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

5. The War Years (1939-1945)

                The outbreak of war on 1 September 1939 directly affected Heisenberg’s scientific career. He was called to military service, and to his surprise was ordered to report to the Army Weapons Bureau (Heereswaffenamt) in Berlin. There he and other leading German atomic scientists, the so-called Uranium Club (Uranverein), were asked to investgate whether the fission of uranium, discovered in December 1938 vy Otto Hahn and Fritz Strassmann in Berlin, could be used for gaining energy on a large scale. Within two months Heisenberg completed a comprehensive report in which he developed the theory of chain reaction with uranium fission by neutrons (6 December 1939); he followed it soon by a second report refining the theory and taking into account recently improved data (29 February 1940).

                The Army Weapons Bureau designated the Kaiser Wilhelm Institute (KWI) für Physik in Berlin-Dahlem as the center of its uranium research. Peter Debye, director of the institute and a Dutch citizen, was prevented as a foreigner from working on the secret research. He placed himself on leave and accepted a guest professorship at Cornell University in January 1940. Kurt Diebner assumed Debye’s office as head of the Berlin uranium project. At the suggestion of C.F. von Weizsäcker and especially Karl Wirtz, two of Debye’s former collaborators now working on the project, Heisenberg acted as scientific advisor. He thus traveled weekly to Berlin from Leipzig and wrote several reviews on the progress of research at the Berlin institute (December 1940 – February 1942). Other, non-secret research begun under Debye (for example on X-ray analysis and on nuclear and low temperature physics) continued at the KWI für Physik under the formal supervision of Max von Laue, vice-director. During 1941 and 1042 Heisenberg also directed a seminar at the institute in which he and other members and guests of the institute discussed the recent progress and problems of cosmic-ray physics. These talks were published in 1943 in the collection Kosmische Strahlung (Cosmic Radiation) dedicated to Sommerfeld on his 75th birthday.

                In spring of 1940 Heisenberg began experiments with Robert and Klara Döpel at the University of Leipzig in which they studied possible arrangements of uranium and neutron moderator substances, since for uranium fission slow neutrons turned out to be most effective. These experiments substantiated the usefulness of heavy water as moderator (28 October 1941) and indicated in early 1942 that a spherical arrangement of natural uranium and heavy water led to a small multiplication of the irradiating neutrons (confirmed in June 1942). Thus the prerequisites for the construction of a functioning reactor were fulfilled. On 4 June 1942 a meeting was held between Albert Speer, Minister for Armament and War Production (Reichsminister für Rüstung und Kriegsproduktion), and other officials and military leaders and the scientists, including O.Hahn, W.Heisenberg, K.Diebner, P.Harteck and K.Wirtz. There it was decided that the uranium project should continue with the goal of constructing a nuclear reactor, but not an atomic bomb. As a result the Army Weapons Bureau returned the KWI für Physik to the Kaiser Wilhelm Society (Kaiser-Wilhelm-Gesellschaft), but the reactor project remained secret. Heisenberg, the chief scientific advisor to the institute since 1940 and theoretical director of the successful Leipzig experiments, was named head of the leading reactor research group in Germany and appointed on July 1942 director at the KWI für Physik (to substitute for the absent director Debye). He was assisted by many of Debye’s assistants, including Horst Korsching and Karl Wirtz, as well as by Erich Bagge and Fritz Bopp, Kurt Diebner, previous head of the Berlin project, went with several collaborators to the army laboratory at Gottow where he set up his own uranium experiments. C.F.von Weizsäcker accepted a professorship for theoretical physics at the University of Strasbourg.

                The work on the uranium project did not exhaust Heisenberg’s activities during the war years. He occupied himself particularly with problems in cosmic-ray physics and developed in several papers a new approach to the theory of elementary particles based on the concept of the scattering matrix. Three papers received in September and October 1942 and May 1944, respectively, were published in the Zeitschrift für Physik. He regularly gave lecture courses at the universities of Leipzig and Berlin (after summer 1942) on topics of theoretical physics. And he traveled, as far as the war restrictions allowed, to foreign countries, e.g. to Budapest (April 1941), to Zurich and Bern (November 1942) and to Leiden and Utrecht (October 1943), to meet with colleagues and friends and to deliver talks on his (non-secret) scientific research or on more general questions of physics and science. A particular visit was paid in October 1941 to Niels Bohr in Copenhagen. Without violating the secrecy of his work on the uranium project, Heisenberg tried to convey the message that in Germany the construction of the atomic bomb was not being considered. The failure of the visit cast a shadow on the future relations between Heisenberg and his teacher and friend.

                Despite the early success, which at first gave the German reactor ptoject a slight edge over Fermi’s American project, progress was very slow. Although the basic scientific and technological problems had been overcome and only the task remained of assembling enough uranium and heavy water to create an energy producing reactor, the project was increasingly hindered by the war conditions. Germany could barely provide enough uranium and not nearly enough heavy water, deficiencies made more acute by the fact that at least three groups performed competing experiments: besides the largest group in Berlin, a group in Hamburg under Paul Harteck and a group in Gottow under Diebner. The increasing air raids on Berlin forced the transfer of all apparatus to a bunker in the Dahlem institute, and finally, in late 1944, the entire project was shipped out of Berlin and housed in a rock bunker under the castle chapel in the southern German town of Haigerloch. The KWI für Physik had already been transferred earlier (since 1943) to the nearby town of Hechingen. In the first months of 1945 the model reactor B8 nearly went critical (the point at which the reaction is self-sustaining). But the lack of a few hundred kilograms of uranium (out of 1.5 tons) and several hundred liters of heavy water prevented success before war’s end.

                At the end of April 1945 members of an American science intelligent unit, the Alsos Mission, reached Tailfingen (where Otto Hahn’s KWI für Chemie had moved), Hechingen and Haigerloch just ahead of the advancing French troops. The German atomic scientists gathered there – Bagge, Hahn, Korsching, von Laue, von Weizsäcker and Wirtz – were taken prisoner. Heisenberg, however, had gone a few days earlier by bicycle to Urfeld to join his family. He was taken prisoner there on 3 May 1945 by a small unit of the Alsos Mission led by Colonel Boris T.Pash. Heisenberg and Walter Gerlach, Reich Commissioner (Reichsbeauftragter) for the reactor project, along with Diebner and Harteck, soon joined the Tailfingen-Hechingen group in France. All ten scientists were transferred to Castle Facquerel in Belgium and finally to England (on 23 July 1945), while the Americans dismantled the Haigerloch reactor and transported all of the apparatus, uranium, heavy water, and numerous documents to the United States.

                                                                                                David C. Cassidy and Helmut Rechenberg