Bell's inequality

In Youtube, I found a video which brilliantly explains Bell's inequality or theorem and how it was eventually proven experimentally. Back in the 1920s and 30's physicists discovered that particles from a common source were entangled at the point of measurement.  For instance when you measure spin direction in one electron of a pair of electrons from an atom, the spin of its twin will always be opposite.  For example if one electron was spin up the other would be spin down.  This is quantum entanglement.  Before they were measured both electrons were in both spin up and spin down, this is known supposition and is best explained by the Schrodinger's cat thought experiment. This entanglement applied no matter how far the two particles were from each other, they could even be on different ends of the universe but the quantum entanglement still applied.

However Albert Einstein, along with fellow physicists Boris Podolsky and Nathan Rosen, pointed out that for this to work there must be faster than light communication between the two particles.  And this cannot be since it breaks Einstein's special relativity that nothing can go faster than the speed of light.  This became known as the EPR paradox.  Einstein and his colleagues had pointed out a fundamental flaw with quantum physics and he described entanglement as "spooky action at a distance". Insead Einstein believed that the spin states of these particles were already set before measurement and they only become known to us at the point of measurement.  These are known as 'local hidden variables' and they imply that nature is pre-set in some ways.  Einstein characterized this by having two boxes and a pair of gloves, and each box contains a glove each.  When you open a box, not only do you see which hand the glove fits but you instantly know which hand is in the other box. So which was correct, quantum physics or local hidden variables?

For decades this issue lingered on, until 1964, when a Northern Irish scientist, John Bell, published a study on how to resolve this.  He came up with an experiment which tests local hidden variables of entangled particles.  Each entangled particle is sent to a polarizer to measure the spin and if the particle's spin and detector agree then the particle is allowed through, it not, it is blocked.  These detectors can be set to varying angles so we can measure the probability of a particle getting through or not.  Bell theorized that if hidden variables exist then there is a minimum probability of the two detectors having the same result.  This probability was found to be 33%, if the result go below this probability then hidden variable theory would be false and quantum theory would be correct.  All this became known as Bell's inequality or Bell's theorem.

It was not until 1972 that Freedman and Clauser in the USA, had the right equipment to perform this test, they found the results in favour of quantum physics.  Later on in 1983, French phyisicist Alain Aspect performed a much rigorous version of this experiment and found that the results proved Bell, and quantum mechanics itself, correct and the probability of both detectors recording the same result was below 33%.

So there is some kind of faster than light communication or relationship between 2 entangled particles.  How it comes about is another matter and yet to be explained. Further John Bell had taken on a question posed by one of the greatest scientists ever, Einstein, and won.