Duality, Enhanced Symmetry and ‘massless Black Holes’

String theory is defined perturbatively through a set of rules for calculating scattering amplitudes. Perhaps the most important outstanding problem is that of finding the non-perturbative theory whose perturbation theory reproduces that of string theory. This seems a daunting task, but recent progress has led to some striking conjectures regarding the non-perturbative structure of the theory that have passed many tests1−12. The picture that seems to be emerging is that there is some as yet unknown theory that, when expanded perturbatively, looks like a perturbative string theory, but which has a surprisingly simple structure at the nonperturbative level which includes U or S duality symmetries relating perturbative states to solitons, and weak coupling to strong. Moreover, there are a number of different coupling constants corresponding to the expectation values of various scalars, and the perturbation expansions with respect to some of these define string theories, but different string theories arise for different coupling constants. This leads to unexpected equivalences between string theories that look very different in perturbation theory: they result from different perturbation expansions of the same theory. In many cases, the strong coupling limit of a given theory with respect to a particular coupling constant is described by the weak coupling expansion of a dual theory, which is sometimes another string theory and sometimes a field theory.