Diana Preston - Before the Fallout

Almost at once Bohr began working on the consequences of fission at the Institute of Advanced Study at Princeton, helped by John Wheeler. In early February, while puzzling over why the rate of uranium fission he was observing was some one hundred times less than he would have expected, he had a burst of inspiration. Perhaps, he reasoned, the two isotopes present in uranium—the dominant U-238 (so named for its 92 protons and 146 neutrons) and the much rarer U-235 (with 92 protons but 143 neutrons)—behaved differently when bombarded with neutrons. If only U-235 (constituting less than 0.7 percent of natural uraniun) was splitting, and not the U-238 (of which natural uranium was almost entirely composed), this would explain the low rate of fission. It was hard for bombarding neutrons to find a suitable target.

Pondering why U-235 should be more susceptible to fission than U-238, Bohr deduced that the reason related to the number of neutrons and protons and the effect this had on the binding energy of a nucleus when a neutron was added. Adding a neutron to U-235 resulted in an even number of neutrons and a tightly bound U-236 compound nucleus, whereas adding a neutron to U-238 resulted in an odd number of neutrons and a less tightly bound U-239 compound nucleus. The tighter binding of U-236 meant that its formation released significantly more energy than was the case for the formation of U-239—and this further agitated the neutrons and protons of the U-236 to the point where they elongated into the wasp waist required for fission in Bohr’s liquid-drop model. So he concluded that neutrons traveling at any speed would fission U-235. Conversely, in the case of U-238, the energy release was insufficient for fission by slow neutrons.

As Bohr chalked row after row of formulae on his blackboard, his underlying hope was that, if he was right and the isotope U-235 was the key to fission, this would make an atomic bomb unviable. A massive industrial effort would be required to separate out sufficient quantities of the isotope from natural uranium. According to Edward Teller, Bohr told a group including Szilard, Wigner, and himself, gathered expectantly in his office at Princeton, “You would need to turn the entire country into a factory.” On 15 March Bohr published his initial conclusions in Physical Review.

Nevertheless, if sufficient U-235 could, after all, be obtained, an atomic bomb remained a possibility. Bohr conceded as much to a meeting of the American Physical Society in April 1939. Speculating about the results of bombarding a small amount of uranium with neutrons, he admitted it might produce a chain reaction or an atomic explosion. The press picked up his remarks and presented apocalyptic visions to their readers. The science writer of the New York Times, William L. Laurence, portrayed uranium as the “philosopher’s stone” and predicted that a tiny quantity could “blow a hole in the earth 100 miles in diameter. It would wipe out the entire City of New York, leaving a deep crater halfway to Philadelphia and a third of the way to Albany and out to Long Island as far as Patchogue.” The Washington Post’s headline was “Physicists Here Debate Whether Experiments Will Blow Up 2 Miles of the Landscape.”

Scientists in the free world faced decisions not only about whether to publicize their work but about how to respond to old friends and acquaintances still working in totalitarian countries. Some were in no doubt what they should do. In February 1939 the American physicist Percy Bridgman announced in the journal Science: “ I have decided from now on not to show my apparatus to or discuss my experiments with the citizens of any totalitarian state. A citizen of such a state is no longer a free individual, but may be compelled to engage in any activity whatever to advance the purposes of that state…. Cessation of scientific intercourse with totalitarian states serves the double purpose of making more difficult the issues of scientific information by these states and of giving the individual opportunity to express abhorrence of their practices.”

Yet no such embargoes affected Werner Heisenberg that summer of 1939 when he was invited to lecture at universities across the United States. Some American scientists speculated openly that his real purpose, as he traversed the country, was to gather intelligence on fission. However, his old friends, many of them émigré Jewish scientists like Hans Bethe with whom personal bonds of trust and affection were still strong, welcomed him. They also urged him again and again to leave Germany. At Ann Arbor, where Heisenberg stayed with Sam Goudsmit, a Dutchman of Jewish extraction who had emigrated to the United States in the 1920s, he met Enrico Fermi, who was attending the annual physics summer school at the University of Michigan. Their friendship too went back a long way—both had attended Max Born’s lively seminars in Gottingen—but while bonds remained, their discussions revealed how much their lives and opinions had diverged.

Heisenberg’s view was that “Italy’s leading physicist” had chosen to “ride out the coming storm” in the United States. He did not seem to recognize that choice had had little to do with it, if Fermi was to protect his wife and children. According to Heisenberg, the two men discussed whether Heisenberg should also emigrate. It would certainly have been easy for him to find a post. George Pegram at Columbia University was one of several only too eager to offer the German Nobel laureate a professorship. Fermi queried why Heisenberg did not stay in America and play his part “in the great advance of science.” “Why renounce so much happiness?” Fermi asked. Heisenberg’s reply was that he had gathered around him a small circle of young people anxious to ensure that “uncontaminated science” could make a comeback in postwar Germany and that “if I abandoned them now, I would feel like a traitor.”

According to a young graduate hired as a bartender at a party attended by Fermi and Heisenberg and who overheard them, Fermi tried to convince Heisenberg that his belief that he “could influence, even guide the [Nazi] government in more rational channels” was a naive illusion. He argued that the fascists had “no principles; they will kill anybody who might be a threat…. You only have the influence they grant you.” Heisenberg’s reply was that “Germany needed him.” According to his own account, Heisenberg also argued, “Every one of us is born into a certain environment, has a native language and specific thought patterns, and if he has not cut himself off from his environment very early in life, he will feel most at home and do his best work in that environment.” He added, with characteristic insouciance, that “people must learn to prevent catastrophes, not to run away from them.”

Heisenberg also recorded how Fermi pressed him on the issue of fission—a subject which, according to Heisenberg, he himself was never the first to raise during his visit. Fermi warned that “there is now a real chance that atom bombs may be built. Once war is declared, both sides will perhaps do their utmost to hasten this development, and atomic scientists will be expected by their respective governments to devote all their energies to building the new weapons.” Heisenberg, however, recalled assuring him that “the war will be over long before the first atom bomb is built.”

Announcing cheerfully to his friends in America that he had to get back for machine-gun practice with the Mountain Rifle Brigade to which he had been assigned for annual military service, Heisenberg sailed home in a nearly empty ship, the Europa, arriving back in Germany in mid-August 1939. He spent the next few weeks helping his wife furnish and prepare the country house he had bought in Urfeld, high in the Bavarian mountains, so that, as he later wrote, she and the children could take refuge from the coming disaster.