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July 14, 2005

Day 4

Beisert gave a beautiful review of progress on spin chains and integrability of both N=4 SYM and string theory in AdS 5×S 5AdS_5\times S^5. I think perhaps we’ll spend some time this Fall, in the Geometry and String Theory Seminar at UT, reviewing this stuff.

Maldacena and Lunin gave talks about their joint work, some of which I blogged about before. I won’t repeat what I wrote previously, but there was one remark from Juan’s talk that struck me as an important insight. From the free Fermion description, one gets a rather concrete picture of “summing over nontrivial topologies” on the gravity side. This corresponds to including nontrivial topologies (disconnected dropplets) of the Fermi surface. Juan suggested that small droplets, near an otherwise regular Fermi surface — things that might be called “spacetime foam” — are really indistinguishable from gravitons. Counting them separately from the ripples on the Fermi surface, which are interpreted as gravitons, is overcounting configurations.

I’ve long been of the opinion that string theory gives zero evidence for the popular idea of spacetime foam. Juan’s remark seems to confirm that.

Posted by distler at July 14, 2005 10:30 AM

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Re: Day 4


Any comment on Witten’s talk? Tried to get hold of his slides from the Strings site in vain.


Posted by: Jeff on July 14, 2005 1:04 PM | Permalink | Reply to this


I mentioned it briefly in Tuesday’s post. In a nutshell, the surprise was that there are no surprises. The situation with respect to axions in string theory has not changed significantly since the old days.

Posted by: Jacques Distler on July 14, 2005 1:14 PM | Permalink | PGP Sig | Reply to this

Re: Axions

But then, which was the goal of the talk? Review?

Posted by: Alejandro Rivero on July 14, 2005 2:28 PM | Permalink | Reply to this

Re: Day 4

“string theory gives zero evidence for the popular idea of spacetime foam”

Well, I’d agree, if it weren’t for Vafa’s topological string-crystal melting analogy, in which he makes a certain definition of spacetime foam quite precise.

Posted by: Michael on July 14, 2005 2:02 PM | Permalink | Reply to this

Re: Day 4

I think this depends on your idea of “space-time foam”. If it means that there are millions of little tube like wormholes and topology gets messier the smaller the scale gets I would agree.

If it just means that the notion of a space-time manifold breaks down then there are of cause many examples (for example from D-geometry). Actually, my response to somebody who wants an example of why stringy geometry is different from classical geometry, usually is to paraphrase results of your paper on D-brane monodromies and what can happen to a D-brane when you carry it around a loop in moduli space which are incompatible with D-branes being submanifolds when you take them to the stringy regime.

Posted by: Robert on July 15, 2005 5:43 AM | Permalink | Reply to this

Re: Day 4

Dear Robert,

it does not depend on what my idea of spacetime foam is, because I’m using Vafa’s idea of it, which, as I mentioned, is quite well defined.


Posted by: Michael on July 15, 2005 1:11 PM | Permalink | Reply to this

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