Relativity - Einstein's work on relativity provides a revolutionary new conceptulization of space, time, and gravity. The Special Theory, published in 1905, abolished the concept of absolute space and time which had formed the basis of Western philosophy and science for 2500 years. In Einstein's view, lengths change and the rate at which time passes is variable, depending on the motion of the observer relative to the event or object being observed. Only the speed of light is constant for all observers. While the effects this model produces are negligible at the speeds we routinely experience, they become dramatic at speeds approaching that of light. The predicted consequences of the theory have been confirmed in thousands of experiments. The Special Theory is mathematically rather simple, although it is limited to situations in which the relative speeds of two observers are constant; i.e., there are no accelerations, and thus, no forces involved. The General Theory, first presented in 1916, removes this limitation with a radically new conception of gravity as caused by the distortion in the fabric of spacetime produced by an object which has mass. The predictions of this theory -- black holes, gravitational waves, the effect of distorted spacetime on Mercury's orbit -- have also been confirmed to high precision. There are literally thousands of books and websites on relativity (more than 91,000 on the General Theory alone on Google) covering all levels of mathematical sophistication. An excellent one by the man himself is "Relativity: the Special and General Theory". Authorized translation by Robert W. Lawson, (New York) H. Holt & Company 1920.