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hung in the balance, every possible loophole, however improbable, had to be considered. For example, the possibility that the detectors might somehow be signalling to each other was later eliminated by the random switching of their orientation while the photons were in mid-flight. Although it fell short of being the definitive experiment, further refinements and other investigations in the years since have led to Aspect's original results being confirmed. Although no experiment has been conducted in which every possible loophole is closed, most physicists accept that Bell's inequality has been violated.

Bell derived the inequality from just two assumptions. First, there exists an observer-independent reality. This translates into a particle having a well-defined property such as spin before it is measured. Second, locality is preserved. There is no faster-than-light influence, so that what happens here cannot possibly instantaneously affect what happens way over there. Aspect's results mean that one of these two assumptions has to be given up, but which one? Bell was prepared to give up locality. 'One wants to be able to take a realistic view of the world, to talk about the world as if it is really there, even when it is not being observed', he said.52

Bell, who died in October 1990 at the age of 62 from a brain haemorrhage, was convinced that 'quantum theory is only a temporary expedient' that would eventually be replaced by a better theory.53 Nevertheless, he conceded that experiments had shown that 'Einstein's world view is not tenable'.54 Bell's theorem tolled for Einstein and local reality.

Chapter 15

THE QUANTUM DEMON

'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