The Second Superstring Revolution

It is my honor and pleasure to speak at this conference in memory of Andrei Sakharov on the occasion of his 75th birthday. Even though I never met with Sakharov personally, I understand from my good friend Professor Fradkin that Sakharov expressed considerable interest in superstring theory during his exile in Gorky. I would have enjoyed discussing the subject with him, but after his return to Moscow he had other more urgent matters to deal with. I admired him for his courage and persistent efforts on behalf of human rights and disarmament. Major advances in understanding of the physical world have been achieved during the past century by focusing on apparent contradictions between wellestablished theoretical structures. In each case the reconciliation required a better theory, often involving radical new concepts and striking experimental predictions. Four major advances of this type are indicated in Figure 1. These advances were the discoveries of special relativity, quantum mechanics, general relativity, and quantum field theory. This was quite an achievement for one century, but it leaves us with one fundamental contradiction that still needs to be resolved, namely the clash between general relativity and quantum field theory. Many theorists are convinced that superstring theory will provide the answer. There have been major advances in our understanding of this subject, which I consider to constitute the “second superstring revolution,” during the past two years. The plan for this brief report is to sketch where things stood after the first superstring revolution (1984-85) and then to describe the recent developments and their implications. There are various problems that arise when one attempts to combine general relativity and quantum field theory. The field theorist would point to the breakdown of renormalizability – the fact that short-distance singularities become so severe that the usual methods for dealing with them no longer work. By replacing point-like particles with one-dimensional extended strings, as the fundamental objects, superstring theory certainly overcomes the problem of perturbative non-renormalizability. A relativist might point to a different set of problems including the issue of how to understand the causal structure of