1851. He completed his PhD in June 1926 with the first thesis on quantum mechanics to be submitted anywhere. He then continued his research in Copenhagen and Göttingen. In the spring of 1929, he was a visiting professor at the University of Wisconsin–Madison. Family In 1937, Dirac married Margit Wigner, a sister of physicist Eugene Wigner and a divorcee. Dirac raised Margit's two children, Judith and Gabriel, as if they were his own. Paul and Margit Dirac also had two daughters together, Mary Elizabeth and Florence Monica. Margit, known as Manci, had visited her brother in 1934 in Princeton, New Jersey, from their native Hungary and, while at dinner at the Annex Restaurant, met the "lonely-looking man at the next table". This account from a Korean physicist, Y. S. Kim, who met and was influenced by Dirac, also says: "It is quite fortunate for the physics community that Manci took good care of our respected Paul A. M. Dirac. Dirac published eleven papers during the period 1939–46... Dirac was able to maintain his normal research productivity only because Manci was in charge of everything else". Personality Dirac was known among his colleagues for his precise and taciturn nature. His colleagues in Cambridge jokingly defined a unit called a "dirac", which was one word per hour. When Niels Bohr complained that he did not know how to finish a sentence in a scientific article he was writing, Dirac replied, "I was taught at school never to start a sentence without knowing the end of it." He criticised the physicist J. Robert Oppenheimer's interest in poetry: "The aim of science is to make difficult things understandable in a simpler way; the aim of poetry is to state simple things in an incomprehensible way. The two are incompatible." Dirac himself wrote in his diary during his postgraduate years that he concentrated solely on his research, and stopped only on Sunday when he took long strolls alone. An anecdote recounted in a review of the 2009 biography tells of Werner Heisenberg and Dirac sailing on an ocean liner to a conference in Japan in August 1929. "Both still in their twenties, and unmarried, they made an odd couple. Heisenberg was a ladies' man who constantly flirted and danced, while Dirac—'an Edwardian geek', as biographer Graham Farmelo puts it—suffered agonies if forced into any kind of socializing or small talk. 'Why do you dance?' Dirac