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Einstein was desperately seeking to change the physics as well; for he was not the conservative relic many thought. He was convinced that the concepts of classical physics would have to be replaced by new ones. Since the macroscopic world is described by classical physics and its concepts, Bohr agued that even to seek to go beyond them was a waste of time. He had developed his framework of complementarity in order to save classical concepts. For Bohr there was no underlying physical reality that exists independently of measuring equipment, and that meant, as Heisenberg pointed out, 'we cannot escape the paradox of quantum theory, namely, the necessity of using the classical concepts'.73 It is the Bohr-Heisenberg call to retain classical concepts that Einstein called a 'tranquilizing philosoph'.74

Einstein never abandoned the ontology of classical physics, an observer-independent reality, but he was prepared to make a decisive break with classical physics. The view of reality endorsed by the Copenhagen interpretation was all the evidence he needed of the necessity to do so. He wanted a revolution more radical than the one offered by quantum mechanics. It was hardly surprising that Einstein and Bohr left so much unsaid.

In January 1939, Bohr returned to Princeton and stayed for four months as a visiting professor at the institute. Although the two men still enjoyed a warm, friendly relationship, their ongoing dispute over quantum reality had inevitably led to a cooling. 'Einstein was only a shadow of himself', recalled Rosenfeld, who had accompanied Bohr to America.75 They did meet, usually at formal receptions, but they no longer talked about the physics that mattered so much to them. During Bohr's stay Einstein gave only one lecture, on his search for a unified field theory. With Bohr in the audience, he expressed the hope that quantum physics would be derivable from such a theory. But Einstein had already made it known that he would rather not discuss the issue further. 'Bohr was profoundly unhappy about this', said Rosenfeld.76 With Einstein unwilling to talk about quantum physics, Bohr found that there were plenty of others in Princeton eager to discuss the latest developments in nuclear physics, given the ominous events in Europe that would lead once again to a world at war.

'No matter how deeply one immerses oneself in work,' Einstein wrote to Queen Elizabeth of Belgium, 'a haunting feeling of inescapable tragedy persists.'77 The letter was dated 9 January 1939, two days before Bohr sailed for America and brought with him the news of a discovery that others had made: the splitting apart of a large nucleus into smaller nuclei, with an accompanying release of energy – nuclear fission. It was during the voyage that Bohr realised it was the uranium-235 isotope that undergoes nuclear fission when it is bombarded by slow-moving neutrons, and not uranium238. At the age of 53, it was Bohr's last major contribution to physics. With Einstein unwilling to debate the nature of quantum reality, Bohr concentrated on working out the details of nuclear fission with the American John Wheeler from Princeton University.

After Bohr returned to Europe, Einstein sent a letter, dated 2 August, to President Roosevelt urging him to examine the feasibility of developing an atomic bomb, given that Germany had stopped the sale of uranium ore from mines it now controlled in Czechoslovakia. Roosevelt replied in October, thanking Einstein for his letter and informing him that he had set up a committee to investigate the issues raised. In the meantime, on 1 September 1939, Germany attacked Poland.

Still a pacifist, Einstein was prepared to compromise until Hitler and the Nazis were defeated. In a second letter, dated 7 March 1940, he urged Roosevelt that more needed to be done: 'Since the outbreak of the war, interest in uranium has intensified in Germany. I have now learned that research there is carried out in great secrecy.'78 Unknown to Einstein, the man in charge