**9:30**

The careful reader will have noticed that yesterday, my blogging got sparser and sparser. This was probably due to increasing boreodom/annoyance on my part. Often, I thought the organisers should have applied the charta of sci.physics.research that forbits contributions that are so vague and speculative that they are not even wrong. I could not stand it anymore and had to leave the room when the speaker (I am not going to say who it was) claimed that "the big bang is just a phase transition".

Today, I give it a new shot. And the plenary talks are promising. Currently, John Baez has been giving a nice overview on various incarnations of spin foam models (he listed Feynman diagrams, lattice gauge theory and topological strings among them although I am under the impression that in the last point he is misguided as topological strings in fact take into account the complex/holomorphic structure of the background). However, starting from the point "what kind of matter do we have to include to have a nice continuum limit" he digressed via a Witten anecdote (who answered the question if he thinks LQG is right said that he hoped not because he hoped (in the 90s) that there is only one unique matter content (ie strings) consistend with quantised gravity) to making fun of string theorists asking them to make their homework to check the 10^whatever vacua in the landscape.

The next speaker will be Dijkgraaf who hopefully will do a better job than did Theissen yesterday in presenting that stringy people have interesting, deep stuff to say about physics.

Unfortunately, electro-magnetism lectures back in Bremen require me to run off at 11:00 and catch the train so I will not be able to follow the rest of the conference closely.

**9:51**

Baez got back on track with a nice discussion of how Lorentzlian Triangulation fit into the scheme of things and what role the prescribed time slicing might have on large scale physics (introducting further terms than Lambda and R in the LEEA). He showed also a supposed to be spin foam version of it.

**9:56**

Oh no. They have grad students and young postdocs as chairpersons. Bianca Dittrich just announced "Our next speaker is Robbert Dijkgraaf" and nothing happened. It seems Dijkgraaf didn't make it here on the early morning plane. Now, I can fulfil the annonymous reader's wish and report on the presentation of Laurent Friedel, the next speaker.

**10:20**

Before I power down my laptop: Friedel looks at effects of quantum gravity on low energy particle actions. In order to do that he couples matter to the Ponzano Regge model and then will probably try to integrate out the gravitational degrees of freedom.

## 2 comments:

You wrote:

"and topological strings among them although I am under the impression that in the last point he is misguided as topological strings in fact take into account the complex/holomorphic structure of the background"

But certainly one can define a 2D TFT without explicitly talking about a target in which these 'topological strings' live. On the other hand, 'background' information enters in terms of the precise data which one chooses in order to define the TFT.

I don't know, however, how such TFTs are interpreted as 'spin foam'. (I could guess, of course... :-)

Best,

Urs

John Baez send me this comment as an email as he had technical difficulties logging in:

Robert wrote:

"and topological strings among them although I am under the impression that in the last point he is misguided as topological strings in fact take into account the complex/holomorphic structure of the background"

Of course I'm misguided, since I'm not doing string theory. But, at least what I said is correct. :-)

Yes, the usual way you folks get ahold of topological string theories crucially involves the geometrical structure of the

target space in which the strings propagate. For example, the A-model uses the symplectic structure of the target Calabi-Yau manifold, while the B-model uses its complex structure.

However, once you've gotten your hands on a topological string theory, a lot of the information about it is contained in an

"open/closed 2d TQFT". This is a gadget defined by the Moore-Segal axioms: it basically has to assign linear operators to 2-dimensional cobordisms going between open and/or closed 1-dimensional manifolds in a well-behaved way. A mathematically

precise treatment can be found in the new paper by Lauda and Pfeiffer: and Pfeiffer discussed this in his talk at Loops '05.

They show that an open/closed 2d TQFT is completely captured by a "knowledgeable Frobenius algebra".

Anyway, a 2-dimensional cobordism is a special case of a 2-dimensional cell complex - a rather boring case where the

neighborhood of every 0-cell is planar. Using this fact, a topological string theory gives rise to a spin foam model that assigns the linear operator ZERO to every spin foam that's not planar.

To make what I'm saying precise, one needs to do some more work, but Lauda and Pfeiffer are doing this work in their next

paper.

Of course you may not enjoy this outlook on topological string theories; that's your right. I just wanted to point out that these guys do give spin foam models.

Anyway, I'm glad you found my talk tolerable. I also found a bunch of the talks too vague, or at least insufficiently muscular. I think the conference would have been better if instead of 87 talks there were about 15, leaving lots more time for discussion. I had some great discussions during breakfast and dinner at the hotel; I wish there had been time for more.

However, lots of people are only able to get travel funds if they deliver a talk, so there's always this terrible pressure to cram in as many talks as possible.

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