Albert Einstein (March 14, 1879 to April 18, 1955) was a German mathematician and physicist who developed the special and general theories of relativity. In 1921, he won the Nobel Prize for physics for his explanation of the photoelectric effect. In the following decade, he immigrated to the U.S. after being targeted by the Nazis. His work also had a major impact on the development of atomic energy. In his later years, Einstein focused on unified field theory. By November 1915, Einstein completed the general theory of relativity. Einstein considered this theory the culmination of his life research. With his passion for inquiry, Einstein is generally considered the most influential physicist of the 20th century.