Einstein's long foray into academic life began in 1908, when he was offered an apprenticeship as a privatdozent at the University of Bern. Although this was merely a teaching job and not a faculty position with a fixed salary, one had to serve in this capacity before becoming appointed to a professorship. Because he was only being paid out of his students' fees, Einstein continued working full- time at the patent office. During this period of Einstein's career, Hermann Minkowski, one of his mathematics lecturers from the Zurich Polytechnic, developed a mathematical interpretation for the theory of special relativity. Minkowski explained Einstein's theory as a form of geometry. He spoke of a multiplicity of "world points" that make up the "world" and explained that Einstein's relativity unfolds in four dimensions, with time as the fourth dimension. It was Minkowski's geometrized version of relativity that most of Einstein's contemporaries first studied his theory.
In July of 1909, Einstein resigned from the patent office and began to work full-time as a theoretical professor at the University of Zuriich. His position was that of "professor extraordinarius," which was a step up from privatdozent but still not a tenured position at the top of the academic echelons; extraordinarius professors still had to teach several hours a week and supervise PhD students. Although Einstein enjoyed these activities, he resented having to spend so much time away from his own research. Only the position of professor ordinarius would free him to devote himself fully to his theories.
Einstein was offered such a position that same year by the University of Prague. In Prague, Einstein and Mileva enjoyed a higher standard of living, but their relationship became increasingly tense: Mileva was unhappy with the move because she felt like an outsider in a foreign land. She also felt increasingly jealous of Einstein's colleagues and friends, especially his cousin Elsa Lowenthal, with whom he was quite close. In 1910, Mileva gave birth to a second son, Eduard, who would be diagnosed with schizophrenia and eventually die in a mental institution. The year and a half that Einstein and Mileva spent in Prague was a very difficult time for their relationship.
In 1912, Einstein and Mileva left Prague because Einstein was appointed professor of theoretical physics at the Zurich Polytechnic, the very same institution that had once rejected him as a student. In Zurich, Einstein spent most of his time trying to determine whether the principle of relativity also applied to systems that are accelerated relative to one another: are the laws of physics are always the same for all observers, even those in non-inertial reference frames? The answer to this question, which Einstein worked on until November 1915, was the general theory of relativity. While developing this theory in Zurich, Einstein benefited from the mathematical assistance of his old friend Marcel Grossman, who was one of the only people in Zurich with whom Einstein could discuss his new ideas. Their collaboration led to the publication of a partially successful version of the general relativity theory in 1913, followed by a more complete version in 1915.
During his stay in Zurich, Einstein was approached by representatives of several foreign universities attempting to lure him away to their own institutions. One of these offers proved too good to resist: Max Planck, one of the most important developers of quantum theory, and Walther Nernst, a brilliant physical chemist, offered Einstein a professorship at the University of Berlin that allowed Einstein to teach only as much as he wanted. Einstein was offered the maximum salary for professors in Prussia, as well as membership in the highly prestigious Prussian Academy of Sciences, Germany's most elite and prestigious scientific institution. Finally, Einstein was also offered the directorship of a new physics research institute that would be called the Kaiser Wilhelm Institute. Einstein told Planck and Nernst that he would greet them the next day bearing red flowers if he chose to accept their offer, and white flowers if he declined. Unable to resist a position that would enable him to focus entirely on his relativity theory, Einstein arrived with red roses.
Einstein moved to Berlin with Mileva and their sons in April 1915. However, by this point their marriage had turned fully sour, and Mileva and the children returned to Zurich in June with the helpful intervention of Einstein's friend Michele Besso. Although Einstein and Mileva did not officially divorce until February 1919, Einstein knew that their 1915 parting was final. He had already begun spending large amounts of time with his cousin Elsa. When Einstein fell into a prolonged sickness in early 1917, Elsa nursed Einstein to health. By the summer of 1917, he had moved into a flat next door to Elsa, a clear sign that it was time to begin divorce proceedings with Mileva. Thus, after nearly a decade of traveling from one country to another to fill a series of increasingly prestigious academic posts, Einstein at last settled with the woman who would soon become his second wife and his lifelong companion in the country from which he had renounced his citizenship.