Lubos Motl

2005-06-15 17:09:58 UTC

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Hep-th today

There are quite a few interesting papers on hep-th today. 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.

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