tried to conduct this talk in such a way as to preclude putting my life into immediate danger.” Heisenberg remembers asking Bohr if it was right for physicists to work on “the uranium problem” in wartime when there was a possibility that such work could lead to “grave consequences in the technique of war.” Bohr, who had returned from the United States convinced that a bomb was practically impossible, “understood the meaning of the question immediately, as I realized from his slightly frightened reaction.” Heisenberg apparently thought Bohr was privy to American secrets and was reacting guiltily to implicit exposure. But Bohr's next response suggests that he had been, rather, stunned at Heisenberg's revelation: he asked Heisenberg if a bomb really was possible. Heisenberg says he answered that a “terrific technical effort” would be necessary, which he hoped could not be realized in the present war. “Bohr was shocked by my reply, obviously assuming that I had intended to convey to him that Germany had made great progress in the direction of manufacturing atomic weapons. Although I tried subsequently to correct this false impression I probably did not succeed… I was very unhappy about the result of this conversation.”
Thus Heisenberg's version of the evening walk. Bohr's is less detailed. His son Aage, a Nobel laureate in his turn and his father's successor as director of the Copenhagen institute, summarizes it in a memoir:
The impression that in Germany great military importance was given to [atomic energy research] was strengthened by the visit to Copenhagen in the autumn of 1941 of Werner Heisenberg and C. F. von Weizsacker… In a private conversation with my father Heisenberg brought up the question of the military applications of atomic energy. My father was very reticent and expressed his scepticism because of the great technical difficulties that had to be overcome, but he had the impression that Heisenberg thought that the new possibilities could decide the outcome of the war if the war dragged on… [Heisenberg's] account [of the meeting] has no basis in actual events.
Robert Oppenheimer, who also had the story direct from Bohr, condenses the meeting to the comment: “Heisenberg and von Weizsacker came over from Germany, and so did others. Bohr had the impression that they came less to tell what they knew than to see if Bohr knew anything that they did not; I believe that it was a standoff.”
The two accounts are not incompatible, but both leave out a crucial fact: that Heisenberg passed to Bohr a drawing of the experimental heavy-water reactor he was working to build. If he did so clandestinely he certainly risked his life. If he did so cynically and with Nazi approval to misdirect Allied intelligence he was certainly no longer attached to Bohr as a father figure, as Elisabeth Heisenberg writes. Whatever his intent, it had the wrong effect on Bohr. Elisabeth Heisenberg thinks “Bohr essentially heard only one single sentence: The Germans knew that atomic bombs could be built. He was deeply shaken by this, and his consternation was so great that he lost track of all else.” But Aage Bohr's and Oppenheimer's accounts imply a further response from Bohr: indignation, even incredulity, that Heisenberg would think Bohr might be willing in any way, for any reason, to cooperate with Nazi Germany. Heisenberg, in turn, was aghast that Bohr would fail to see and credit his reservations, would not understand, as his wife writes, that his “bond to his country and its people was not tantamount to a bond to the regime.” To the contrary, she adds, “Bohr told Heisenberg that he understood completely that one had to use all of one's abilities and energies for one's country in time of war.” Not surprisingly, since it implied Bohr thought the worst of him — that he was willing to work for the Nazis — “Heisenberg was deeply shocked by Bohr's reply.”
The meeting, and especially the drawing Heisenberg passed, gave Bohr more to worry about, but he continued to doubt that any nation could afford sufficient industrial capacity, especially in wartime, to pursue isotope separation. He must have been pained at what he took to be the treachery of a brilliant and formerly devoted protege. Heisenberg for his part found himself, says his wife, in “a state of confusion and despair.” Even at risk he had not convinced Bohr of his sincerity nor in any way begun a dialogue to avert possible catastrophe. In the absence of such dialogue he had only managed potentially to alarm Germany's most powerful enemy further with news of progress in approaching the chain reaction. That news must necessarily accelerate Allied efforts to build a bomb. As Rudolf Peierls writes of this period in Heisenberg's life, “he had agreed to sup with the devil, and perhaps he found that there was not a long enough spoon.”
Arthur Compton sent draft copies of the third National Academy of Sciences report to Vannevar Bush and Frank Jewett before the weekend of November 1. The new report was brief — six double-spaced typewritten pages (with forty-nine pages of technical appendices and figures) — and finally and emphatically to the point: “The special objective of the present report is to consider
This time the report knew what it was about:
The NAS estimate of destructiveness was low compared to the MAUD Report estimate, some 30 tons of TNT equivalent per kilogram of U235 (for 25 pounds, 300 tons compared to MAUD's 1,800 tons), but the American report attempted to compensate for its doubts about the efficacy of an intense energy release from a small amount of matter by emphasizing that the destructive effects on life of a bomb's radioactivity “may be as important as those of the explosion itself.”
The centrifuge and gaseous diffusion programs were noted to be “approaching the stage of practical test.” Fission bombs might be available “in significant quantity within three or four years.” Like its predecessors the report stressed not the German challenge but the long-term prospect: “The possibility must be seriously considered that within a few years the use of bombs such as described here, or something similar using uranium fission, may determine military superiority. Adequate care for our national defense seems to demand urgent development of this program.”
In detailed appendices Compton calculated the critical mass of a bomb heavily constrained in tamper at no more than 3.4 kilograms; Kistia-kowsky debated whether a fission explosion would be as destructive in terms of energy produced as the explosion of an equivalently energetic mass of TNT and confirmed the feasibility of firing together two pieces of uranium at a speed of several thousand feet per second; and a senior physicist on Compton's committee reported favorably on the isotope-separation systems then under consideration and recommended “the principle of
Notably missing from the third report was any mention of the uranium-graphite work going on at Columbia or of plutonium. Compton remembers that a U235 bomb looked “more straightforward and more certain of accomplishment” than a plutonium bomb, but the omission also measures the extent to which Briggs' judgment of priorities, and Briggs himself, had been set aside. Bush writing Jewett before he met with Compton had already mentioned “leaving Briggs in charge of a section devoted as it is at the present time to physical measurements” — small potatoes indeed — and constituting “a new group under a full-time head to handle development.” He was considering Ernest Lawrence but still thought Lawrence talked too much: “The matter… would have to be handled under the strictest sort of secrecy. This is the reason that I hesitate at the name of Ernest Lawrence.”
If the third and last NAS report only rationalized a previous presidential decision, it at least served to check the British findings independently and to commit the American physics community to the cause. The United States had finally set its wheels to the bomb track. Its inertia was proportional to the juggernaut of its scientific, engineering and industrial might. Acceleration overcoming inertia, it now began to roll.
No document Franklin Delano Roosevelt signed authenticates the fateful decision to expedite research toward an atomic bomb that Vannevar Bush reported in his October 9 memorandum to James Bryant Conant: the archives divulge no smoking gun. The closest the records come to a piece of paper that changed the world is a banality. Bush personally delivered the third National Academy of Sciences report to the President on November 27, 1941. Roosevelt returned it to him two months later with a note on White House stationery written in black ink with a broad-nibbed pen, a note that would communicate only a commonplace of the housekeeping of state secrets except for the authority of its first vernacular expression and the initials it bears:
