As the Nazis intensified their intolerant racial policies, Bohr felt a moral obligation to speak out. Thus, when he was invited to address the International Congress of Anthropological and Ethnological Sciences, Bohr took advantage of the opportunity. Because of Denmark's tradition of neutrality, however, he could not make a public attack on Hitler. Instead, he chose his words carefully and made a scientific argument for cultural harmony and the advantages of accepting different backgrounds. While guarded, the meaning of his speech was clear, and Bohr's became one of the strongest European voices to confront the Nazis. At this conference Bohr managed to exchange a few private words with Fermi, who indicated his desire to leave Italy. As he was about to win the Nobel Prize, Fermi used this trip to Stockholm to escape to Copenhagen until he and his family could leave for the United States.

Before Fermi left, the Fascist government had already announced his discovery of a new element, No. 93, though Fermi himself was not certain. Work on this unknown substance led the chemist Otto Hahn to results that could be explained only by the conclusion that he had split the uranium nucleus—something that sounded ridiculous at the time. Moreover, Bohr had established the liquid-drop model of the nucleus, which did not allow the nucleus to be broken. But as Lise Meitner, Hahn's former partner who had been forced to relocate due to the Nazi occupation of Austria, and her nephew Frisch pondered the possibilities, they recognized that a drop could conceivably elongate to a point where it became separated into two drops. The forces that would resist—just as the surface tension of water would resist—could possibly be overcome by the electric charge of nuclei. The splitting would lead to a release of approximately 200 million electron volts. Frisch rushed to Copenhagen to tell Bohr, who responded by slapping his forehead and wondering how they could have missed it. But he was elated nevertheless and pushed Frisch to hurry with their paper.

In this paper, Meitner and Frisch called the division of the nucleus "fission." Uranium had been split into barium and possibly krypton, but since barium was so far down the table, at No. 56, Fermi had failed to identify it as such. Immediately speculation began over the uses of atomic energy, both productive and destructive. Scientists everywhere attempted to duplicate the experiment and found themselves astonished at their success. Bohr himself contributed to the excitement with his perception that the rare isotope of uranium with atomic weight 235, because of its uneven number of neutrons, was more likely to be capable of fission.

The problem still remained that more energy was required to cause the split than could be produced by it. But further experiments by Bohr, Fermi, and others revealed the possibility that a chain reaction could be set off by the neutrons released in fission. The March day in 1939 when Fermi sent his findings to be published, he also met with the chief of naval operations to inform him of the significance his results could have in the creation of an explosive of unprecedented power. Meanwhile, Fermi's partner in the experiment, Leo Szilard, met with Bohr and others and discussed the implications of their work. No one present at the meeting doubted that the scientists remaining in Germany were capable of reaching similar results, if they hadn't already. Szilard thus urged that the work be pushed forward with urgency, and also that this work be undertaken with secrecy. At this latter suggestion Bohr was troubled. Openness had been a fundamental principle of scientific procedure—voluntary censorship had never seemed applicable and ran contrary to his instinct.

At this time Bohr was residing at Princeton's Institute for Advanced Study. Many urged him not to return to Denmark and instead to bring his family to the United States. But Bohr refused, feeling that his duty required his presence at his own Institute, which remained one of the few havens from the Nazi's expanding domain. In 1940, the Nazis invaded Denmark, and Denmark yielded without much of a fight. Another offer came from the American embassy suggesting that Bohr come to the United States, but Bohr remained resolute. He felt that his presence was needed not only at the Institute, but symbolically for his country. His mother had been a member of a prominent Jewish family, and his well-known support for Jewish scientists made him a symbol of strength. During this time, the Danish Society that Bohr had helped to found asked him to write an introduction for a definitive book on Danish culture. Bohr wrote this essay with pride, drawing on a lifetime of experiences that had never seemed so clear as now, when the nation's identity was under attack. Bohr himself was under constant supervision from the Gestapo, but he never let himself get caught doing anything that could get him arrested.

As conditions in Denmark worsened, more people offered to aid Bohr's escape. H began consider these offers more seriously, as his arrest could come at any moment, endangering his family as well. It was only on the day when he had been informed that the Nazis were coming to arrest him that Bohr finally made the escape to Sweden by fishing boat, successfully eluding the Nazi surveillance. Once there, he learned that arrests of Denmark Jews were about to begin, and he acted fast. Lending support to the Danish resistance movement, Bohr aided in the rescue of almost 6,000 Jews, who reached sanctuary in Sweden. About 300 were captured or killed in rescue attempts, and 472 were sent to concentration camps. Forty-three died, but without the work of the resistance movement and Sweden's cooperation, that number would have been much greater.

Meanwhile, just as many scientists were writing off uranium's potential as an explosive, Frisch returned with new calculations that brought them closer than ever.

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