What Did Heisenberg Tell Bohr about the Bomb?; The Science of War: Nuclear History; Exclusive Online Issues; by Jeremy Bernstein; 4 Page(s)

In September 1943 Niels Bohr learned that the gestapo in Copenhagen intended to arrest him. A few weeks later, on the 29th, he, his wife and several others hoping to escape from Denmark crawled in complete darkness to a beach outside Carlsberg. There they boarded a boat and crossed the ¿resund in secret to Sweden. On October 6 the British flew Bohr alone from Sweden to Scotland. Later that same day he traveled to London and in the evening met with Sir John Anderson, the physical chemist in charge of the nascent British atomic bomb project. Anderson gave the Danish physicist a briefing on the Anglo-American program. According to Bohr's son Aage, who followed his father to England a week later and was his assistant throughout the war, Bohr was deeply surprised--shocked may be a better description--by how far the Anglo-American program had already progressed.

Bohr's alarm very likely had two sources. First, during the 1930s, when nuclear physics was developing, Bohr had said on several occasions that he thought any practical use of nuclear energy was all but impossible. That view was reinforced in the spring of 1939, when he realized an important detail concerning the fission of uranium. In December 1938 the German physical chemists Otto Hahn and Fritz Strassmann had discovered that uranium could be fissioned if it was bombarded with neutrons. (Hahn's former assistant Lise Meitner and her nephew Otto Frisch conjectured that the uranium nucleus had actually been split in the experiments and so coined the name "fission" for the process.) The experiments used natural uranium, 99 percent of which is in the isotope uranium 238. About seven tenths of a percent is in the isotope uranium 235, whose nucleus contains three fewer neutrons.