Albert Einstein was the most prominent man to take refuge in the United States during the twentieth century. He won the Nobel Prize in Physics in 1921 for his work on the photoelectric effect, but is best known for his theories of relativity. When Adolf Hitler and the Nazi Party seized power in Germany in 1933, he was in Princeton, New Jersey, at the Institute for Advanced Study. He elected to remain in the United States, becoming a U.S. citizen in 1940. He never returned to Germany. Alarmed by the rising power of Hitler's Germany, he wrote his most famous letter, a plea to President Franklin D. Roosevelt urging that the United States fund research into the possibility of the making of an atomic bomb.