Infeld was interested in the theory of relativity. He was awarded a doctorate at the Jagiellonian University (1921), worked as an assistant and a docent at the University of Lwów (1930–1933) and then as a professor at the University of Toronto between 1939 and 1950. He collaborated with Albert Einstein at Princeton University (1936–1938). The two scientists jointly formulated the equation describing star movements as well as concurrently writing the book The Evolution of Physics.