The Hungarian American physicist Leo Szilard first conceived of the nuclear chain reaction—a crucial milestone in the development of the atomic bomb—on a gray London morning in September 1933 while waiting for a traffic light to change from red to green. “It suddenly occurred to me that if we could find an element…which would emit two neutrons when it absorbed one neutron,” he wrote, this element could “liberate energy on an industrial scale, and construct atomic bombs.” In his book on the history of the bomb, historian Richard Rhodes writes that “as he crossed the street time cracked open before him and he saw a way to the future, death into the world.”