Introduction

Although the roots of string theory go back to the late 1960's, the first connection between string theory and gravity was noticed in 1974 independently by Yoneya (1974) and by Scherk and Schwarz (1974). By the early 1980's it became clear that the recently developed superstring theory was an excellent candidate for our first perturbatively finite quantum theory of gravity. One loop calculations were shown to be finite and general arguments suggested that this should hold to all orders. (For a review of what was known at that time see Schwarz (1982).) Since then, an enormous amount of work has been done and progress made in our understanding of string perturbation theory. The evidence for finiteness has grown stronger and stronger (see e.g. D'Hoker and Phong (1988); Atick, Moore and Sen (1988); La and Nelson (1989), and references therein). Although there is always a chance of some unexpected results, no one who works on this subject doubts that it is true.

Conspicuously absent from this brief history is the remarkable explosion of interest in string theory beginning in the fall of 1984 and the almost equally remarkable drop in interest in the past year or so. This mood swing had nothing to do with string theory providing a consistent quantum theory of gravity. Rather, it resulted from the hope that the “uniqueness” of string theory would lead to definite low energy predictions in a simple way. Unfortunately, this hope has not been fulfilled.