(1879-1955), German-born U.S. physicist. His theory of general relativity revolutionized scientific thought and served as the theoretical foundation for later exploitation of atomic energy. He won a Nobel Prize in 1921 for his work explaining the photoelectric effect. He became a Swiss (1905) and later a U.S. citizen (1940). He joined other physicists in writing to President Franklin Roosevelt to warn him that Germany could possibly make an atomic bomb.