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exclusive experimental conditions.48 Given the quantum mechanical nature of reality, he argued, light was neither a particle nor a wave. It was both, and sometimes it behaved like a particle and sometimes like a wave. On any given occasion, nature's answer to whether it was a particle or a wave simply depended on the question asked – on the type of experiment performed. An experiment to determine through which slit in S2 a photon passed was a question that solicited a 'particle' answer and therefore no interference pattern. It was the loss of an independent, objective reality and not probability, God playing dice, that Einstein found unacceptable. Quantum mechanics, therefore, could not be the fundamental theory of nature that Bohr claimed it to be.

'Einstein's concern and criticism provided a most valuable incentive for us all to re-examine the various aspects of the situation as regards the description of atomic phenomena', recalled Bohr.49 A major point of contention, he stressed, was 'the distinction between the objects under investigation and the measuring instruments which serve to define, in classical terms, the conditions under which the phenomena appear'.50 In the Copenhagen interpretation the measuring instruments were inextricably linked with the object under investigation: no separation is possible.

While a microphysical object such as an electron was subject to the laws of quantum mechanics, the apparatus obeyed the laws of classical physics. Yet Bohr had to retreat in the face of Einstein's challenge as he applied the uncertainty principle to a macroscopic object, the first screen S1. By doing so, Bohr had imperiously consigned an element of the large-scale world of the everyday to the realm of the quantum as he failed to establish where is 'the cut' between the classical and the quantum worlds, the border between the macro and micro. It would not be the last time that Bohr played a questionable move in his game of quantum chess with Einstein. The spoils for the victor were just too high.

Einstein spoke only once more during the general discussion, when he asked a question. De Broglie recalled later that 'Einstein said hardly anything beyond presenting a very simple objection to the probability interpretation' and then 'he fell back into silence'.51 However, with all the participants staying at the Hotel Metropole, it was in its elegant art deco dining room that the keenest arguments took place, not in the conference room at the Institute of Physiology. 'Bohr and Einstein,' said Heisenberg, 'were in the thick of it all.'52

Surprisingly for an aristocrat, de Broglie spoke only French. He must have seen Einstein and Bohr deep in conversation in the dining room, with the likes of Heisenberg and Pauli listening closely. As they spoke in German, de Broglie did not realise that they were engaged in what Heisenberg called a 'duel'.53 The acknowledged master of the thought experiment, Einstein would arrive at breakfast armed with a new proposal that challenged the uncertainty principle and with it the much-lauded consistency of the Copenhagen interpretation.

The analysis would begin over coffee and croissants. It continued as Einstein and Bohr headed to the Institute of Physiology, usually with Heisenberg, Pauli and Ehrenfest trailing alongside. As they walked and talked, assumptions were probed and clarified before the start of the morning session. 'During the meeting and particularly in the pauses we younger people, mostly Pauli and I, tried to analyse Einstein's experiment,' Heisenberg said later, 'and at lunch time the discussions continued between Bohr and the others from Copenhagen.'54 Late in the afternoon, following further consultations among themselves, the collaborative effort would yield a rebuttal. During dinner back at the Metropole, Bohr would explain to Einstein why his latest thought experiment had failed to break the limits imposed by the uncertainty principle. Each time Einstein could find no fault with the Copenhagen response, but they knew, said Heisenberg, 'in his heart he was not convinced'.55