In 1935, Einstein and two of his colleagues, Podolsky and Rosen, proposed a thought experiment, subsequently called the EPR experiment, that was designed to show that quantum theory is incomplete and needs an underlying theory of hidden variables in order to tell the whole truth.
In the EPR experiment an atomic particle would split into two pieces, A and B, which would fly apart from each other. Since they originated from the same particle, A and B are correlated. They would have the same speed and same distance from their origin at any given instant—but in opposite directions. Quantum theory says that if we measure a particle’s velocity then we cannot measure its position, and furthermore, its position has no status in reality. Einstein conceded that we cannot measure the velocity and position at the same time, but that a particle possesses, nevertheless, both a real velocity and a real position at the same time.
We can determine A’s velocity by direct measurement (thereby rendering its position unknown) but determine its position indirectly by measuring B’s position (leaving B’s velocity unknown) since A and B are located the same distance from their origin but in opposite directions. We now know both the velocity and the position of each particle. What we can measure, directly or indirectly, is real, everybody would agree.
While he was at it, Einstein decided to go after the specters of particles. Since A and B are correlated, if A were to spin clockwise, B would have to spin counterclockwise. Now if you believe in particle ghosts, the A particle actually has a pair of clockwise and counterclockwise ghosts accompanying it, and so does B. If we take a measurement of A’s spin and by so doing promote its clockwise ghost into ...1
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