Branes and string duality
We have studied the main properties of string theory in the framework of perturbation theory. However, on physical grounds we expect the theory to have interesting non-perturbative dynamics; for instance, because the low-energy limit of string theory contains gauge theories, which do have non-perturbative effects. Although a complete definition of string theory at the non-perturbative level is lacking, there is a considerable body of knowledge on some of its properties. In this chapter we introduce certain non-perturbative extended states known as branes, and review their role in non-perturbative string theory and in string duality.
6.1 D-branes in string theory
We start by uncovering the existence in string theory ...