Perhaps the two most important conceptual breakthroughs in twentieth century physics are relativity & quantum mechanics. Developing a theory that combines the two seamlessly is a difficult & ongoing challenge. This accessible book contains intriguing explorations of this theme by the distinguished physicists Richard Feynman & Steven Weinberg. Richard Feynmans contribution examines the nature of antiparticles & in particular the relationship between quantum spin & statistics. In his essay Steven Weinberg speculates on how Einsteins theory of gravitation might be reconciled with quantum theory in the final laws of physics. Both these Nobel laureates have made huge contributions to fundamental research in physics as well as to the popularization of science. Anyone interested in the development of modern physics will find this a fascinating book.