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Terrible Secret. The Nazis were not the only terror loose in the world. There was something else that only the physicists suspected. With their new mathematical tools they had been delving deep into atomic secrets, and they had come to realize that atomic nuclei hold enormous stores of potentially destructive energy.
Early in 1939, before the start of World War II, Bohr made a trip to the U.S. Just as his ship was about to leave Copenhagen, two German refugee physicists, Lise Meitner and O. R. Frisch, rushed aboard with a dismaying report. They had just heard that German Chemists Otto Hahn and Fritz Strassmann in Berlin had split the uranium atom. This was atomic fission, and with it the Nazis might soon be able to build an atomic bomb.
Bohr took the terrible news with him to New York and passed it along to U.S. physicists whom he trusted. By then the U.S. was well supplied with first-rank physicists, many of them Bohr's former students; they understood only too well the implications of his message. Soon confirming experiments were in full swing. Bohr himself worked for a while at Princeton. And there, one snowy night as he walked from his club to a laboratory, a problem that he had been puzzling over was unexpectedly resolved and the facts fell into place. Bohr realized that it was the rare uranium isotope U-235 that fissions. That knowledge was a signal contribution to further U.S. research.
He returned to Copenhagen before the Nazis overran Denmark in April 1940. At first they did not bother Bohr, despite his part-Jewish ancestry. Then, in 1943, he learned that he was slated for arrest. That same night Bohr, his wife and his son Aage sneaked aboard the fishing boat Sea Star and escaped to Sweden. (He was the kind of man about whom absent-minded professor stories are told, and legend has it that he had kept a bottle of heavy water, then important for atomic research, hidden in his refrigerator; in his hasty departure he left the heavy water behind and rescued an ordinary bottle of beer.)
Soon after Bohr reached Sweden, a British bomber arrived to pick him up. During the dangerous flight, while the bomber dodged German fighters, he almost died of asphyxiation from a faulty oxygen mask. From England he went on to the U.S., where the news that he had brought in 1939 had already mushroomed into the enormous Manhattan Project for constructing the first atom bomb.
First Bomb. At Los Alamos, Bohr, whose face was familiar to just about every physicist alive, was introduced with transparent secrecy as Mr. Nicholas Baker.
Though he probably did as much as any other man to ensure the success of the Manhattan Project, once the first bomb was built, he would not wait to see the first test explosion at Alamogordo. For the rest of his life, all nuclear weapons were objects of horror to him. His fondest hope was to find a way to abolish them.