When Germany invaded Poland in 1939, Heisenberg was already drafted into a reserve mountain infantry unit. With the outbreak of war he and other physicists received military orders, not to the front, but to the Army Weapons Bureau (Heereswaffenamt) in Berlin. Here they were asked to explore the prospects for the practical utilization of a new discovery: nuclear fission. Nuclear fission involved the splitting of nuclei with the release of enormous amounts of energy. Under the right circumstances, the fission process in uranium can be controlled, leading to a heat-producing reactor that can be harnessed to the production of electricity. In other circumstances, if the reaction is uncontrolled, the energy is released extremely rapidly, producing an enormous explosion--an atomic bomb.
"For the present I believe that the war will be over long before the first atom bomb is built."
—Heisenberg, recollection of a statement in 1939
Heisenberg seized the initiative in German fission research, sending the Weapons Bureau within three months a secret two-part survey on its prospects. Until 1942 he headed a small reactor research group in Leipzig and advised a second, larger group in Berlin, splitting his weeks evenly between Leipzig and Berlin. With his appointment in 1942 to the Kaiser Wilhelm Institute for Physics, the main reactor research group, and to the University of Berlin, Heisenberg took up a bachelor existence in the capital city. His wife, his mother, a maid, and his by then six growing children all crowded for safety into their Bavarian summer home for the duration of the war.
Heisenberg remained a leading figure in the German nuclear fission project, maintaining administrative ties to the end of the war. The frightening possibility that this German effort might succeed in providing Hitler with a nuclear weapon was one of the driving forces of the Manhattan Project in the United States, which produced the nuclear weapons dropped on Japan in August 1945.
Heisenberg's leading role in German nuclear research during the war has been the subject of heated controversy ever since. Part of the controversy centers on why Heisenberg was involved with this potentially dangerous project. Another, related issue is why the German results were so minimal relative to what they could have accomplished. After all, nuclear fission was a German discovery. In addition, at the outbreak of war, Germany had the only military research effort in place; and the conquests of the German army had yielded the world's richest supplies of uranium, heavy water, and other materials and equipment. Yet by the end of the war, Germany was nowhere near attaining a bomb and had not even achieved a chain reaction. A U.S. reactor had achieved that in Chicago more than two years earlier.
The conclusions regarding these complicated and emotional issues vary widely in the historical literature. One assertion (which in the opinion of most researchers is not well supported) is that Heisenberg participated in the project in order to sabotage it, and that is why Germany's achievement was so meager. Another assertion is that Heisenberg, as a theorist without much interest or ability in experimental work, was ill-suited for this practical project. He made a lot of errors in his initial work on fission, and because of his sense of superiority, his lack of vision, and his theoretical-physics background, he never recognized his errors and never saw that the project could progress more efficiently.
Still another view, less critical of Heisenberg as a physicist, is that German nuclear research was doomed to insignificance by the circumstances of the war. Although it is true that Heisenberg failed to calculate how to build a nuclear bomb with only a few kilograms of pure uranium-235 (he thought it would take tons), the correct calculation was far from obvious. No German scientist gave the matter much attention--after all, getting even a few kilograms would have required a titanic long-range project. They could scarcely imagine asking their government to undertake that in the midst of a dire war. The fate of nuclear work was sealed by the military's eventual decision not to give it generous support, but to concentrate on rockets and jet aircraft.
Finally, there is the view, by no means incompatible with some of the above interpretations, that Heisenberg saw this project only as a means to an end, rather than as an end in itself. It helped him prove to the authorities his own worth and the worth of theoretical physics. After Heisenberg's appointments in 1942 to the top of the physics profession in Berlin, along with numerous government concessions regarding his teaching and research, Heisenberg no longer needed to participate in this engineering project. Although he remained involved in the project as an administrator, instead of devoting his efforts to improving his initial calculations and reactor designs, he turned instead to his real interest: high-energy particle physics. He published a series of important papers on this subject beginning in 1942.
A focus of the controversy over Heisenberg's role is his 1941 meeting with Bohr in Copenhagen.
Heisenberg has also been criticized because during the war he traveled to German-occupied countries as a cultural representative of the German Reich. He is reported to have made some insensitive and even outrageous pro-German statements while visiting these oppressed countries. On the other hand, Heisenberg and others have pointed to the amelioration of working conditions for some physicists in these countries and to his assistance to some of those in personal difficulties.
At the end of the war an Allied science intelligence unit captured Heisenberg and other German nuclear scientists, along with most of their papers and equipment. After interrogations, American and British authorities detained Heisenberg and nine other German scientists for six months at an English country manor, Farm Hall near Cambridge, where their private conversations were secretly recorded, transcribed, and translated in part. The fascinating transcripts of their conversations, especially surrounding the news of the atomic bombing of Japan, have been declassified and published. They not only provide new insights, but they also add further fuel to the controversies surrounding Heisenberg and German fission research during World War II.
An excerpt from the Farm Hall transcripts.