Superstring Theories as Candidates for Unification

Abstract

String theories are promising candidates for a unified theory of all fundamental forces and elementary particles. In perturbation theory they are free from the ultraviolet divergences that plague all other attempts to formulate a quantum theory of gravity. Mathematical consistency appears to require a supersymmetric theory in ten dimensions. The E ₈×E ₈ heterotic string theory may be capable, after spontaneous compactification of six dimensions, of producing realistic low-energy physics. It certainly accounts in a natural way for the occurrence at low mass of chiral families of quarks and leptons. Superstring theory is being developed very rapidly now that it has become so popular. However, it will probably take a long time to full understand its nonperturbative behaviour as a quantum theory and to make a convincing case that it is the theory that nature has chosen.