on "Forces in Molecules", based on a topic assigned by John C. Slater, who was sufficiently impressed by the paper to have it published. Today, its main result is known as the Hellmann–Feynman theorem. In 1939, Feynman received a bachelor's degree and was named a Putnam Fellow. He attained a perfect score on the graduate school entrance exams to Princeton University in physics—an unprecedented feat—and an outstanding score in mathematics, but did poorly on the history and English portions. The head of the physics department there, Henry D. Smyth, had another concern, writing to Philip M. Morse to ask: "Is Feynman Jewish? We have no definite rule against Jews but have to keep their proportion in our department reasonably small because of the difficulty of placing them." Morse conceded that Feynman was indeed Jewish, but reassured Smyth that Feynman's "physiognomy and manner, however, show no trace of this characteristic". Attendees at Feynman's first seminar, which was on the classical version of the Wheeler–Feynman absorber theory, included Albert Einstein, Wolfgang Pauli, and John von Neumann. Pauli made the prescient comment that the theory would be extremely difficult to quantize, and Einstein said that one might try to apply this method to gravity in general relativity, which Sir Fred Hoyle and Jayant Narlikar did much later as the Hoyle–Narlikar theory of gravity. Feynman received a PhD from Princeton in 1942; his thesis advisor was John Archibald Wheeler. In his doctoral thesis entitled "The Principle of Least Action in Quantum Mechanics", Feynman applied the principle of stationary action to problems of quantum mechanics, inspired by a desire to quantize the Wheeler–Feynman absorber theory of electrodynamics, and laid the groundwork for the path integral formulation and Feynman diagrams. A key insight was that positrons behaved like electrons moving backwards in time. James Gleick wrote: One of the conditions of Feynman's scholarship to Princeton was that he could not be married; nevertheless, he continued to see his high school sweetheart, Arline Greenbaum, and was determined to marry her once he had been awarded his PhD despite the knowledge that she was seriously ill with tuberculosis. This was an incurable disease at the time, and she was not expected to live more than two years. On June 29, 1942, they took the ferry to Staten Island, where they were married in the city office. The ceremony was attended by neither family nor friends and was witnessed