String Theory, Unification and Quantum Gravity

TL;DR

This paper reviews the development of string theory as a candidate for unifying quantum gravity with other fundamental forces, highlighting its solutions to ultraviolet divergences and the emergence of M-theory.

Contribution

It provides an overview of how string theory and M-theory unify quantum gravity with particle physics through duality symmetries and extended objects.

Findings

String theory naturally resolves ultraviolet divergences.

M-theory incorporates various extended-object states.

Duality symmetries are central to unification.

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 candidate 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 extended 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 r\^ole played by a web of nonperturbative duality symmetries suggested by the nonlinear structures of the field-theoretic supergravity limits of string theory.