Albert Einstein (14 March 1879 - 18 April 1955) was a German-born theoretical physicist and philosopher of science. He developed the general theory of relativity, one of the two pillars of modern physics (alongside quantum mechanics). He is best known in popular culture for his mass-energy equivalence formula E = mc2 (which has been dubbed 'the world's most famous equation'). He received the 1921 Nobel Prize in Physics 'for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect'. The latter was pivotal in establishing quantum theory. Einstein was visiting the United States when Adolf Hitler came to power in 1933 and, being Jewish, did not go back to Germany, where he had been a professor at the Berlin Academy of Sciences. He settled in the USA, becoming an American citizen in 1940. On the eve of World War II, he endorsed a letter to President Franklin D. Roosevelt alerting him to the potential development of 'extremely powerful bomb