Introduction to M Theory

This is an introduction to some recent developments in string theory and M theory. We try to concentrate on the main physical aspects, and often leave more technical details to the original literature. In the past four years, a series of exciting developments in the area of supersymmetric field theories and string theory has completely changed the landscape of these subjects. Duality has been the central theme of these developments. By now, it is a common belief that different string theories all have the same origin, although this unique theory still remains somewhat mysterious. This theory is dubbed M theory [1]. It appears that all degrees of freedom, given enough supersymmetries, are in our possession, and the future effort will be directed toward finding out a nonperturbative formulation of M theory. Though abstract and seemingly remote from the real world, M theory already has found many useful applications, in particular to supersymmetric gauge theories in various dimensions [2], and to quantum properties of black holes [3]. String theory is the most promising approach to quantum gravity [4]. The primary motivation for many string theorists is to understand how the universally accepted theory