Quantum: Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar (2009)



'I thought a hundred times as much about the quantum problems as I have about general relativity theory', Einstein once admitted.1 Bohr's rejection of the existence of an objective reality as he tried to understand what quantum mechanics was telling him about the atomic world was a sure sign for Einstein that the theory contained, at best, only a part of the whole truth. The Dane insisted that there is no quantum reality beyond what is revealed by an experiment, an act of observation. 'To believe this is logically possible without contradiction,' Einstein conceded, 'but it is so very contrary to my scientific instinct that I cannot forgo the search for a more complete conception.'2 He continued to 'believe in the possibility of giving a model of reality which shall represent events themselves and not merely the probability of their occurrence'.3 Yet, in the end, he failed to refute Bohr's Copenhagen interpretation. 'About relativity he spoke with detachment, about quantum theory with passion', recalled Abraham Pais, who had known Einstein in Princeton.4 'The quantum was his demon.'

'I think I can safely say that nobody understands quantum mechanics', said the celebrated American Nobel laureate Richard Feynman in 1965, ten years after Einstein's death.5 With the Copenhagen interpretation as firmly established as the quantum orthodoxy as any papal edict issued ' from Rome, most physicists simply followed Feynman's advice. 'Do not keep asking yourself, if you can possibly avoid it, "but how can it be like that?"' he warned.6'Nobody knows how it can be like that.' Einstein never thought it was like that, but what would he have thought of Bell's theorem and the experiments showing that it tolled for him?

At the core of Einstein's physics was his unshakeable belief in a reality that exists 'out there' independently of whether or not it is observed. 'Does the moon exist only when you look at it?' he asked Abraham Pais in an attempt to highlight the absurdity of thinking otherwise.7 The reality that Einstein envisaged had locality and was governed by causal laws that it was the job of the physicist to discover. 'If one abandons the assumption that what exists in different parts of space has its own independent, real existence,' he told Max Born in 1948, 'then I simply cannot see what it is that physics is meant to describe.'8 Einstein believed in a realism, causality, and locality. Which, if any, would he have been prepared to sacrifice?

'God does not play dice', said Einstein memorably and often.9 Just like any modern-day advertising copywriter, he knew the value of an unforgettable tagline. It was his snappy denunciation of the Copenhagen interpretation and not a cornerstone of his scientific worldview. This was not always clear, even to someone like Born who knew him for almost half a century. It was Pauli who eventually explained to Born what really lay at the heart of Einstein's opposition to quantum mechanics.

During Pauli's two-month stay in Princeton in 1954, Einstein gave him a draft of a paper written by Born that touched on determinism. Pauli read it and wrote to his old boss that 'Einstein does not consider the concept of "determinism" to be as fundamental as it is frequently held to be.'10 It was something that Einstein told him 'emphatically many times' over the years.11 'Einstein's point of departure is "realistic" rather than "deterministic",' explained Pauli, 'which means that his philosophical prejudice is a different one.'12 By 'realistic' Pauli meant that Einstein assumed that electrons, for example, have pre-existing properties prior to any act of measurement. He accused Born of having 'erected some dummy Einstein for yourself, which you then knocked down with great pomp'.13 Surprisingly, Born, given their long friendship, had never fully grasped that what really troubled Einstein was not dice-playing, but the Copenhagen interpretation's 'renunciation of the representation of a reality thought of as independent of observation'.14

One possible reason for the misunderstanding may be that Einstein first said that God 'is not playing at dice' in December 1926 when he tried to convey to Born his unease at the role of probability and chance in quantum mechanics and the rejection of causality and determinism.15 Pauli, however, understood that Einstein's objections went far beyond the theory being expressed in the language of probability. 'In particular it seems to me misleading to bring the concept of determinism into the dispute with Einstein', he warned Born.16

At the heart of the problem,' wrote Einstein in 1950 of quantum mechanics, 'is not so much the question of causality but the question of realism.'17 For years he had hoped that he 'may yet work out the quantum puzzle without having to renounce the representation of reality'.18 For the man who discovered relativity, that reality had to be local, with no place for faster-than-light influences. The violation of Bell's inequality meant that if he wanted a quantum world that existed independently of observers, then Einstein would have had to give up locality.

Bell theorem cannot decide whether quantum mechanics is complete or not, but only between it and any local hidden variables theory. If quantum mechanics is correct – and Einstein believed it was, since it had passed every experimental test in his day – then Bell's theorem implied that any hidden variables theory that replicated its results had to be non-local. Bohr would have regarded, as others do, the results of Alain Aspect's experiments as support for the Copenhagen interpretation. Einstein would probably have accepted the validity of the results testing Bell's inequality without attempting to save local reality through one of the loopholes in these experiments that remained to be closed. However, there was another way out that Einstein might have accepted, even though some have said that it violates the spirit of relativity – the no signalling theorem.

It was discovered that it is impossible to exploit non-locality and quantum entanglement to communicate useful information instantaneously from one place to another, since any measurement of one particle of an entangled pair produces a completely random result. After performing such a measurement, an experimenter learns nothing more than the probabilities of the outcome of a possible measurement on the other entangled particle conducted at a distant location by a colleague. Reality may be non-local, allowing faster-than-light influences between entangled pairs of particles in separate locations, but it is benign, with no 'spooky communication at a distance'.

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 theory asserting that, on the atomic scale, 'this objective world of time and space did not even exist'.25 Born believed that Einstein 'could no longer take in certain new ideas in physics which contradicted his own firmly held philosophical convictions'.26 While acknowledging that his old friend had been 'a pioneer in the struggle for conquering the wilderness of quantum phenomena', the fact that 'he kept himself aloof and sceptical' about quantum mechanics, Born lamented, was a 'tragedy', as Einstein 'gropes his way in loneliness, and for us who miss our leader and standard-bearer'.27

As Einstein's influence waned, Bohr's grew. With missionaries like Heisenberg and Pauli spreading the message among their own flocks, the Copenhagen interpretation became synonymous with quantum mechanics. When he was a student in the 1960s, John Clauser was often told that Einstein and Schrödinger 'had become senile' and their opinions on matters quantum could not be trusted.28 'This gossip was repeated to me by a large number of well-known physicists from many different prestigious institutions', he recalled years after becoming the first to test Bell's inequality in 1972. In stark contrast, Bohr was deemed to possess almost supernatural powers of reasoning and intuition. Some have even suggested that while others needed to perform calculations, Bohr did not.29 Clauser recalled that during his student days 'open inquiry into the wonders and peculiarities of quantum mechanics' that went beyond the Copenhagen interpretation was 'virtually prohibited by the existence of various religious stigmas and social pressures, that taken together, amounted to an evangelical crusade against such thinking'.30 But there were unbelievers prepared to challenge the Copenhagen orthodoxy. One of them was Hugh Everett III.

When Einstein died in April 1955, Everett was 24 and studying for his master's degree at Princeton University. Two years later he obtained a PhD with a thesis entitled 'On the Foundations of Quantum Mechanics' in which he demonstrated that it was possible to treat each and every possible outcome of a quantum experiment as actually existing in a real world. According to Everett, for Schrödinger's cat trapped in its box this would mean that the moment the box was opened the universe would divide, leaving one universe in which the cat was dead and another in which it was still alive.

Everett called his interpretation the 'relative state formulation of quantum mechanics' and showed that his assumption that all quantum possibilities exist led to the same quantum mechanical predictions for the results of experiments as the Copenhagen interpretation.

Everett published his alternative in July 1957 with an accompanying note from his supervisor, the distinguished Princeton physicist John Wheeler. It was his very first paper and it went virtually unnoticed for more than a decade. By then disillusioned by the lack of interest, Everett had already left academia and was working for the Pentagon, applying game theory to strategic war planning.

'There is no question that there is an unseen world', the American film director Woody Allen once said. 'The problem is how far is it from midtown and how late is it open?'31 Unlike Allen, most physicists balked at the implications of accepting an infinite number of co-existing parallel alternative realities in which every conceivable outcome of every possible experimental result is realised. Sadly, Everett, who died of a heart attack aged 51 in 1982, did not live to see the 'many worlds interpretation', as it became known, taken seriously by quantum cosmologists as they struggled to explain the mystery of how the universe came into being. The many worlds interpretation allowed them to circumvent a problem to which the Copenhagen interpretation had no answer – what act of observation could possibly bring about the collapse of the wave function of the entire universe?

The Copenhagen interpretation requires an observer outside the universe to observe it, but since there is none – leaving God aside – the universe should never come into existence but remain forever in a superposition of many possibilities. This is the long-standing measurement problem writ large. Schrödinger's equation that describes quantum reality as a superposition of possibilities, and attaches a range of probabilities to each possibility, does not include the act of measurement. There are no observers in the mathematics of quantum mechanics. The theory says nothing about the collapse of the wave function, the sudden and discontinuous change of the state of a quantum system upon observation or measurement, when the possible becomes the actual. In Everett's many worlds interpretation there was no need for an observation or measurement to collapse the wave function, since each and every quantum possibility coexists as an actual reality in an array of parallel universes.

'This problem of getting the interpretation proved to be rather more difficult than just working out the equations', said Paul Dirac 50 years after the 1927 Solvay conference.32 The American Nobel laureate Murray Gell-Mann believes part of the reason was that 'Niels Bohr brain-washed a whole generation of physicists into believing that the problem had been solved'.33 A poll conducted in July 1999 during a conference on quantum physics held at Cambridge University revealed the answers of a new generation to the vexed question of interpretation.34 Of the 90 physicists polled, only four voted for the Copenhagen interpretation, but 30 favoured the modern version of Everett's many worlds.35 Significantly, 50 ticked the box labelled 'none of the above or undecided'.

The unresolved conceptual difficulties, such as the measurement problem and the inability to say exactly where the quantum world ends and the classical world of the everyday begins, have led to an increasing number of physicists willing to look for something deeper than quantum mechanics. 'A theory that yields "maybe" as an answer,' says the Dutch Nobel Prize-winning theorist Gerard 't Hooft, 'should be recognized as an inaccurate theory.'36 He believes the universe is deterministic, and is in search of a more fundamental theory that would account for all the strange, counterintuitive features of quantum mechanics. Others like Nicolas Gisin, a leading experimenter exploring entanglement, 'have no problem thinking that quantum theory is incomplete'.37

The emergence of other interpretations and the claim to completeness of quantum mechanics being in serious doubt have led to a reconsideration of the long-standing verdict against Einstein in his long-running debate with Bohr. 'Can it really be true that Einstein, in any significant sense, was as profoundly "wrong" as the followers of Bohr might maintain?' asks the British mathematician and physicist Sir Roger Penrose. 'I do not believe so. I would, myself, side strongly with Einstein in his belief in a submicroscopic reality, and with his conviction that present-day quantum mechanics is fundamentally incomplete.'38

Although he never managed to deliver a decisive blow in his encounters with Bohr, Einstein's challenge was sustained and thought-provoking. It encouraged men like Bohm, Bell and Everett to probe and evaluate Bohr's Copenhagen interpretation when it was all-prevailing and few distinguished theory from interpretation. The Einstein-Bohr debate about the nature of reality was the inspiration behind Bell's theorem. The testing of Bell's inequality directly or indirectly helped spawn new areas of research including quantum cryptography, quantum information theory, and quantum computing. Among the most remarkable of these new fields is quantum teleportation, which exploits the phenomena of entanglement. Although it appears to belong to the realm of science fiction, in 1997 not one but two teams of physicists succeeded in teleporting a particle. The particle was not physically transported, but its quantum state was transferred to a second particle located elsewhere, thereby effectively teleporting the initial particle from one place to another.

After having been marginalised during the last 30 years of his life because of his criticism of the Copenhagen interpretation and his attempts to slay his quantum demon, Einstein has been vindicated, in part. Einstein versus Bohr had little to do with the equations and numbers generated by the mathematics of quantum mechanics. What does quantum mechanics mean? What does it say about the nature of reality? It was their answers to these types of questions that separated the two men. Einstein never put forward an interpretation of his own, because he was not trying to shape his philosophy to fit a physical theory. Instead he used his belief in an observer-independent reality to assess quantum mechanics and found the theory wanting.

In December 1900, classical physics had a place for everything and almost everything in its place. Then Max Planck stumbled across the quantum, and physicists are still struggling to come to terms with it. Fifty long years of 'conscious brooding', said Einstein, had not brought him any closer to understanding the quantum.39 He kept trying to the end, taking solace in the words of the German playwright and philosopher Gotthold Lessing: 'The aspiration to truth is more precious than its assured possession.'40