Albert Einstein, with his students Nathan Rosen and Boris Podolsky, were the first to point out these apparent connections; they used them to argue that the quantum theory must be incomplete. In an article known as the EPR paper4, published in 1935, they pointed out that by making a measurement of the momentum of one particle, it is possible to accurately guage the momentum of another with which it has previously interacted. This implies at least one of two things: Either the quantum theory is incomplete, and incorrect in its assertion that the second particle does not have a definite momentum before it is measured; or else the measurement of the first particle somehow determines the state of the other, however far away it is. Einstein called this 'spooky action at a distance' - spooky because there is no known mechanism for such an interaction, and because it would entail that things can be affected by events which, in some frame of reference, haven't happened yet.

– Physics & Philosophy group at Bristol University

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