1. Youth
(1901-1920)

2. Student and Postdoctoral
Years (1920-1925)

3. The Development of Quantum
Mechanics (1925-1927)

4. Professor in Leipzig
(1927-1942)

5. The War Years
(1939-1945)

6. The period of Reconstruction
and Renewal (1946-1958)

7. The Munich Years
(1958-1976)

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Heisenberg-Society

4. Professor in Leipzig (1927-1942)

                Through his fundamental contributions to quantum mechanics Heisenberg soon became known far beyond the circle of the Munich, Göttingen and Copenhagen physicists as a leading representative of the new, successful atomic theory. At the Meeting of the German Scientists and Physicians in Düsseldorf (19-25 September 1926) he delivered the main address on quantum mechanics. In October 1927 Heisenberg, then only 25 years old, was called from Copenhagen to the chair for theoretical physics at the University of Leipzig, where Peter Debye had just accepted the chair for experimental physics. In spring 1929 Friedrich Hund joined Heisenberg’s theoretical institute, and in 1931 the mathematician Bartel Leendert van der Waerden, who developed group theoretical methods in quantum mechanics, came to Leipzig as professor of mathematics.

                Together Heisenberg and his colleagues created in Leipzig a leading center for research on atomic and quantum physics that attracted numerous talented students and collaborators. Felix Bloch, Rudolf Peierls and Edward Teller were among Heisenberg’s first students. Hans Euler, Leon Rosenfeld, Victor Weisskopf, Carl Friedrich von Weizsäcker and Gian Carlo Wick also studied under Heisenberg. As part of their research program Debye and Heisenberg set up an annual symposium (until 1933) on current research in physics, the famous Leipzig University Week (Universitätswoche). The first of these took place on 18-23 June 1928 with, among others, Paul Dirac speaking on his relativistic electron equation and Fritz London on the theory of the chemical bond.

                Heisenberg’s research in Leipzig concentrated on applications and extensions of quantum mechanics. In May 1928 he provided an explanation of ferromagnetism from first principles in which he showed that the quantum mechanical exchange integral that had played a decisive role in his solution to the helium problem could account for the strong molecular magnetic field in the interior of ferromagnetic materials. A little later Felix Bloch supplemented Heisenberg’s theory of ferromagnetism with the invention of spin waves. During the period 1928-1931 Heisenberg and his pupils presented a series of basic contributions to the theories of molecules and metals. At the same time Heisenberg collaborated with Wolfgang Pauli, who held the chair in theoretical physics at the E.T.H (Swiss Polytechnic) in Zurich since spring 1928, on the formulation of relativistic quantum field theory. In March and September 1929 they completed two basic papers that allowed a systematic treatment of interacting quantum fields, but that also indicated the existence of severe difficulties and divergences in the theory, such as infinite self-energies of electrons. The discovery of the neutron in early 1932 occasioned Heisenberg’s development of the neutron-proton model of the nucleus (independently of Dimitri Iwanenko), in which he introduced the concept of nuclear exchange force and the formalism of isotopic spin (June and July 1932). In recognition of his work on quantum mechanics and its applications Heisenberg received the Nobel Prize for physics in 1933 (for the year 1932).

1929 Chicago University: front from left: Heisenberg, Dirac, Gale, Hund, Compton; backrow: Monk, Eckardt, Mulikan, Hoyt

                Besides his intensive research during the early years in Leipzig, Heisenberg undertook a number of trips abroad. In spring of 1929 he delivered a series of lectures at the University of Chicago on “The Physical Principles of Quantum Theory” which were published as his first book. He made the return trip with Dirac, who had also come to the United States, through the American West (August 1929) to Japan and India (September – November 1929). In 1932 he lectured at the Ann Arbor summer school. Heisenberg participated as well in numerous conferences, such as the Solvay congress of 1927 (24-29 October), 1930 (20-25 October) and 1932 (22-29 October) in Brussels, the centenary meeting of the British Association in London (29 September 1931) and the nuclear physics conference in Rome (October 1931).

                Throughout the 1930s Heisenberg very frequently visited Niels Bohr in Copenhagen and occasionally Wolfgang Pauli in Zurich; his close relations with both men and their colleagues were for him irreplaceable in the difficult period.

1930 Conference in Copenhagen; front row from left: Oscar Klein, Niels Bohr, Werner Heisenberg, Wolfgang Pauli, George Gamow, Lev Landau, Hans Kramers

                Quantum mechanics led to entirely new conceptions in the description of the microscopic world and to a revision of some basic physical and philosophical notions, such as the principle of causality. After the completion of the theory by Born, Bohr and Heisenberg, the latter two as did others, devoted considerable attention to obtaining many of the intellectual consequences of the new theory. These consequences often went far beyond the realm of physics, extending into chemistry, biology, and even into social and ethical phenomena. To many laymen, however, the new ideas seemed as strange and disturbing as the revisions brought about earlier by the relativity theory – if not more so. In order to explain the new conceptions to non-specialists and to relieve the public of any bewilderment, which – as with relativity theory – grew dangerously infected with ideology, Heisenberg wrote for and spoke before wide audiences of laymen interested in science during the 1930s. His widely read first collection of essays, Wandlungen in den Grundlagen der Naturwissenschaft (1935) contained two such addresses.

                Despite the political situation in Germany, which also affected his collaborators, Heisenberg’s physics continued to flourish. The main focus of his research concentrated during the 1930s on the analysis of problems in relativistic quantum field theory. A major component of this work involved the analysis of elementary processes in high energy cosmic-ray interactions, where the new elementary particles positron and meson were discovered in 1932 and 1937, respectively. For the solution of problems in field theory and the explanation of cosmic-ray phenomena, fundamentally new ideas appeared to Heisenberg unavoidable. In cosmic-ray physics he argued in favor of the existence of particular multiple processes, which he called “explosion showers”,  that could be comprehended only by means of a nonlinear quantum field theory (June 1936, May 1939). He further claimed that the divergence difficulties in quantum field theory had to be resolved by assuming a fundamental length that limited the validity of field theory at very small distances (June 1938).

                 In the years following 1933 Heisenberg became the main spokesman in Germany for modern theoretical physics. Although the numers of his students decreased, Heisenberg presented systematic lectures on all fields of theoretical physics, including the politically disfavored relativity – both the special and the general theories. He defended relativity theory even in an article published in the Nazi Party newspaper Völkischer Beobachter on 26 February 1936. Together with two colleagues in experimental physics, Hans Geiger in Berlin and Max Wien in Jena, Heisenberg authored a memorandum to the Reich Minister for Education and Science (Reichsminister für Erziehung und Wissenschaft) opposing the official discrediting of theoretical physics and emphasizing its great importance for the training of needed physicists. His opponents, who gathered around Philipp Lenard and Johannes Stark, sought to dismiss relativity and quantum theory as “Jewish” and “non-German”.

                After Arnold Sommerfeld reached retirement age in 1934, attacks by representatives of the so-called “German physics” greatly increased when Heisenberg was named by the faculty of the University of Munich as the first candidate to succeed him. On 25 July 1937 an article entiteledWeisse Juden in der Wissenschaft” (“White Jews in Science”) and signed by Stark appeared in Das schwarze Korps, the journal of the SS. It contained wild accusations against modern theoretical physics and theoreticians, such as Heisenberg, and closed with the demand that Heisenberg and his colleagues be made to “disappear” like the Jews. Heisenberg responded with a letter to head of the SS, Heinrich Himmler, urgently requesting a termination of the campaign against him. After a long serie of investigations and interrogations by the SS, Heisenberg attained his goal in a letter from Himmler on 21 July 1938. The campaign ceased, but the Munich chair went in December 1939 to the aerodynamicist Wilhelm Müller. Heisenberg managed, however, to have an article on the significance of modern theoretical physics published in the organ of the Reich student’s organization. The article, entitled “Die Bedeutung der ‘modernentheoretischen Physik” (“The meaning of ‘modern’ theoretical physics”), finally appeared in 1942 after considerable delay, and was eagerly read by many young people.

Summer 1942: Last hiking excursion of his Leipzig seminar: from left Blass, Heisenberg, Trefftz, and Hund.

In spite of the official discrediting of theoretical physics in the Third Reich, Heisenberg and Hund managed to keep an unusually stimulating atmosphere in their institute, made even more lively by many foreign guests. Besides the German members of the institute (E.Bagge, S.Flügge, B.Kockel and H.Voltz), there were a number of visitors from America (R.S.Mulliken), J.C.Slater, J.H. Van Vleck, C.Zener), Italy (U.Fano, G.Gentile), Japan (K.Ariyama, Y.Fujioka, S.Kikuchi, S.Tomonaga, K.Umeda, S.Watanabe), Scandinavia (B.O.Grönblom, H.Wergeland), Hungary (L.Tisza) and Yugoslavia (I.Supek), to name only a few. Heisenberg also continued his relations with foreign physicists through lectures and travels to scientific conferences. In 1937 he attended the Galvani Conference in Bologna (18-21 October), in April 1934 and March 1938 he traveled to England (to Cambridge and Manchester, repectively), and in summer of 1939 he went to America where he presented his latest results on the theory of cosmic-ray showers at the Symposium on Cosmic Rays at the University of Chicago (27-30 June). Although he received several tempting offers of positions in America, which he declined to return to Germany.

1937: Werner and Elisabeth visit Bohr in Copenhagen.

                In April 1937 Heisenberg married Elisabeth Schumacher, daughter of Hermann Schumacher, a professor of economics at the University of Berlin. In early 1938 the twins Maria and Wolfgang were born; the family was later increased by Jochen (1939), Martin (1940), Barbara (1942), Christine (1944) and Verena (1950). For a secure place of refuge during the imminent war the Heisenbergs bought in 1939 a summer house in Urfeld on Lake Walchen, south of Munich, that had previously belonged to the painter Lovis Corith. When Heisenberg transferred to Berlin in 1942 to assume directorship of the Kaiser Wilhelm Institute für Physik, his family did not accompany him but moved to Urfeld rather than to Berlin, for safety against bombing raids.

                                                                                                David C. Cassidy and Helmut Rechenberg