Albert Einstein (1879–1955) was a German-born theoretical physicist, best known for developing the theory of relativity. His mass–energy equivalence formula E = mc² has been called "the world's most famous equation." He received the 1921 Nobel Prize in Physics for his services to theoretical physics, particularly for his discovery of the law of the photoelectric effect. Born in the Kingdom of Württemberg, he moved to Switzerland in 1895, acquiring Swiss citizenship in 1901.