Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [182]
Whereas Aspect's team and others who tested Bell's inequality ruled out either locality or an objective reality but allowed a non-local reality, in 2006 a group from the universities of Vienna and Gdansk became the first to put non-locality and realism to the test. The experiment was inspired by the work of the British physicist Sir Anthony Leggett. In 1973 and not yet knighted, Leggett had the idea of amending Bell's theorem by assuming the existence of instantaneous influences passing between entangled particles. In 2003, the year he won the Nobel Prize for his work on the quantum properties of liquid helium, Leggett published a new inequality that pitted non-local hidden variable theories against quantum mechanics.
The Austrian-Polish group led by Markus Aspelmeyer and Anton Zeilinger measured previously untested correlations between pairs of entangled photons. They found that the correlations violated Leggett's inequality, just as quantum mechanics predicted. When the results were published in the journal Nature, in April 2007, Alain Aspect pointed out that the philosophical 'conclusion one draws is more a question of taste than logic'.19 The violation of Leggett's inequality implies only that realism and a certain type of non-locality are incompatible; it did not rule out all possible non-local models.
Einstein never proposed a hidden variables theory, even though he seemed to implicitly advocate such an approach in 1935 at the end of the EPR paper: 'While we have thus shown that the wave function does not provide a complete description of the physical reality, we left open the question of whether or not such a description exists. We believe, however, that such a theory is possible.'20 And as late as 1949, in a reply to those who had contributed to a collection of papers to mark his 70th birthday, Einstein wrote: 'I am, in fact, firmly convinced that the essentially statistical character of contemporary quantum theory is solely to be ascribed to the fact that this [theory] operates with an incomplete description of physical systems.'21
The introduction of hidden variables to 'complete' quantum mechanics seemed to be in accordance with Einstein's view that the theory is 'incomplete', but by the beginning of the 1950s he was no longer sympathetic to any such attempt to complete it. By 1954 he was adamant that 'it is not possible to get rid of the statistical character of the present quantum theory by merely adding something to the latter, without changing the fundamental concepts about the whole structure'.22 He was convinced that something more radical was required than a return to the concepts of classical physics at the sub-quantum level. If quantum mechanics is incomplete, only a part of the whole truth, then there must be a complete theory waiting to be discovered.
Einstein believed that this was the elusive unified field theory that he spent the last 25 years of his life searching for – the marriage of general relativity with electromagnetism. It would be the complete theory that would contain within it quantum mechanics. 'What God has put asunder, let no man join together', was Pauli's caustic judgement on Einstein's dream of unification.23 Although at the time most physicists ridiculed Einstein as out of touch, the search for such a theory would become the holy grail of physics as the discoveries of the weak nuclear force responsible for radioactivity and the strong nuclear force that held the nucleus together brought the number of forces that physicists had to contend with to four.
When it came to quantum mechanics there were those, like Werner Heisenberg, who simply accused Einstein of being 'unable to change his attitude' after a career spent probing the 'objective world of physical processes which runs its course in space and time, independent of us, according to firm laws'.24 It was hardly surprising, Heisenberg implied, that Einstein found it impossible to accept a