In the last few years, physicists have learned that the different string theories discovered and studied in different ways are limiting cases of a single, more powerful theory, known as M theory. "M" stands for magic, mystery, or matrix, according to taste. Some of these developments will be explained in this lecture.

Edward Witten, professor of physics at the Institute for Advanced Study in Princeton, N.J., is arguably the premier theoretical physicist of our time. Renowned for his many contributions to particle physics and string theory, Witten has almost single-handedly constructed a new branch of mathematical physics For his achievements, he has been awarded mathematics' highest prize, the 1990 Fields Medal. A member of the National Academy of Sciences, he is a recipient of a MacArthur Prize, the Dirac Medal, and many other honors.