Albert Einstein (14 March 1879 – 18 April 1955) was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history. 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 within physics. A childhood portrait of Albert Einstein and his sister Maja Albert Einstein on his 75th birthday, March 15, 1954, in Princeton, New Jersey Einstein visiting the observatory of Mount Wilson, which at that time operated the largest telescope Albert Einstein escorts his daughter, Margot, and Dimitri Marianoff' to the civil registry office for their wedding The famous tongue image Einstein walks through the campus of Princeton University Einstein gives a speech at the London Albert Hall Einstein, his secretary Helen Dukas, left, and his daughter Margaret Einstein take the oath of U.S. citizenship Einstein visiting Mt. Wilson Observatory