Diana Preston - Before the Fallout

When it landed on his desk in July 1941, Vannevar Bush found the draft Maud Report compelling reading. Not only did it give a cogent summary of the underlying science, it defined a concrete program for taking the project forward that chimed with his own desire for action. By the time that, on 3 October, Conant and Bush received an official copy of the Maud Report, they had already decided to show it to the president and urge close collaboration between the United States and Britain. Mark Oliphant had traveled to Berkeley specifically to brief Ernest Lawrence and had spoken to him frankly about both the British work and the German threat. Further energized by these discussions, Lawrence had then gone out of his way to assure Bush and Conant that American experimental results confirmed the British conclusions.

To give themselves even more ammunition, Bush and Conant asked the National Academy of Sciences to carry out a fresh review to validate the British claims that an atomic bomb was feasible. On 9 October, while still awaiting the results of this review, Bush took the Maud Report to Roosevelt. He underlined the British conviction that a bomb with a destructive power equivalent to eighteen hundred tons of TNT could be made with just twenty-five pounds of active material, that the first bombs could be available by the end of 1943, but that achieving this feat, in particular building a plant to separate sufficient quantities of U-235, would require a huge and expensive industrial effort.

Impressed by Bush’s advocacy, the president endorsed “complete interchange with Britain on technical matters.” He also agreed that Bush could expand current fission research and assured him of sufficient funding from a special source without the need for explicit congressional approval. Anxious to ensure political control of the program and to restrict the scientists to their own sphere, Roosevelt decided to limit consideration of policy to a tiny inner circle. In addition to himself, Bush, and Conant, members were Vice President Henry Wallace; the secretary of war, the white-haired, seventy-seven-year-old Henry Stimson; and the army chief of staff, General George C. Marshall. It became known as the Top Policy Group.

Bush was well pleased with the outcome. Roosevelt had recognized the imperative of determining the feasibility of an atomic bomb, had agreed to make the necessary resources available, and had sanctioned collaboration with the British. On 11 October 1941 the president offered Britain a partnership deal, writing to Winston Churchill, “It appears desirable that we should soon correspond or converse concerning the subject which is under study by your Maud Committee and by Dr. Bush’s organization in this country in order that any extended efforts may be coordinated or even jointly conducted.”

In Germany that autumn the initiative rested with Germany’s scientists rather than with their political masters. As Werner Heisenberg later told a British historian, “It was from September 1941 that we saw an open road ahead of us, leading to the atomic bomb.” A month earlier, in August 1941, Manfred von Ardenne had suddenly decided to circulate Fritz Houtermans’s report “On Triggering a Nuclear Chain Reaction,” revealing that atom bomb fuel in the form of plutonium could be made in a reactor. Heisenberg claimed that this had induced a “panic reaction” in him. Not only had this sensitive information been widely revealed among the German scientific community, but it made Heisenberg worry whether scientists abroad had discovered the same information and were, even then, planning massive plants to manufacture plutonium. A letter to a friend revealed Heisenberg’s nervous frame of mind. He wrote, “Perhaps we humans will recognize one day that we actually possess the power to destroy the earth completely, that we could very well bring upon ourselves a ‘last day’ or something closely related to it.”

Houtermans’s findings, in fact, caused few ripples in Germany, where, in the late summer of 1941 as their troops advanced ever deeper into Russia, most people were convinced of an early and victorious end to the war. However, Heisenberg later claimed, the progress of nuclear research forced him to confront certain moral issues at that time. Should he and others disengage from fission research? Alternatively, should they try to ensure that their efforts focused on nuclear power, not nuclear weapons? In his memoirs Heisenberg wrote, “We all sensed that we had ventured onto highly dangerous ground.” He also recalled a conversation with Carl-Friedrich von Weizsacker, during which the two men discussed their worries. According to Heisenberg, “Von Weizsacker said something like ‘At present, we don’t have to worry about atom bombs, simply because the technical effort seems quite beyond our resources. But this could easily change. That being so, are we right to continue working here? And what may our friends in America be doing? Can they be heading full steam toward the atom bomb?’ ”

Heisenberg remembered that he tried to put himself into their position, acknowledging that refugee scientists must be firmly convinced that they were “fighting for a just cause” and that “even the good fight invariably involves some bad means.” However, he suggested, “is there not a point beyond which [the scientist] cannot go under any circumstances?” “All in all,” he concluded, “I think we may take it that even American physicists are not too keen on building atom bombs.” But, he added, “they could, of course, be spurred on by the fear that we may be doing so.”

It was then, according to Heisenberg, that von Weizsacker suggested a solution to their dilemma: “‘It might be a good thing,’ Carl-Friedrich told me, ‘if you could discuss the whole subject with Niels in Copenhagen. It would mean a great deal to me if Niels were, for instance, to express the view that we are wrong and that we ought to stop working with uranium.’”

There had been no direct contact between Heisenberg, von Weizsacker, and Bohr for nearly a year after the occupation of Denmark. Then, in March 1941, von Weizsacker had been invited to Copenhagen to lecture at the newly opened German Cultural Institute—a propaganda organization to promote Germanic “values” among the conquered Danes. There he had met Cecil von Renthe-Fink, the German plenipotentiary in Denmark, a friend of von Weizsacker’s high-ranking father in the German foreign office, who left the visitor in no doubt of Bohr’s uncompromising attitude toward his country’s occupiers. Bohr, he said, would have absolutely nothing to do with the Germans.

Nevertheless, Heisenberg sought a way to engineer a private meeting with Bohr. He later claimed that, without revealing his true purpose, he asked the German Embassy in Copenhagen to organize a visit for him, and officials arranged for him to speak at a lecture series on astrophysics at the German Cultural Institute. The series—a propaganda exercise intended to follow up von Weizsacker’s March visit—had, in fact, been in gestation for some months, possibly before Houtermans’s discoveries about plutonium, and Heisenberg’s participation had already been discussed. It is therefore unclear which came first—Heisenberg’s decision that he had to speak to Bohr or the invitation to visit occupied Denmark. What is clear from the records is that the authorities were initially wary of allowing Heisenberg out of the country. Only when the German foreign office, perhaps at von Weizsacker senior’s prompting, suggested that allowing him to go to Copenhagen would be a good test of his suitability to lecture at other propaganda events in occupied Europe was permission finally given.

On 14 September 1941, accompanied by von Weizsacker, Heisenberg caught the night train to Copenhagen for a meeting that would damage a twenty-year-long friendship and spawn enduring controversy over what he actually said and why.