Scientists in the 1930s, using machines that could break apart the nuclear cores of atoms, confirmed Einstein's formula E=mc² . The release of energy in a nuclear transformation was so great that it could cause a detectable change in the mass of the nucleus. But the study of nuclei -- in those years the fastest growing area of physics -- had scant effect on Einstein. Nuclear physicists were gathering into ever-larger teams of scientists and technicians, heavily funded by governments and foundations, engaged in experiments using massive devices. Such work was alien to Einstein's habit of abstract thought, done alone or with a mathematical assistant. In return, experimental nuclear physicists in the 1930s had little need for Einstein's theories.

In August 1939 nuclear physicists came to Einstein, not for scientific but for political help. The fission of the uranium nucleus had recently been discovered. A long-time friend, Leo Szilard, and other physicists realized that uranium might be used for enormously devastating bombs. They had reason to fear that Nazi Germany might construct such weapons. Einstein, reacting to the danger from Hitler's aggression, had already abandoned his strict pacifism. He now signed a letter that was delivered to the American president, Franklin D. Roosevelt, warning him to take action. This letter, and a second Einstein-Szilard letter of March 1940, joined efforts by other scientists to prod the United States government into preparing for nuclear warfare. Einstein played no other role in the nuclear bomb project. As a German who had supported left-wing causes, he was denied security clearance for such sensitive work. But during the war he did perform useful service as a consultant for the United States Navy's Bureau of Ordnance.

A postwar reconstruction of the signing of the letter.