NONLOCAL CORRELATION by two particles is demonstrated in the Franson experiment which sends two photons to separate but identical interferometer. Each photon may take a short route or a longer 'detour' at the first beam splitter They may leave through the upper or lower exit ports. A detector looks at the photons leaving the upper exit ports. Before entering its interferometer, neither photon knows which way it will go. After leaving, each knows instantly and nonlocally what its twin has done and so behaves accordingly. Although in these experiments the photons were separated by only a few feet, quantum mechanics predicts that the correlations would have been observed no matter how far apart the two interferometers were.

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One of the oddities of quantum particles is that their properties only take on definite values when measured. The electron and positron, for instance, are both effectively spinning. Either particle's spin is equally likely to be clockwise (known as "up") or anticlockwise ("down")-but you won't know which unless you measure it. Until that measurement is made the particle is in a weird indefinite state, a "superposition" of both spins. What is definite, however, is that in an entangled state, the spins of the two particles are intimately linked. Since the original pion had no spin, the positron and electron must always spin in opposite senses so that their net spin remains zero. If you find the electron's spin to be "up", you'll find the positron's to be "down", and vice versa. So it is as if the two entangled particles, no matter how far they are apart, are not really separate at all. Measure one, and as its spin becomes definite this triggers the other to respond. Its indeterminate spin also becomes definite, in the opposite direction to that of its partner. What is astonishing and disturbing is that this response happens instantaneously even if the particles are separated by huge distances. Consequently, quantum theory requires action at a distance. What happens in one part of the Universe can have instantaneous "nonlocal" consequences in other parts, no matter how far away they might be. And this poses a problem, because instantaneous action at a distance is a punch in the nose for Einstein. His theory of relativity-the comerstone of physics -claims that our Universe has an absolute speed limit.

Nothing, according to Einstein, can travel faster than light. So you might wonder-do we really need to swallow this nonlocal quantum weirdness? Perhaps there is a better theory that accounts for these entanglements without action at a distance? Think of this: if someone separated a pair of your shoes by a great distance and then weighed one, they would immediately have a good estimate of the weight of the other. There's no mystery here. Nothing nonlocal. Shoes have weight. And if they come from a pair, their weights are correlated from the outset. Could something similar be true for entangled particle pairs? Despite what quantum theory says, perhaps the particles do have definite spins, arranged oppositely at all times, and measurements merely reflect this pre-existing situation. This is an obvious possibility. It might even be true. The trouble is, it doesn't cushion the blow for relativity. In 1964, physicist John Bell of CERN, the European Laboratory for Particle Physics, examined this line of argument in detail and proved a famous theorem which fellow physicist Henry Stapp of the Lawrence Berkeley Laboratory in California calls "the greatest discovery of all science". Bell first supposed that quantum theory doesn't say all there is to say about quantum particles. He then proved that if any more complete theory - any theory imaginable - were to give predictions in agreement with quantum theory, it would necessarily still contain the same kind of nonlocal influences as ordinary quantum theory. "What Bell gave us," says philosopher David Albert of Columbia University in New York, "is a proof that there is a genuine nonlocality in the workings of nature, however we attempt to describe it, period." Every conceivable story about entangled states has to be nonlocal. There is no escape. Unless, of course, entangled states don't really exist, and quantum theory is wrong.

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