Perhaps the two most important conceptual breakthroughs in 20th century physics are relativity and quantum mechanics. Developing a theory that combines the two seamlessly is a difficult and ongoing challenge. This book contains explorations of this theme by the physicists Richard Feynman and Steven Weinberg. Richard Feynman's contribution examines the nature of antiparticles, and in particular the relationship between quantum spin and statistics. In his essay, Steven Weinberg speculates on how Einstein's 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 should find this book of interest.