Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [136]
In February 1927, as Bohr was edging towards complementarity, Einstein had given a lecture in Berlin on the nature of light. He argued that instead of either a quantum or a wave theory of light, what was needed was 'a synthesis of both conceptions'.28 It was a view he had first expressed almost twenty years earlier. Where he had long hoped to see some sort of 'synthesis', Einstein now heard Bohr imposing segregation through complementarity. It was either waves or particles depending on the choice of experiment.
Scientists had always conducted their experiments on the unspoken assumption that they were passive observers of nature, able to look without disturbing what they were looking at. There was a razor-sharp distinction between object and subject, between the observer and observed. According to the Copenhagen interpretation, this was not true in the atomic realm, as Bohr identified what he called the 'essence' of the new physics – the 'quantum postulate'.29 It was a term he introduced to capture the existence of discontinuity in nature due to indivisibility of the quantum. The quantum postulate, said Bohr, led to no clear separation of the observer and the observed. When investigating atomic phenomena, the interaction between what is measured and the measuring equipment meant, according to Bohr, that 'an independent reality in the ordinary physical sense can neither be ascribed to the phenomenon nor to the agencies of observation'.30
The reality Bohr envisaged did not exist in the absence of observation. According to the Copenhagen interpretation, a microphysical object has no intrinsic properties. An electron simply does not exist at any place until an observation or measurement is performed to locate it. It does not have a velocity or any other physical attribute until it is measured. In between measurements it is meaningless to ask what is the position or velocity of an electron. Since quantum mechanics says nothing about a physical reality that exists independently of the measuring equipment, only in the act of measurement does the electron become 'real'. An unobserved electron does not exist.
'It is wrong to think that the task of physics is to find out how nature is', Bohr would argue later.31 'Physics concerns what we can say about nature.' Nothing more. He believed that science had but two goals, 'to extend the range of our experience and to reduce it to order'.32 'What we call science,' Einstein once said, 'has the sole purpose of determining what is.'33 Physics for him was an attempt to grasp reality, as it is, independent of observation. It is in this sense, he said, that 'one speaks of "physical reality"'.34 Bohr, armed with the Copenhagen interpretation, was not interested in what 'is', but in what we can say to each other about the world. As Heisenberg later stated, unlike objects in the everyday world, 'atoms or the elementary particles themselves are not as real; they form a world of potentialities or possibilities rather than one of things or facts'.35
For Bohr and Heisenberg, the transition from the 'possible' to the 'actual' took place during the act of observation. There was no underlying quantum reality that exists independently of the observer. For Einstein, a belief in the existence of an observer-independent reality was fundamental to the pursuit of science. At stake in the debate that was about to begin between Einstein and Bohr was the soul of physics and the nature of reality.
After Bohr's contribution, three others had already spoken when Einstein indicated to Lorentz that he wanted to break his self-imposed silence. 'Despite being conscious of the fact that I have not entered deeply enough into the essence of quantum mechanics,' he said, 'nevertheless I want to present here some general remarks.'36 Quantum mechanics, Bohr had argued, 'exhausted