vol.2.. Superstring theory and beyond

When I first decided to write a book on string theory, more than ten years
ago, my memories of my student years were much more vivid than they
are today. Still, I remember that one of the greatest pleasures was finding
a text that made a difficult subject accessible, and I hoped to provide the
same for string theory.
Thus, my first purpose was to give a coherent introduction to string
theory, based on the Polyakov path integral and conformal field theory.
No previous knowledge of string theory is assumed. I do assume that the
reader is familiar with the central ideas of general relativity, such as metrics
and curvature, and with the ideas of quantum field theory through non
Abelian gauge symmetry. Originally a full course of quantum field theory
was assumed as a prerequisite, but it became clear that many students
were eager to learn string theory as soon as possible, and that others had
taken courses on quantum field theory that did not emphasize the tools
needed for string theory. I have therefore tried to give a self-contained
introduction to those tools.
A second purpose was to show how some of the simplest four
dimensional string theories connect with previous ideas for unifying the
Standard Model, and to collect general results on the physics of four
dimensional string theories as derived from world-sheet and spacetime
symmetries. New developments have led to a third goal, which is to intro
duce the recent discoveries concerning string duality, M-theory, D-branes,
and black hole entropy.
In writing a text such as this, there is a conflict between the need to
be complete and the desire to get to the most interesting recent results
as quickly as possible. I have tried to serve both ends. On the side of
completeness, for example, the various path integrals in chapter 6 are
calculated by three different methods, and the critical dimension of the
bosonic string is calculated in seven different ways in the text and exercises