developed the mathematical basis of his ideas, derived familiar formulae and advanced new ones. While papers by others initially cited Schwinger, papers citing Feynman and employing Feynman diagrams appeared in 1950, and soon became prevalent. Students learned and used the powerful new tool that Feynman had created. Computer programs were later written to evaluate Feynman diagrams, enabling physicists to use quantum field theory to make high-precision predictions. Marc Kac adapted Feynman's technique of summing over possible histories of a particle to the study of parabolic partial differential equations, yielding what is now known as the Feynman–Kac formula, the use of which extends beyond physics to many applications of stochastic processes. To Schwinger, however, the Feynman diagram was "pedagogy, not physics". By 1949, Feynman was becoming restless at Cornell. He never settled into a particular house or apartment, living in guest houses or student residences, or with married friends "until these arrangements became sexually volatile". He liked to date undergraduates, hire prostitutes, and sleep with the wives of friends. He was not fond of Ithaca's cold winter weather, and pined for a warmer climate. Above all, at Cornell, he was always in the shadow of Hans Bethe. Despite all of this, Feynman looked back favorably on the Telluride House, where he resided for a large period of his Cornell career. In an interview, he described the House as "a group of boys that have been specially selected because of their scholarship, because of their cleverness or whatever it is, to be given free board and lodging and so on, because of their brains". He enjoyed the house's convenience and said that "it's there that I did the fundamental work" for which he won the Nobel Prize. Caltech years Personal and political life Feynman spent several weeks in Rio de Janeiro in July 1949. That year, the Soviet Union detonated its first atomic bomb, generating concerns about espionage. Fuchs was arrested as a Soviet spy in 1950 and the FBI questioned Bethe about Feynman's loyalty. Physicist David Bohm was arrested on December 4, 1950 and emigrated to Brazil in October 1951. Because of the fears of a nuclear war, a girlfriend told Feynman that he should also consider moving to South America. He had a sabbatical coming for 1951–52, and elected to spend it in Brazil, where he gave courses at the Centro Brasileiro de Pesquisas Físicas. In Brazil, Feynman was impressed