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Heisenberg arrived in Leipzig in October 1927, the new head of its Institute for Theoretical Physics. Leipzig had grown outdated, as its aging professors were failing to keep up with the rapid pace of new developments in relativity and quantum mechanics. Heisenberg and his experimentalist counterpart, Peter Debye (who also headed the Institute for Physics, of which Heisenberg's was a subdivision), revitalized the school by virtue of their presence alone, attracting many new students who were attracted by the excitement of modern physics. Indeed, several of Heisenberg's courses set school enrollment records. Because of his young age and perpetually youthful lifestyle, Heisenberg got along well with his students, and even took them on trips similar to his youth movement excursions. In Leipzig he also fit effortlessly into the elite social circles, consisting largely of book publishers, university professors, judges, and attorneys–a combination of wealth and culture. Here, Heisenberg's musical prowess served him well, for music served as one of the primary cultural uniting factors among the elite.
In the late 1920s and early 1930s, the central questions of physics began to change. First was the problem of how to reconcile quantum mechanics with relativity–the search for a relativistic quantum field theory–a question that still occupies physicists today. Heisenberg helped lead the way on this problem, working directly with Pauli. The two proposed a theory in 1929, in a paper entitled "On the Quantum Dynamics of Wave Fields." Their theory was far from complete, however, as it left several matters unresolved.
While Heisenberg was struggling with these problems, his father passed away in 1930. Their relationship had become rather distant; while Heisenberg appreciated his father's support, he also became annoyed at his attempts to meddle in his career. Perhaps what moved Heisenberg most at the time of his father's death were feelings of his own mortality. He retreated for a short time from the big questions to pursue teaching and other work more earnestly, feeling that answers would come only with further experimental evidence.
Heisenberg would not have to wait long. In early 1932, James Chadwick discovered the neutron, and shortly afterward Heisenberg published the first modern theory of nuclear physics. The existence of the neutron helped him explain a number of different forces, largely concerned with the previously unexplained stability of the nucleus.
Only weeks after Heisenberg completed the third paper of his nuclear physics trilogy, Hitler was appointed Chancellor of Germany. Within months, thousands of Jews and political opponents lost their jobs and left the country. Heisenberg, still firm in his belief that science was above politics and that anti-Semitism was a purely political issue, made no active response. However, as his colleagues in the scientific world began to lose their jobs or resign, he turned to the influential Max Planck for advice. Though the two men, separated by over forty years of age, made an unlikely team, they did everything they could to slow down the mass emigration of scientists, promoting the hope that conditions would improve soon. However, the situation only worsened and they could do little to prevent it. Heisenberg even became angry at Schrödinger for resigning from Berlin despite being non-Jewish and therefore out of harm's way. Heisenberg was apparently more concerned with the well being of German physics than any broader principles. Though Heisenberg and Planck's intentions may have been good, they were also shortsighted. The men failed to recognize that their continued support of German physics constituted an implicit acceptance of the Nazi regime.
Records show that Heisenberg never joined the Nazi party, but he performed its required duties without complaint. On a personal level, he likely disapproved of certain Nazi policies as excessive, but he never felt driven to protest. The only explanation of his behavior is the same that applies to many Germans facing the same dilemma at the time. Heisenberg's life was bound up with the culture and people of Germany, and though the new regime disoriented him, he was not moved to the point of breaking away from his homeland. Like his fellow citizens, he simply found ways to rationalize his choices.
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