hep-th/9507121, IASSNS-HEP-95-63
Some Comments On String Dynamics
arXiv:hep-th/9507121 v1 23 Jul 1995
1
Edward Witten School of Natural Sciences, Institute for Advanced Study Olden Lane, Princeton, NJ 08540, USA Three subjects are considered here: a self-dual non-critical string that appears in Type IIB superstring theory at points in K3 moduli space where the Type IIA theory has extended gauge symmetry; a conformal eld theory singularity at such points which may signal quantum eects that persist even at weak coupling; and the rich dynamics of the real world under compactication, which may be relevant to some attempts to explain the vanishing of the cosmological constant.
July, 1995
1
To appear in the proceedings of Strings '95, USC, March 1995
My lecture at Strings '95 focussed on determining the strong coupling behavior of various string theories in various dimensions. Among the main points were the following: U -duality of Type II superstrings requires that the strong coupling limit of the Type IIA make sense of heterotic string dynamics in ve, six, and seven dimensions and deduce Sto the Type IIA theory on R6 × K3. The detailed arguments have appeared elsewhere [1] superstring in ten dimensions is eleven-dimensional supergravity (on R10 × S1 ); one can
duality in four dimensions if one assumes that the heterotic string on R6 × T4 is equivalent and will not be repeated here. Instead I will try to clarify a few related issues, in some cases involving questions that were asked at the meeting. The issues I will discuss in sections one and two involve mainly the extended gauge section one, I analyze how the Type IIB theory behaves when Type IIA has extended symmetry of the Type IIA superstring on R6 × K3 at certain points in moduli space. In
gauge symmetry, and in section two, I discuss the nature of the singularity that occurs in conformal eld theory at these points. In section three, I consider instead some issues involving the behavior of the real world under dimensional reduction; these issues may be relevant to the vanishing of the cosmological constant.

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