First news from the Corfu School: Baez TWF 280

**marcus** · Sep28-09, 03:55 PM

MTd2 spotted this one and called our attention to it:
http://math.ucr.edu/home/baez/week280.html

The brief summary announcement he put at the n-Category Café group blog says:

==quote==
This Week’s Finds in Mathematical Physics (Week 280)
Posted by John Baez

In week280 of This Week’s Finds, hear about the courses taught by Ashtekar and Rovelli at the quantum gravity summer school in Corfu. Ashtekar spoke about loop quantum cosmology, and how it could turn the Big Bang into a Big Bounce. Rovelli spoke about spin foam models, and how the new EPRL model cures many problems of the old Barrett-Crane model. The graviton propagator seems to work!

==endquote==

Then if you go to TWF 280 you find an eight-page report that explains a lot of stuff from the QG school in a way that makes it easy to (begin to) understand. Here's how he starts off:

"I have a lot to talk about, since I just got back from a quantum gravity summer school in Corfu. I felt a bit like Rip Van Winkle, the character who fell asleep for 20 years and woke to find everything changed. I gave up working on quantum gravity about 4 years ago because so many problems seemed intractable. Now a lot of these have been solved, or at least seen some progress. It was great!

There school featured courses by Ashtekar, Barrett, Rivasseau, Rovelli and myself - we each gave 5 hours of lectures. ..."

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

If you want to go to the Corfu QG school website and see the summary of the various lectures and the list of workshop talks then
what I find the easiest to remember is to google "QG meetings"
The first hit is the list of conferences workshops schools organized/supported by the QG Network.
You don't need a password, the google thing is mistaken about that. You just click on the first hit and scroll down to "schools" and you find this:
http://www.physics.ntua.gr/corfu2009/qg.html

So far they have not posted PDF slide sets for the talks.

**MTd2** · Sep28-09, 07:02 PM

What matters now it is that John is back too Loop Quantum Gravity, and he says he's doing 2 articles about this! :)

**marcus** · Sep28-09, 08:56 PM

I share your delight, it's good news for several reasons.

**marcus** · Sep28-09, 11:55 PM

There's a casual gem of an explanation in TWF 280 which smoothly slips comprehension of the Holst action into your head---something JB is good at. It comes at the end, after the main message, which is about those 'special' reps of the Lorentz cover group SL(2,C) that helped solve the pre-2007 problems.
[In case the notation is unfamiliar, an element of the group SL(2,C) is just 4 complex numbers a,b,c,d in a box---a 2X2 matrix---with the determinant ad-bc = 1. And a rep(resentation) is mainly just a way for that humble group of matrices like that to act on a vector space.]

This series of 'special' reps corresponds to the half-integers j. Like a different flavor of spin numbers (caramel spins instead of chocolate?). You take a positive real number gamma which turns out later to be the Immirzi, and for each half-integer j, you form the pair (j, k) where k simply equals gamma times j. There is a commonly used recipe for taking any pair (j, k) where j is half-integer and k is some real number and cranking out a representation of the group. JB gives a bit of intuition about this.

The gist of it is that the sequence of spin labels j = 0, 1/2, 1, 3/2,... can also be thought of as a sequence of SL(2,C) labels.

Baez frames this in a simple story. Before 2007 there were spinnetworks describing (quantum states of) 3D geometry and spinfoams describing 4D evolution of geometry as evolution of spinnetworks. There was a mismatch because the networks were labeled with spins j, but the foams were labeled with any old SL(2,C) reps--not limited to the special kind.

Baez describes the troubles A) B) and C) which arose largely because of that mismatch.

Then in 2007 it was realized that the foam labels should actually be chosen from the 'special' reps (j, k) where k is always some fixed real number gamma multiplied by j. The troubles went away. It's an easy instructive story. Told like that it helps get you into the subject.

However besides telling about the change in Loop that happened around 2007, there is also this very nice discussion of the Holst action which is somewhat more intriguing. Thing about Baez is if he thinks you might need to know something and starts explaining it, your ears perk up because you realize you probably do need to understand that, and you can trust that this is the simplest explanation. This trust removes obstacles. Can call it "mathematical rapport" for lack of anything better. Not all writers have it to the same extent.

This happened to me with the Holst action, as it was explained in TWF 280. I got the feeling that the Holst action was right for me, and meant to be. This is just a subjective matter, but subjective comfort can make a difference.

==================
EDIT TO RESPOND TO LATER POST
Tom, I don't know about JBs plans to continue the sequence. Much of his internetting seems to be at n-Category Café now. Hope he continues TWF. Can't speculate.

**tom.stoer** · Sep29-09, 01:23 AM

The Holst action has to be completed when fermions are present (Nieh-Yan invariant). There was already a series of papers over the last couple of years. So it seems that spin foams are finally understood: Holst action, Immirzi parameter, matching is spin networks, graviton propagator etc.

Two puzzles remain (at least for me):
- what can one deduce for the LQG Hamiltonian; are its ambiguities resolved?
- what about the (special) value for the Immirzi parameter? Or the parameter as a scalar, axion-like field?

**tom.stoer** · Sep29-09, 01:26 AM

John is posting this as week 280 of his "this week finds".

Checking his page the table of contents still terminates with 270. Week 280 is not properly linked. Will he continue with TWF now?

**marcus** · Sep29-09, 01:44 AM

Here is a listing of TWFs
http://math.ucr.edu/home/baez/twfshort.html
that includes links to #276, 277, 278, 279 etc.

It could be the table of contents is not up-to-date.

The announcement about TWF 280 which appeared in n-Category Cafe had this illustration of a universe collapsing and then re-expanding from a state of very high density. Like two cones point-to-point. The illustration came from New Scientist and is posted on the Penn State physics outreach website.
http://gravity.psu.edu/outreach/articles/bigbounce.pdf

**ccdantas** · Sep30-09, 08:20 AM

Hi Marcus,

I have posted a question over at n-Cat, so far no response. I will copy my question here in case someone has some information about it or is willing to elaborate on the question. It is a concern about the "area gap" in LQC, its nature, etc. Thanks.

=========
I was wondering whether there was any further work on the nature of the “area gap”. I mean, as symmetry-reduced versions of LQG, LQC models assume certain results directly from full LQG. For instance, the smallest nonzero area eigenvalue of LQG is the assumed step size (the so called “area gap”) of the LQC difference equation. For far this can be made more formally justifiable (on either physical and/or mathematical grounds)?

What is the connection of that problem (in terms of interpretation and consequences) with the fact that the Wheeler-de Witt equation can be made to agree with LQC (in the case of a k = 0, Λ = 0 FRW cosmological model, coupled to a massless scalar field), as the area gap diminishes (but taking in consideration that the approximation is not uniform in the chosen interval of “internal time”)? And also, with the fact that if the area gap is set to zero, LQC does not admit any limit, being an intrinsically discrete theory?
=========

**marcus** · Sep30-09, 10:35 AM

Christine,
thanks for letting us know. That's an extremely good question to be putting to JB. I wish Carlo Rovelli or Etera Livine were participants in a group blog like that, so one could put that kind of question where they might see it.

I will find your question, over at n-Cat Cafe, and check to see if it is answered. If it is answered, and I miss it, could you let us know?

I went looking over there. Some neat discussion going on!
Here is JB's reply to a question by MTd2:
http://golem.ph.utexas.edu/category/...1.html#c027130

and here is your question, Christine, but i see no answer to it yet:
http://golem.ph.utexas.edu/category/...1.html#c027076

**ccdantas** · Sep30-09, 12:43 PM

Typo in my previous comment:

"For far this can be..." -> "How far this can be..."

Thanks, if I learn something, I'll post here.

**marcus** · Sep30-09, 01:15 PM

Originally Posted by ccdantas

Thanks, if I learn something, I'll post here.

I wish I could give you a pure Platonic hug
without disrespect to your married and matronly status.

Basically I am a bit awed by the n-Cat Café venue and feel more comfortable here at Beyond. So I am very happy if you go over there and bug them for answers while I stay here and take it easy.

I wonder is that Greek website ever going to post PDF slides for the Corfu QG school, as they indicated they would?
If you click "Online Lectures" you get:
http://www.physics.ntua.gr/corfu2009/lectures.html
which says "Lectures are expected to appear on the conference website as soon as they become available."

**ccdantas** · Sep30-09, 02:29 PM

Originally Posted by marcus

I wish I could give you a pure Platonic hug
without disrespect to your married and matronly status.

Originally Posted by marcus

I am very happy if you go over there and bug them for answers

I have no problem in "bugging" people, at least if it is reasonable and not an exaggeration. John Baez is not the only one that I am "bugging" right now but that is another story!

Baez is a nice person, and has an amazing gift for explaining difficult stuff in a clear manner. Sometimes he reminds me of Asimov's style. I was happy to help him out with drawing figures and "bugging" him with questions over at his QG Seminar some years ago. So I have no problem in commenting at n-Cat, although the material there is mostly over my head.

Originally Posted by marcus

I wonder is that Greek website ever going to post PDF slides for the Corfu QG school

What a suspense! Yeah, we must all remember that this school was the one that made John Baez get interested again in quantum gravity. I certainly look forward to that updated material.

**ccdantas** · Oct4-09, 07:27 AM

John Baez replied.

**marcus** · Oct7-09, 05:44 PM

There has been a change of format at the Corfu school website:
http://www.physics.ntua.gr/corfu2009...eekSchool.html

press "refresh" on your browser to make sure you get the new version of the page.

It is a much better listing of the talks, by day and time.
My hunch is that some of these talk titles will eventually become links to PDF, and we'll get to share a little bit in the school.

Having done the "refresh" and gotten the new page, I know the titles of some talks that were TBA before. For example---

Steve Giddings: "What does high energy scattering teach us about the quantum physics of gravity?"

Shahn Majid: "Noncommutative differentials on U_q(su₂) and 3D quantum gravity"

The following is extensively edited to compress it down to something I can eyeball and get an impression of. The original was all spread out and took a lot of scrolling up and down to get the picture.
=========quote=======
Monday 14th Sep (School:)
09.00 - 10.00 Loop Quantum Gravity---A. Ashtekar
10.00 - 11.00 Spin networks and quantum gravity---J. Barrett
11.30 - 12.30 Categorification in Fundamental Physics---J. Baez
12.30 - 13.30 Loop Quantum Gravity---A. Ashtekar
16.00 - 16.45 Discussion session
(Workshop:)
16.45 – 17.30 Asymptotic Safety and Background Independence in Quantum Gravity---M. Reuter
17.30 – 18.00 Reduced phase space quantization of Loop Quantum Gravity---K. Giesel
18.30 – 19.15 Geometry and observables in (2+1)-gravity---C. Meusburger
19.15 – 19.45 Noncommutative black holes---P. Schupp
19.45 – 20.15 Asymptotics of four-dimensional spin foam models---W. Fairbairn

Tuesday 15th September (School:)
09.00 - 10.00 Categorification in Fundamental Physics---J. Baez
10.00 - 11.00 Spin networks and quantum gravity---J. Barrett
11.30 - 12.30 Spin networks and quantum gravity---J. Barrett
12.30 - 13.30 Loop Quantum Gravity---A. Ashtekar
16.00 - 16.45 Discussion session
(Workshop:)
16.45 – 17.30 Noncommutative differentials on U_q(su₂) and 3D quantum gravity---S. Majid
17.30 – 18.00 Kappa-deformed oscillators and kappa-statistics---J. Lukierski
18.30 – 19.00 Equivariant Dimensional Reduction and Quiver Gauge Theory---B. Dolan
19.00– 19.30 Quantum Geometry of the M5-brane in a Constant C-Field from Multiple Membranes---C.– S. Chu
19.30 – 20.00 Hopf algebra symmetry and Diffeomorphism in String---S. Watamura
20.00 – 20.30 Gravitational anomaly and fundamental forces---J. van der Bij

Wednesday 16th September (School:)
09.00 - 10.00 Spin networks and quantum gravity---J. Barrett
10.00 - 11.00 Loop Quantum Gravity---A. Ashtekar
11.30 - 12.30 Loop Quantum Gravity---A. Ashtekar
12.30 - 13.30 Categorification in Fundamental Physics---J. Baez

Thursday 17th September (School:)
09.00 - 10.00 Spin networks and quantum gravity---J. Barrett
10.00 - 11.00 Covariant loop quantum gravity and its low-energy limit---C. Rovelli
11.30 - 12.30 Covariant loop quantum gravity and its low-energy limit---C. Rovelli
12.30 - 13.30 Renormalization in Fundamental Physics---V. Rivasseau
16.00 – 17.00 Renormalization in Fundamental Physics---V. Rivasseau
17.00 – 17.30 Discussion session
(Workshop:)
17.30 – 18.15 What does high energy scattering teach us about the quantum physics of gravity?---S. Giddings
18.15 - 18.45 Noncommutative gravity solutions---P. Aschieri
ESF QG network Steering Committee meeting starts at 18.45.

Friday 18th September (School:)
09.00 - 10.00 Covariant loop quantum gravity and its low-energy limit---C. Rovelli
10.00 - 11.00 Renormalization in Fundamental Physics---V. Rivasseau
11.30 - 12.30 Renormalization in Fundamental Physics---V. Rivasseau
12.30 - 13.30 Categorification in Fundamental Physics---J. Baez
16.00 - 16.45 Discussion session
(Workshop:)
16.45 – 17.30 Noncommutative renormalizable Quantum Field Theory---H. Grosse
17.30 – 18.00 Matrix Models, Quantum Spaces, and Gravity---H. Steinacker
18.30 – 19.00 UV/IR duality in noncommutative quantum field theory---R. Szabo
19.00– 19.30 Fuzzy Riemann surfaces---J. Hoppe
19.30 – 20.00 Asymptotically flat spacetimes revisited---G. Barnich
20.00 – 20.30 Emergent and Evaporating Geometry---D. O’Connor

Saturday 19th September (School:)
09.00 - 10.00 Renormalization in Fundamental Physics---V. Rivasseau
10.00 - 11.00 Covariant loop quantum gravity and its low-energy limit---C. Rovelli
11.30 - 12.30 Categorification in Fundamental Physics---J. Baez
12.30 - 13.30 Covariant loop quantum gravity and its low-energy limit---C. Rovelli
16.00 - 16.45 Discussion session
(Workshop:)
16.45 – 17.30 Spin-foams from the LQG point of view---J. Lewandowski
17.30 – 18.00 2-group representations and spin foam models---A. Baratin
18.30 – 19.00 On asymptotics of spin networks---R. Littlejohn
19.00– 19.30 Gauge symmetry breaking in matrix models---F. Lizzi

**marcus** · Oct9-09, 09:29 PM