ABSTRACT
This chapter describes reasons to believe that string theories are important and perhaps present the master-key to the most fundamental problems. It begins with the project of grand unification on the string basis and also describes the 3-D Ising model and 4-D chromodynamics. The chapter considers the NSR-string, the wave functional of which is annihilated by the supercurrent and energy momentum tensor. It discusses critical exponents of strings—a subject intimately connected with their extrinsic geometry. A unique property of the critical strings is that they describe gravitons. The bosonic strings are not totally consistent, since they contain tachyons. In ordinary closed strings modular invariance is automatic, since non-chiral determinants can be regularized in a manifestly covariant way. All massive modes of strings are presumed to be very heavy, and therefore unobservable. The only way to ensure scale invariance in space-time is to have a massless dilaton in the string spectrum.
