Introduction

In the previous chapters we have studied the upper-left- and upper-right-hand boxes of Fig. 20.1 that concern the standard perturbative formulation of string theory and the effective actions of the ten-dimensional string theories (and M theory). We have also learned a bit about the existence of some non-perturbative states in the string spectrum, in particular D-branes and KK and winding modes in compactified theories (the lower-lefthand box of Fig. 20.1). We have studied in the three cases the existence of dualities that related various theories and how these dualities are realized in the worldsheet action (when this is possible, i.e. for T duality) and in the effective actions. We have also mentioned that S dualities and T dualities imply the existence of new solitonic states in the string spectrum.

In this chapter and the next we are going to study systematically the lower-right-hand and central boxes of Fig. 20.1, that is, the solitonic solutions of the string effective field theories and their worldvolume actions. We will study the implications that the various dualities have for them (which are evidently related to the effects of dualities on the effective actions) and for the non-perturbative string spectrum. This chapter will be devoted to a general introduction to extended objects, and in Chapter 25 we will deal specifically with those that occur in string/M theory.

These are subjects with many facets that are related in many ways to each other and to the subjects of the previous chapters. Therefore, it is hopeless to try to give a complete, or even half-complete, account of them in the space that we have at our disposal. Our aim will be to cover the basic material and the essential results and solutions in a unified system of conventions (like the rest of the book), giving pointers to the literature for further developments.