Albert Einstein was a German-born theoretical physicist who developed the theory of relativity—one of the two pillars of modern physics. He lived from 1879 to 1955. In 1921, he received the Nobel Prize in physics. While Einstein was visiting the United States in 1933, Adolph Hitler came into power. Because he was a Jew, Einstein did not return to Germany, and subsequently, in 1940, he became a US citizen.