.php?t=84585

http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/panel.htmland I remember Atiyah at Santa Barbara interrupted and almost derailed as he tried to get across his "old man's crazy thoughts".

https://www.physicsforums.com/showthread.php?t=96806

http://online.kitp.ucsb.edu/online/strings05/atiyah/

and as if to compensate there is Gerard 't Hooft's response after listening to some strange and quite possibly wrong ideas from Atiyah---That sounds like physics!

marcus · Nov 6, 2005

There may be hints of slow shift in research interest from string to non-string approaches to quantum gravity. The latter include Causal Dynamical Triangulations (CDT), spinfoams, Loop Quantum Gravity (LQG) and others on the Loops '05 conference programme.
this shift in research activity, if it exists, is hard to verify and measure statistically. here is one indicator---by itself not conclusive but something to watch along with the rest.
Last month at selfAdjoint's suggestion I included "heterotic, superstring" in the list of keywords and did a search using the Harvard ADS abstract service engine.
https://www.physicsforums.com/showthread.php?p=789185#post789185

I'll be glad to try. The main thing is just to have a fixed set of keywords you can apply year by year, to get the trend in papers with those keywords.
In accordance with your suggestion, I checked how many papers were published each year (October thru September) with the words "brane" or "M-theory" or "AdS/CFT" or "superstring" or "heterotic" in the abstract.
Code:
2001   1202
2002   1097
2003    970
2004    959

For continuity I tried the same check today. This is now the papers November thru October, year by year, with any of the same keywords in the abstract.

Code:

2001   1220
2002   1083
2003    972
2004    938

Part of this could certainly have nothing to do with a concurrent increase in QG research output in the non-string lines of investigation. It is very iffy and difficult to link the two trends! But at a level of anecdotal evidence one does encounter cases of people who have switched over.

Since the effort in non-string QG is still small compared with string, this shift (if it is occurring) could be viewed simply as diversification. One could take it NOT AS A SIGN THAT ONE THING IS RIGHT AND ANOTHER WRONG but that for whatever reason people are branching out in more directions, and trying non-string ones.

I will try to get some figures on non-perturbative QG research output trends.

marcus · Nov 8, 2005

non-string QG research seems to have increased during the same period.
For a rough indication of this I use the keyword search engine at arxiv.org, to find the number of preprints submitted each year with certain terms in the abstract. It gets some papers it shouldn't (that just happen to have the right keywords) and it misses some. Here the results:

Code:

In case anyone is interested here are links to these arxiv.org searches, and to some others just to have them handy.

Year 2001:
http://arXiv.org/find/grp_physics/1...m+AND+OR+triply+doubly+special/0/1/0/2001/0/1
Year 2002:
http://arXiv.org/find/grp_physics/1...m+AND+OR+triply+doubly+special/0/1/0/2002/0/1
Year 2003:
http://arXiv.org/find/grp_physics/1...m+AND+OR+triply+doubly+special/0/1/0/2003/0/1
Year 2004:
http://arXiv.org/find/grp_physics/1...m+AND+OR+triply+doubly+special/0/1/0/2004/0/1
Last twelve months:
http://arXiv.org/find/grp_physics/1...m+AND+OR+triply+doubly+special/0/1/0/past/0/1
Year to date, 2005:
http://arXiv.org/find/grp_physics/1...m+AND+OR+triply+doubly+special/0/1/0/2005/0/1

BTW here I have been looking only at the 2001-2004 period. Can we say anything about current trends? Well it may take a while for the 2005 numbers to stabilize and it may be too soon to say anything much. But I think nonperturbative QG research is experiencing LOLL-SHOCK and is in a temporary lull where people are considering re-directing their efforts more in line with Causal Dynamical Triangulations (because of recent seemingly important results).

John Baez has been very frank about this. Last month (October) he presented an overview of Spinfoam where he pointed out CDT results and asked could Spinfoam be modified (introducing an analogous causality structure?) to be more like CDT and could it get similar results, and then maybe it would surpass CDT because of inherent advantage in some other department.
this was the talk he gave at Loops '05 and posted at his website.

There is a lot of new stuff to digest right now, besides CDT there is Thiemann single constraint program, another approach which is not standard LQG and which look attractive to some LQG people, and there is Freidel's result that highlights DSR as a possibly necessary feature of QG.

For whatever reason there has been almost no QG appearing on arxiv.org since the 10-14 October conference. Loll-shock is my best guess but their could be other reasons.

marcus · Dec 4, 2005

Here is a great talk by Sundance Bilson-Thompson given 16 November 2005 at Perimeter Institute

http://streamer.perimeterinstitute....rType=WM64Lite&mode=Default&shouldResize=true

it is split screen, slides and video, he occasionally goes to the blackboard to explain stuff and the camera gets that too.
he is talking about his preon model (simple basis for a sketchy approximation of the standard model) which he and Lee Smolin are currently trying to connect with the spin networks of LQG.

Please let me know if this link does not work for you. It worked for me when I tried it.

marcus · Dec 13, 2005

Freidel wraps up 3D, starts on 4D

Loll aside, this is my guess as to the most influential quantum gravity paper of 2005:
http://arxiv.org/abs/hep-th/0512113
Effective 3d Quantum Gravity and Non-Commutative Quantum Field Theory
Laurent Freidel, Etera R. Livine
9 pages
"We show that the effective dynamics of matter fields coupled to 3d quantum gravity is described after integration over the gravitational degrees of freedom by a braided non-commutative quantum field theory symmetric under a kappa-deformation of the Poincaré group."

This paper (summarizing rapid development during the past year or so) pretty well takes care of the 3D case. In the conclusions Freidel discusses the extension to 4D, and cites a paper by Freidel and Baratin in preparation, called Hidden quantum gravity in Feymnan graphs.
but this was also the title of the talk which Aristide Baratin gave at October Loops '05. And we can download the slides. This has a link to the video as well, but i cannot get it to work:
http://loops05.aei.mpg.de/index_files/abstract_baratin.html
Here are Baratin slides for his talk "Hidden quantum gravity in Feymnan graphs".

http://loops05.aei.mpg.de/index_files/PDF_Files/baratin.ppt

Who is Aristide Baratin? Perimeter lists him as one of their grad students. I think Baratin finished Lycee in Paris around 1997 (won an essay prize) and went to Ecole Normal in Lyon (where Freidel works part-time). Perhaps he did his Bac at Lyon and after becoming a grad student moved from Lyon to Perimeter in 2005. I think probably he was born around 1980 and is now in mid Twenties. Anyway he is collaborating with Freidel on the paper where Freidel starts seriously to work on 4D.

There was already some preparation in that paper of Freidel with Artem Starodubtsev, which was also about 4D. I will get some quotes where Freidel gives an idea of what his strategy is.

marcus · Dec 14, 2005

this gives an idea of the success in 3D which Freidel intends to duplicate, if it is possible, in 4D.

--quote from Freidel paper hep-th/0512113--

CONCLUSIONS AND OUTLOOK

To sum up, we have shown that the 3d quantum gravity amplitudes, defined through the Ponzano-Regge spinfoam model, are actually the Feynman diagram evaluations of a (braided) non-commutative quantum field theory. This effective field theory describe the dynamics of the matter field after integration of the gravitational degrees of freedom. This applies as well in the Euclidean case as in the true Lorentzian theory. The noncommutative action is invariant under a kappa-deformation of the Poincaré algebra, which acts non-trivially on the many-particle states.

This is an explicit realization of a quantum field theory in the framework of deformed special relativity. Moreover, the theory naturally comes with a momentum cut-off which does not break the Poincaré symmetry but only deforms it.

This alternative to dimensional regularization was originally proposed by Snyder[15]. However, here we have explicitly shown that the gravitational field acts as such a regulator in the Euclidean sector. Moreover, we have checked that the quantum gravity amplitudes for the trivial topology reduces in the nogravity limit kappa -> 0 to the standard QFT amplitudes, as expected in the semi-classical limit of the theory.

We can see two natural extensions. The first one is that the spinfoam amplitudes provide a generalization of the Feynman rules for non-trivial topology. The second one is the generalization to the four-dimensional space-time. Indeed, we propose a new point of view. We have shown how to write the 3d Feynman evaluations as expectation values of certain observables of a topological (abelian) theory. This (abelian) theory was then identified as a particular limit of the quantum gravity theory. This result suggests that the Feynman evaluations of 4d QFT could be reformulated as expectation values of a 4d topological model. This is supported by the fact that 4d gravity becomes topological in the G ->0 limit [16]. This would be interpreted as the zeroth order of the spinfoam model for 4d quantum gravity [17]
---endquote---

REF [16] IS TO FREIDEL STARODUBTSEV
REF [17] IS TO FREIDEL BARATIN (Hidden quantum gravity in Feynman graphs).

marcus · Dec 14, 2005

Freidel's talk at Loops '05

Freidel talk at October conference was essentially this paper that he just posted, but with a differerent title

The paper is
http://arxiv.org/abs/hep-th/0512113
Effective 3d Quantum Gravity and Non-Commutative Quantum Field Theory

the talk was
http://loops05.aei.mpg.de/index_files/abstract_freidel.html
Effective Field theory from quantum gravity
"The Coupling of matter fields to spin foam models of quantum gravity will be discussed. We will show in the case of three dimensional gravity how the integration of quantum gravity degrees of freedom coupled to matter can be explicitely described in terms of an effective field theory. This theory is a new non commutative field theory obeying the principle of doubly special relativity. We will conclude on the extension of this approach to the four dimensional case."

the slides to the talk are not available, but there was a video. But I can't get it to download. too bad. I think AEI-Golm posted the video files only a limited time. I downloaded a number of them and have them on my hard-drive, but I find that I cannot download the same ones now. the link that USED to work, to get the freidel talk video was:
http://loops05.aei.mpg.de/index_files/Video_freidel.wmv

marcus · Jan 2, 2006

updating post #88

marcus said:
...
Last month at selfAdjoint's suggestion I included "heterotic, superstring" in the list of keywords and did a search using the Harvard ADS abstract service engine.
https://www.physicsforums.com/showthread.php?p=789185#post789185

For continuity I tried the same check today. This is now the papers November thru October, year by year, with any of the same keywords in the abstract.
Code:
2001   1220
2002   1083
2003    972
2004    938
... it NOT AS A SIGN THAT ONE THING IS RIGHT AND ANOTHER WRONG but that for whatever reason people are branching out in more directions, and trying non-string ones.
...

this time of year data becomes available and we can update. this is now for JANUARY THRU DECEMBER year by year----the same keywords
(heterotic, superstring, M-theory, brane, AdS/CFT)

Code:

2001   1204
2002   1188
2003   1081
2004   1012
2005    843

http://adsabs.harvard.edu/physics_service.html

these are figures on stuff that has gotten PUBLISHED in the indicated year. there is also the current rate of preprint postings---here is an arxiv link that may sometimes be useful
http://lanl.arxiv.org/find/nucl-ex,...brane+abs:+OR+M-theory+p-brane/0/1/0/2006/0/1

marcus · Jan 2, 2006

Early in 2005, Freidel and Livine posted
http://arxiv.org/abs/hep-th/0502106
Ponzano-Regge model revisited III: Feynman diagrams and Effective field theory
Laurent Freidel, Etera R. Livine
46 pages
"We study the no gravity limit G_{N}-> 0 of the Ponzano-Regge amplitudes with massive particles and show that we recover in this limit Feynman graph amplitudes (with Hadamard propagator) expressed as an abelian spin foam model. We show how the G_{N} expansion of the Ponzano-Regge amplitudes can be resummed. This leads to the conclusion that the dynamics of quantum particles coupled to quantum 3d gravity can be expressed in terms of an effective new non commutative field theory which respects the principles of doubly special relativity. We discuss the construction of Lorentzian spin foam models including Feynman propagators"

In October Freidel's Loops '05 talk was
http://loops05.aei.mpg.de/index_files/abstract_freidel.html
Effective Field theory from quantum gravity
"The Coupling of matter fields to spin foam models of quantum gravity will be discussed. We will show in the case of three dimensional gravity how the integration of quantum gravity degrees of freedom coupled to matter can be explicitely described in terms of an effective field theory. This theory is a new non commutative field theory obeying the principle of doubly special relativity. We will conclude on the extension of this approach to the four dimensional case."

In December 2005 a related paper appeared
http://arxiv.org/abs/hep-th/0512113
Effective 3d Quantum Gravity and Non-Commutative Quantum Field Theory
"We show that the effective dynamics of matter fields coupled to 3d quantum gravity is described after integration over the gravitational degrees of freedom by a braided non-commutative quantum field theory symmetric under a kappa-deformation of the Poincaré group."

Laurent Freidel's most recent paper was with Shahn Majid:
http://arxiv.org/abs/hep-th/0601004
Noncommutative Harmonic Analysis, Sampling Theory and the Duflo Map in 2+1 Quantum Gravity
54 pages, 2 figs
We show that the *-product for U(su_2) arising in [1] in an effective theory for the Ponzano-Regge quantum gravity model is compatible with the noncommutative bicovariant differential calculus previously proposed for 2+1 Euclidean quantum gravity using quantum group methods in [2]. We show that the effective action for this model essentially agrees with the noncommutative scalar field theory coming out of the noncommutative differential geometry. We show that the required Fourier transform essentially agrees with the previous quantum group Fourier transform. In combining these methods we develop practical tools for noncommutative harmonic analysis for the model including radial quantum delta-functions and Gaussians, the Duflo map and elements of `noncommutative sampling theory' applicable to the bounded SU_2,SO_3 momentum groups. This allows us to understand the bandwidth limitation in 2+1 quantum gravity arising from the bounded momentum. We also argue that the the anomalous extra `time' dimension seen in the noncommutative differential geometry should be viewed as the renormalisation group flow visible in the coarse graining in going from SU_2 to SO_3. Our methods also provide a generalised twist operator for the *-product."

[1] refers to the two 2005 Freidel and Livine papers
[2] is a 2003 paper of Majid and Batista

for quotes and more discussion of this series of papers please scroll back three or four posts to post #91 and #92
at the moment I am trying to connect the dots and understand a little better what is happening here. please help if you think you can explain. It looks like Freidel has found a connection at the 3D level between Regge gravity, Feynman diagrams for particles, and DSR (deformed Poincaré). the same laws that govern gravity also govern particles and produce Feynman vertex amplitudes in the flat limit. It looks like Freidel together with Shahn Majid and others are preparing to try to raise this setup to the 4D level. A fifth dimension has appeared and is being interpreted as a renormalization group parameter (instead of a second dimension of time). If you ask me it is real hard to comprehend, but maybe others will have an easier time with it.

marcus · Jan 3, 2006

If I am right in my guess about where QG is going, then this paper should serve as a kind of FILTER---

marcus said:

Early in 2005, Freidel and Livine posted
http://arxiv.org/abs/hep-th/0502106
Ponzano-Regge model revisited III: Feynman diagrams and Effective field theory
Laurent Freidel, Etera R. Livine
46 pages
"We study the no gravity limit G_{N}-> 0 of the Ponzano-Regge amplitudes with massive particles and show that we recover in this limit Feynman graph amplitudes (with Hadamard propagator) expressed as an abelian spin foam model. We show how the G_{N} expansion of the Ponzano-Regge amplitudes can be resummed. This leads to the conclusion that the dynamics of quantum particles coupled to quantum 3d gravity can be expressed in terms of an effective new non commutative field theory which respects the principles of doubly special relativity. We discuss the construction of Lorentzian spin foam models including Feynman propagators"

...

what I mean is that for the immediate future a good way to find interesting QG papers is to LOOK FOR THOSE WHICH HAVE CITED THIS ONE AS A REFERENCE.

if this is an important germinal paper then it makes our job easier, we can just look here
http://www.citebase.org/cgi-bin/citations?id=oai:arXiv.org:hep-th/0502106

and it says that this February 2005 paper has 11 citations (including some ones that are new to me---that I hadn't noticed when I was scanning arxiv) and these citations are
http://arxiv.org/cits/hep-th/0502106
(but this only gives the titles/authors and not the abstracts)
to see the same list with brief summaries:
http://www.citebase.org/cgi-bin/search?type=identifier&rank=Citations+(Paper)&submit=Cited+By&identifier=oai:arXiv.org:hep-th/0502106

Great! I have been trying to observe QG, and using these search tools, for several years and I am only just learning some laborsaving ways to use the tools.

the thing is: this February 2005 Freidel-Livine paper treats QG DYNAMICS AND MATTER ON THE SAME PLATFORM and unites Feynman diagrams (for the matter) with quantum Gen Rel dynamics for the spacetime

and this paper does it for 3D, so probably the future of QG will be doing similar stuff in 4D-----so this paper filters out future interesting QG papers

if a future paper cites this one, then it has a chance of being interesting.
so we have a new window----namely this citebase.org link.

BTW another link similar to the one a couple of posts back
http://lanl.arxiv.org/find/grp_phys...SCFT+abs:+OR+string+braneworld/0/1/0/2006/0/1

Sauron · Feb 8, 2006

Sorry by the slight off-topic.

I am currently reading the complete thread "intuitive content of loop..." that i had followed discontinously until now.

These one seems to be much of the same stuff ¿Why two duplicate threads?

¿Are topics covered here and uncovered in the other?

marcus · Feb 8, 2006

Sauron said:

Sorry by the slight off-topic.

I am currently reading the complete thread "intuitive content of loop..." that i had followed discontinously until now.

These one seems to be much of the same stuff ¿Why two duplicate threads?

¿Are topics covered here and uncovered in the other?

One difference is I try to make the older, larger thread more INCLUSIVE.
I suppose the other thread gets about 10 times more links per month, although I do not know the exact numbers.
I guess that in a month where I add 2 links to this more selective thread, I might add perhaps 20 links to the more inclusive one.

In the other thread, I try to put in any new QG paper that I think someone of us might be interested in. Even if I am not sure about it, I may put it in just so that it is less likely to be lost---and just in case someone else might be interested. So my own personal bias and interest does not influence it so much.

In this thread the links I put in reflect my own personal judgment of what is most essential and what are the most interesting directions in QG research. I am not always consistent. Last year I focused a lot of my attention on Loll Triangulations gravity (CDT). This year, so far, I am focusing mainly on the Freidel Spinfoam gravity (with its connection to matter and to DSR).

I would like to put in this thread only things that I think are of the utmost importance---and if a month or two pass by with nothing new that is OK, it just means that for me nothing especially important came out for a couple of months. But not being omniscient and clairvoyant, how can I tell what is really important? So in the other thread, to be on the safe side, I include stuff that I think has a chance to be of interest.

If you would like to contribute to our bibliography, Sauron, perhaps you would follow the same plan and preserve the "short list/long list" idea.

marcus · Feb 10, 2006

QG introductory course and QG blogs

(non-string) Quantum Gravity is emerging as a field and becoming more articulate. One of the major milestones marking this is Lee Smolin's course of lectures Introduction to Quantum Gravity
Notice that the title is not LOOP quantum gravity, but more general. Lecture #7 that I watched yesterday was BF theory----Lecture #8 touched on Chern-Simons. In response to a question Smolin also spent some of that day's session introducing the inclusion of matter in QG (by Laurent Freidel and others).

Besides this video course in QG, another signpost is Christine Dantas blog
http://christinedantas.blogspot.com
and her QG reading list.
http://christinedantas.blogspot.com/2006/02/basic-curriculum-for-quantum-gravity.html

As a kind of footnote, I will keep an eye out for other QG BLOGS, which in some sense indicate the growing self-awareness of the field.

One of the members of the Nottingham QG group has
http://realityconditions.blogspot.com
http://realityconditions.blogspot.com/2006/02/monday-quantum-gravity-group-meeting.html
Nottingham University has a major QG research group headed by John Barrett ("Barrett-Crane spinfoam model") and Kiril Krasnov (spinfoam and GFT parallels Freidel's)
http://www.maths.nottingham.ac.uk/QG/QG.html
and the blogger belongs to that group.

the "Reality Conditions" blogger Alejandro Katz just flagged Christine's QG reading list
http://realityconditions.blogspot.com/2006/02/study-guide-for-lqg.html

likewise Nigel's LQG blog
http://lqg.blogspot.com/2006/02/dr-dantas-has-lqg-reading-list.html
and also Victor Rivelles blog "SUM OVER HISTORIES", a mostly in Portuguese QG blog that I did not know of until today.
http://rivelles.blogspot.com/2006/02/road-to-loop-quantum-gravity.html

To get to the video of the Smolin Lectures go to
http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False
and scroll down to Introduction to Quantum Gravity around item #22 on the sidebar menu. It is currently one of the most recent additions to Perimeter online resources and so is near the bottom

marcus · Mar 4, 2006

as of now, the best short overview of LQG so far, in my opinion,
is Rovelli's Slides for a talk he gave in January 2006 at Lyon
it would be great if we had the audio that went with the slides
but the slides are fairly self-explanatory especially you have heard other Rovelli talkshttp://www.cpt.univ-mrs.fr/~rovelli/Lyon2006II.pdfthese 59 slides make an excellent summary of the essential ideas, with lots of sketches and diagrams (pointers to the history of how QG developed are given parenthetically by listing names and dates so you could look it up, but the emphasis is on giving intuitive grasp of the subject at present)

there are other resources at Rovelli homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
including a link to a downloadable draft of his book "Quantum Gravity" published by Cambridge University Press in 2004.

the SMOLIN LECTURES available as streaming video here
http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False
(scroll down to Introduction to Quantum Gravity on the sidebar menu)
now comprise 12 lectures-----the table of contents is on two pages so you have to flip the page to find lectures 11 and 12.

the Smolin Lectures are more technical and in-depth, so they complement Rovelli's easy conceptual overview. it helps to look at it from both perspectives

marcus · Apr 8, 2006

check back here later

so as not to lose track of the link, we have a forecast poll on the most influential first-quarter 2006 paper
https://www.physicsforums.com/showthread.php?t=116791

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

this is the thread that has updates of the TOC of the book by Daniele Oriti "Towards QG----a new understanding of space and time" or words to that effect.

last time i checked arXiv had preprints for 7 of the chapters, may be more now
https://www.physicsforums.com/showthread.php?t=113282

in particular see post #3 and 4
https://www.physicsforums.com/showthread.php?p=931982#post931982

marcus · Apr 30, 2006

marcus · May 22, 2006

there is always a danger of losing rare threads, after they are done, unless you keep a list

Mike2 started this one
Quantum foam producing both particles and spacetime
https://www.physicsforums.com/showthread.php?t=121527
it has John Baez telling Carlo Rovelli's Gedankenexperiment where a bolt of gravity wave comes and knocks down the Eiffel Tower

and also it has selfAdjoint's reply to a Zen koan posed by Carl Brannan.
Carl said "What is Energy made of?"
and sA said "Cobordisms"
In this case the answer is as bad as the question.

I forget why this one is good. It has some John Baez remarks but I forget what exactly
Non-string QG positions in US (some data)
https://www.physicsforums.com/showthread.php?t=121501
Oh yeah, I remember several interesting things---about FQX and Huck Finn and various such.
(apparently Baez was holed up with a bad cold in an apartment in Waterloo Ontario and had some time so he posted on this thread)

Baez and Perez beef strings and branes
https://www.physicsforums.com/showthread.php?t=120985
in the first week this got 34 replies and 774 views
must have generated some controversy. Kea was active IIRC and a bunch of others.
Kea has apparently been writing a PhD thesis and hasnt been posting much.
I guess everybody realizes without my needing to say it that the topic of
this thread is potentially kind of important. matter arising in a 4D model

Please help me build a list of references
https://www.physicsforums.com/showthread.php?t=120779
I confess to gross negligence here. We should have done a better job of responding to Kakarukeys.
Kakarukeys (Jiang-Fung Wong) is starting a QG group at Singapore U. He started this thread to get
links for his reading list. Francesca helped. What a serious person she is, never goofing off, just being helpful. Maybe she will become a professor of Quantum Gravity at an Italian university some day.

Nominations 2nd quarter M.I.P. prediction poll
https://www.physicsforums.com/showthread.php?t=117269

there are a lot more but you can't be complete or the list would be useless

marcus · Aug 4, 2006

In case anyone is interested, here are links to these arxiv.org searches.

Year 2001:
http://lanl.arXiv.org/find/grp_phys...m+AND+OR+triply+doubly+special/0/1/0/2001/0/1

Year 2002:
http://lanl.arXiv.org/find/grp_phys...m+AND+OR+triply+doubly+special/0/1/0/2002/0/1

Year 2003:
http://lanl.arXiv.org/find/grp_phys...m+AND+OR+triply+doubly+special/0/1/0/2003/0/1

Year 2004:
http://lanl.arXiv.org/find/grp_phys...m+AND+OR+triply+doubly+special/0/1/0/2004/0/1

Year 2005:
http://arXiv.org/find/grp_physics/1...m+AND+OR+triply+doubly+special/0/1/0/2005/0/1

Year 2006:
http://lanl.arXiv.org/find/grp_phys...m+AND+OR+triply+doubly+special/0/1/0/2006/0/1

Past 12 months:
http://lanl.arXiv.org/find/grp_phys...m+AND+OR+triply+doubly+special/0/1/0/past/0/1

arxiv stats link (thanks to arivero):
http://arxiv.org/Stats/hcamonthly.html

Harvard adsabs link (published articles only)
[e.g. kywd heterotic, superstring, M-theory, brane, AdS/CFT, jan-jun 2001]

http://adsabs.harvard.edu/cgi-bin/n...txt_wgt=YES&ttl_sco=YES&txt_sco=YES&version=1)

[e.g. kywd heterotic, superstring, M-theory, brane, AdS/CFT, jan-jun 2006]

http://adsabs.harvard.edu/cgi-bin/n...txt_wgt=YES&ttl_sco=YES&txt_sco=YES&version=1)

marcus · Aug 5, 2006

Wikipedia articles:
http://en.wikipedia.org/wiki/Background_independence

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

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

A recent contributor to Wikipedia active since early 2006 is
147.188.42.5
who, among others, seems to have worked hard and made a significant improvement----I see less evidence of hostile obfuscation than in prior versions.

Introduction To Loop Quantum Gravity

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