String Theory, Unification and Quantum Gravity Imperial/TP/12/KSS/01

Abstract An overview is given of the way in which the unification program of particle physics has evolved into the proposal of superstring theory as a prime can-didate for unifying quantum gravity with the other forces and particles of nature. A key concern with quantum gravity has been the problem of ultraviolet divergences, which is naturally solved in string theory by replacing particles with spatially ex-tended states as the fundamental excitations. String theory turns out, however, to contain many more extended-object states than just strings. Combining all this into an integrated picture, called M-theory, requires recognition of the rôle played by a web of nonperturbative duality symmetries suggested by the nonlinear structures of the field-theoretic supergravity limits of string theory. 1 Introduction: Th