In what way are the quantum correlations in an entangled quantum state
different from correlations in a classical system? If a measurement of a quan-
tum state yields a probabilistic outcome, could we not assume that the ob-
servable measured has a definite value that was merely uncovered by the
measurement? Then the probabilities encoded in a quantum state would be
like classical probabilities, in that they indicate the lack of knowledge we have
about the system. The correlations we just saw in the entangled pair would be
just like those in classical systems. For instance, say I have a bag with pairs of
socks of random colors, each in paired, unlabelled packets. Now suppose you
pull out a packet at random, and give one of the pair each to Alice and Bob.
If Alice finds she got a red sock then immediately she can tell that Bob has
a red sock too! Perfect correlation! As another example, if Alice found a left
sock then she knows Bob has a right sock, without Bob looking at his sock:
perfect anticorrelation.
In what way is this (anti)correlation different when we talk about a pair
of quantum particles in a Bell state? Can it not be that the particles simply
possess spin values that are the same (or opposite, depending on the state),
and the measurements just discover these values? The question here is a subtle
one: we will ask it again in a different way. Can these correlations between en-
tangled particles be explained by some hidden properties that are not evident
in quantum theory, that are assigned specific values at the time of production?
This feature has been thoroughly examined by many scholars. Most prac-
tising physicists follow the practical school of thought, known as the Copen-
hagen School, so-called after the city of famous physicist Niels Bohr, its
main proponent. According to this school, Nature is nothing more than the
experimental results, and there is no place for assumed hidden properties.
In other words, the spin of the atom in the entangled pair doesn’t have an
objective existence until brought into being by a measurement
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