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Quantum Mechanics
Closing the EinsteinBohr EPR debate: the 1972 experiment by Clauser and Freedman tests Bell's theorem: physical systems obey quantum mechanics

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from http://www.quantumphysicslady.org/glossary/quantumentanglement/
Einstein was a great believer in realism. He is quoted as saying, “I think that a particle must have a separate reality independent of the measurements. That is, an electron has spin, location and so forth even when it is not being measured. I like to think that the moon is there even if I am not looking at it.”
So, the mathematics of entanglement seemed to violate both locality and realism unless hidden variables could be found that would explain correlations of distant particles. At the time that the EPR paper was written, it was not possible to run an experiment that would test whether entanglement actually occurred. The experimental equipment was not sufficiently refined. And it was not possible to test for hidden variables.
In 1964, the physicist, John Bell, published a mathematical paper proposing a way to test for whether hidden variables could account for entanglement. His work is called “Bell’s Theorem” or “Bell’s Inequalities.” In 1972, John Clauser and Stuart Freedman put Bell’s mathematical work to the test. They created an experiment that tested for entanglement and whether hidden variables were at play. Their experiment confirmed the reality of entanglement. They also found that, according to Bell’s mathematics, hidden variables could not explain the correlations in the behavior of entangled particles.
from https://en.wikipedia.org/wiki/Bell%27s_theorem
The first rudimentary experiment designed to test Bell's theorem was performed in 1972 by John Clauser and Stuart Freedman; more advanced experiments, known collectively as Bell tests, have been performed many times since. Often, these experiments have had the goal of "closing loopholes", that is, ameliorating problems of experimental design or setup that could in principle affect the validity of the findings of earlier Bell tests. To date, Bell tests have consistently found that physical systems obey quantum mechanics and violate Bell inequalities; which is to say that the results of these experiments are incompatible with any local hidden variable theory.
