Historical gravity conference in progress---Loops '05

[Martin]

the field of quantum gravity has made a lot of advances recently

and they are being reported this week at the Loops 05 conference in Potsdam.

 

here's some blog news

 

http://www.math.columbia.edu/~woit/wordpress/?p=279

 

One of the leaders in quantum gravity, Lee Smolin, has reported from the conference, after the third day (it's evening already in Germany), in the comments. Scroll down to comment #5 here:

 

http://www.math.columbia.edu/~woit/wordpress/?p=279#comments

 

the full program for the week, with the abstract summary of each talk, is here

http://loops05.aei.mpg.de/index_files/Programme.html'>http://loops05.aei.mpg.de/index_files/Programme.html

 

the homepage for the conference is here

http://loops05.aei.mpg.de/

 

some video of the talks will probably eventually be made available, but so far we just have the talk-summaries, and reports like those of Lee Smolin and Robert Helling

[Severian]

Why is this more historical than, say, Loops '04?

[Martin]

In his post on Peter Woit's blog, Smolin listed a halfdozen or so significant advances, made into a list here by spacing. Here is an exerpt, with some parts highlighted.

------quote------

... We are not afraid of emphasizing open problems but we do hope that people notice when they are solved.

Hence, I would have hoped you noticed and reported that major progress was described concerning the problem of showing that classical spacetime honestly emerges from background independent theories.

 

1. Rovelli derived the graviton propagator and hence Newton’s law.

 

2. Freidel and Livine showed in detail that 2+1 quantum gravity with matter has a low energy limit which is an effective QFT on a non-commutative geometry with deformed Poincare invariance. Both results were derived from spin foam models.

Hence, one can no longer say that there is no understanding of how classical spacetime and low energy qft emerges from these theories.

There was still more.

 

3. Perez showed how regularization ambiguities in the Hamiltonian constraint may be resolved.

 

4. Markopoulou discussed a new approach to the low energy limit based on her new paper with Kribs.

 

5. Loll announced major results showing that 3+1 spacetime emerges from causal dynamical triangulations and that at short distances the theory scales as a 1+1 dimensional theory.

 

6. Livine and Terno showed how to derive the log(area) corrections to black hole entropy and estimate the rate of Hawking radiation.

 

7. Starodubtsev reported on work with Freidel in which quantum gravity is defined by a perturbation expansion around a topological quantum field theory where the expansion parameter is G Lambda. This is a very promising direction as there are indications (no proof yet) that this new pert. theory is renormalizable and the low energy limit reproduces QFT in DeSitter spcetime……

 

These, and other results were based on detailed calculations and are solid results. I would have hoped that your report would have focused on the presentation of these major result.

In the face of this kinds of impressive progress, I was not embaressed to emphasize open issues or speculate a bit about future directions. So my talk was certainly not representative. By the way, I would have thought that as a particle physicist you would have recognized that what I presented was just a preon model. It was translated into the language of LQG with the help of recent work by Bilson-Thompson. This is new stuff and much remains to be done. But you can also no longer say that there is no proposal for unification of matter and geometry in LQG.

Still to come are new results on rigorous formulations of the theory and at least one striking new results on quantum cosmology, of relevence for upcoming observations.

 

----endquote----

 

the original post by Smolin is here: scroll down to comment #5:

http://www.math.columbia.edu/~woit/wordpress/?p=279#comments

[Martin]

Why is this more historical than, say, Loops '04?

 

great question! I have to go out now, but will be back hopefully in an hour or so.

big difference between this and the May 2004 conf at Marseille, love to discuss!

 

EDIT: I'm back and can reply at least briefly. Maybe have more to say on this later. there are several reasons it's historical

 

The short answer to your question, though, is you ask

"Why is this more historical than, say, Loops '04?" and the obvious reply is

because there wasn't any Loops '04!

 

This is the first year there has been seen a need for establishing an ANNUAL conference in non-perturbative/background independent Quantum Gravity.

 

before this there were two conferences of nearly this scope and size, that I know of. One was in Poland in the late 1990s and one was in Marseille in May of 2004.

 

About 100 people attended the Marseille conference last year. I think it was probably there that people got the feeling that there was a lot happening in QG and that it would be appropriate to have an annual conference

 

About 150 or 160 people are attending Loops '05.

 

for me the sense that this conference represents a landmark, or turningpoint, is based on another perception, which I will describe shortly. have to attend to something, back later.

[Martin]

Why is this more historical than, say, Loops '04?

As I said there wasnt any Loops '04 because this year's is the first annual conference for nonperturbative/background independent Quantum Gravity research.

 

But it is still interesting to look at what WOULD have been Loops '04, if they had thought last year that they would be having conferences like this every year.

 

last year it was an apparently one-time workshop in LQG/spinfoams organized by Carlo Rovelli, at Marseille, that about 100 people attended.

 

The big surprise last year was Renate Loll paper (not even in LQG or spinfoams, but something called "causal dynamical triangulations" or CDT).

 

Here is the main webpage for last year’s conference:

http://web.lpta.univ-montp2.fr/users/philippe/quantumgravitywebsite/

 

The official name was

Non Perturbative Quantum Gravity:

Loops and Spin Foams

It took place 3-7 May at Marseille.

 

There is a lot of overlap in the international organizing committee. Roughly the same people organized the two, except in 2005 the effort was to be more inclusive: not just LQG and spinfoams but ALL the non-string, or nonperturbative/background independent, approaches to QG they could think of. They included Fay Dowker's Causal Sets approach, for example, and Martin Reuter's QEG (quantum einstein gravity).

 

Roughly the same people participated, except this year MORE:

Last year there were 101 registered participants

http://web.lpta.univ-montp2.fr/users/philippe/quantumgravitywebsite/

This year there are 156 participants.

http://loops05.aei.mpg.de/index_files/Participants.html'>http://loops05.aei.mpg.de/index_files/Participants.html

 

The name “Loops ‘05? is very much a shorthand expression. On the main webpage for this year’s conference

http://loops05.aei.mpg.de/

they say: “Loops ‘05…[this year] the annual international meeting on non-perturbative/background independent quantum gravity takes place from …”

 

So the "Loops" conference is not really limited to narrowly defined Loop Quantum Gravity. It is an annual meeting of researchers in non-perturbative/background independent quantum gravity, but that is too long to say, so they call it by the shorthand “Loops” but include several different approaches, not just LQG but also spinfoams, CDT, QEG, causalsets.

 

I think

1. we are seeing the emergence of a field called Quantum Gravity that includes these approaches

 

2. all these approaches are currently making comparatively rapid progress, the pace has picked up

 

3. and for the time being they show signs of CONVERGING---something John Baez discussed in his talk Tuesday.

 

various of these approaches are

 

A. addressing the inclusion of matter whereas before the focus was on quantizing spacetime geometry with little or no matter

 

B. getting Newton gravity, showing signs of having Gen Rel as largescale limit. (this was an important agenda item for many years)

 

C. showing how a 4D universe can emerge from a microscopic spacetime dynamic that is not specified to be 4D----that is getting a grip on why 4 dimensional spacetime. (Loll's 2004 research)

[Martin]

Ratzinger, who might or might not be the head of the Roman Catholic Church, happened to be in Germany at the same time as the Loops '05 conference and he has provided a report of Lee Smolin's public lecture

This was today, Wednesday 12 October, at the Urania Science Center in downtown Berlin

http://www.urania-berlin.de/

 

---quote---

I'm in Berlin right now and found out that Lee Smolin is giving a public lecture, named "The unfinished revolution: finishing what Einstein started". So how could I resist?

 

He started with the question "what is at stake?". Answer: all the big questions (what is time, space, physical law? why is the universe hospitable to life?). After going through the three revolutions (Aristotele, Newton, Einstein) and their notions of space, time, etc., he stressed the importance of relationality in present-day world view, both in qm and gr.

 

(He said Leibniz was right about relationality, but he had no workable physical theory, so scientist followed Newton for 200 years. Mach and then Einstein rediscovered relational thinking.)

 

So what are the approaches to attack quantum gravity? There are two, according to Smolin.

1. Einsteins way (rethinking the concepts of space and time, and especially overworking qm)

 

2. everybody elses way (string theory, loops, etc.)

 

He admitted that he researches everybody elses way, but pointed out the need for radical and rebellious thinking the Einstein way. He also praised Penrose in that context.

 

 

He ended with saying that physicist are standing in the lights and shadows of Einsteins. Lights: Einstein's theories, that they work with. Shadows: neglecting for what is at stake and not sharing Einstein vision of a complete understanding of the universe by rejecting qm.

 

Edit: 1. I think Renata Loll was also sitting in the audience.

2. Smolin was very excited by the coming representation of Winkler and another guy on Friday.

---endquote---

 

the original Ratzinger report can be found at

http://www.physicsforums.com/showthread.php?p=785298#post785298

[timo]

I didn´t have the time to read much of your posts Martin but still I have two questions (one not-so-serious one and one which is a bit of personal interest for me):

 

1) Why did you put this under "Quantum Mechanics" and not under "Modern/Theoretical Physics"? Just because it´s not about classical electromagnetism?

 

2) I printed out the paper about the graviton propagator but I didn´t read it, yet (I´ll most probably have the time to do so this weekend, though). Nevertheless, I´m rather surprised that the graviton propagator should be a big deal. I´ve seen that in diploma theses (hep-ph/0306182, paragraph 2.5) already. Sure, the paper I posted is about Kaluza Klein gravitons but the general idea of constructing a spin-2 particle out of spin-1 particles via the Clebsch Gordan coefficients should work quite straightforwardly for any graviton. So what am I missing here?

[Martin]

...

2) I printed out the paper about the graviton propagator but I didn´t read it' date=' yet (I´ll most probably have the time to do so this weekend, though). Nevertheless, I´m rather surprised that the graviton propagator should be a big deal. ...[/quote']

 

I'm glad that you got interested and plan to read Rovelli's paper. I think you mean this one:

 

http://www.arxiv.org/abs/gr-qc/0508124

Graviton propagator from background-independent quantum gravity

Carlo Rovelli

6 pages

 

"We study the graviton propagator in euclidean loop quantum gravity, using the spinfoam formalism. We use boundary-amplitude and group-field-theory techniques, and compute one component of the propagator to first order, under a number of approximations, obtaining the correct spacetime dependence. In the large distance limit, the only term of the vertex amplitude that contributes is the exponential of the Regge action: the other terms, that have raised doubts on the physical viability of the model, are suppressed by the phase of the vacuum state, which is determined by the extrinsic geometry of the boundary."

 

 

We might wait until this weekend when you have had a chance to look at rovelli's paper before discussing this. He may answer your question.

 

Perhaps the essential difference, with nonperturbative quantum gravity (or with background independent quantum gravity) is that one comes to gravitons from a different direction.

 

one does not normally take for granted a flat background space, or a fixed curved background geometry

 

so things which may come easily if one takes, say, Minkowski space, for granted, may in background independent QG turn out to be difficult to achieve.

 

but first, when you have time, please have a look at the paper.

 

John Baez also has an up-to-date survey of (non-pert.) QG at his website. there is a list of links to recent papers by other people, and there are his notes for the talk he gave Tuesday at Loops '05 at Potsdam

 

http://math.ucr.edu/home/baez/loops05/

 

You can see that the Rovelli paper is one of those that Baez included, but he refers to others. It could be a useful short list

[timo]

Yes, that´s the paper I meant. I am not really convinced that reading the paper will be much of a help to me since my background knowledge about LQG is exactly none.

But the answer that the paper I mentioned only handles the graviton as a pertubation to a flat minkowsky space seems as a reasonable shorthand explanation to me so thanks for now.