In 1915, Albert Einstein presented his masterwork to the Prussian Academy of Sciences: a theory incorporating gravity, matter, space, and time—his general theory of relativity. In it, he describes black holes, the evolution of the universe, why clocks run slower on the surface of the earth than in space, and even the possibility of time travel. Although Einstein's special theory of relativity from 1905 contains the famous equation E=mc2, astrophysicist John Gribbin makes the case here that 1915 better deserves to be considered the scientist's high-water mark, and he puts Einstein's astonishing breakthrough in the context of his life and work.