Later Life
When British eclipse expeditions in 1919 confirmed his predictions about the general theory of relativity, Einstein was bombarded by the popular press. Einstein's personal ethics also fired public imagination. Einstein, who after returning to Germany in 1914 did not reapply for German citizenship, was one of only a handful of German professors who remained a pacifist and did not support Germany's war aims. After the war, when the victorious allies sought to exclude German scientists from international meetings, Einstein--a Jew traveling with a Swiss passport--remained an acceptable German envoy. Einstein's political views as a pacifist and a Zionist pitted him against conservatives in Germany, who branded him a traitor and a defeatist. The public success accorded his theories of relativity evoked savage attacks in the 1920s by the anti-Semitic physicists Johannes Stark and Philipp Lenard, men who after 1932 tried to create a so-called Aryan physics in Germany. Just how controversial the theories of relativity remained for less flexibly minded physicists is revealed in the circumstances surrounding Einstein's reception of a Nobel Prize in 1921--awarded not for relativity but for his 1905 work on the photoelectric effect.
With the rise of fascism in Germany, Einstein moved, in 1933 to the United States and abandoned his pacifism. He reluctantly agreed that the new menace had to be put down through force of arms. In this context Einstein sent a letter, in 1939, to President Franklin D. Roosevelt that urged that the United States proceed to develop an atomic bomb before Germany did. The letter, composed by Einstein's friend Leo Szilard, was one of many exchanged between the White House and Einstein, and it contributed to Roosevelt's decision to fund what became the Manhattan Project.
As much he appeared to the public as a champion of unpopular causes, Einstein's central concerns always revolved around physics. At the age of 59, when other theoretical physicists would long since have abandoned original scientific research, Einstein and his co-workers Leopold Infeld and Banesh Hoffmann achieved a major new result in the general theory of relativity.
Until the end of his life Einstein sought a unified field theory, whereby the phenomena of gravitation and electromagnetism could be derived from one set of equations. After 1920, however, while retaining relativity as a fundamental concept, theoretical physicists focused more attention on the theory of quantum mechanics, as elaborated by Max Planck, Niels Bohr, Werner Heisenberg, and others, and Einstein's later thoughts went somewhat neglected for decades. This picture has changed in more recent years. Physicists are now striving to combine Einstein's relativity theory with quantum theory in a "theory of everything," by means of such highly advanced mathematical models as superstring theories.