## Wednesday, June 15, 2005

### Hep-th today

There are quite a few interesting papers on hep-th today and many of us, including Peter Woit, could enjoy them. Some examples:
• hep-th/0506118 by Hamilton, Kabat, Lifschytz, Lowe. They use the example of AdS2 as a prototype for constructing the bulk local operators in terms of the boundary operators. One of their conclusions is that only operators at the points on the boundary that are spacelike separated from the given point in the bulk are used in global AdS.
• hep-th/0506104 by Cornalba and Costa. They argue - well - that the closed time-like curves may be consistent with unitarity for "right" values of Newton's constant - or, equivalently, the angular momentum of the black hole (integer or half-integer). One may imagine that closed time-like curves are OK if their periodicity is a multiple of the wavelength, but it is tougher to preserve these special properties with interactions included. They argue that although the closed curves break unitarity order by order in perturbation theory, the whole result is OK because it is dominated by graviton exchange where the graviton has the right wavelength. It's hard to believe it, but they have some evidence.
• hep-th/0506106 by Nieto. Matroids and M-theory - or M(atroid) theory. Nieto has written many papers about the subject. An oriented matroid is a finite set E of objects together with a function taking values in {-1,0,1} defined for every subset of E with r (rank) elements that is completely antisymmetric and satisfies other properties. Obviously, it is a kind of a discrete counterpart of differential forms or elementary simplices of homology, but how it can tell us something realistic about M-theory is not clear to me so far. Comments welcome, once again.
• hep-th/0506110 by Emparan and Mateos. Virtually all calculations of black hole (or black "object") entropy in string theory reduce to Cardy's formula. They argue that it is possible to interpret this formula geometrically in the bulk using "Komar integrals" that are equal to the "dimension" entering the Cardy formula if one evaluates them at the horizon. Everything is about the 3D BTZ black holes that are kind of found in all calculable examples. The quantity that becomes the "dimension" is typically a squared angular momentum, and therefore the square root - that appears in the Cardy formula - can give you the Bekenstein-Hawking entropy. It's still not clear to me whether they argue that they understand why the result must be "A/4G" for all the known examples.