John Stewart Bell is one of my favourite physicists. His day job involved accelerator physics at CERN, but he also dabbled in quantum fundamentals, and it was via that dabbling that he settled a dispute that traces all the way back to Einstein.

It relates to quantum entanglement. This is where two (or more) quantum particles in close proximity entangle such that they are forevermore interrelated. If you now make a measurement on one of those particles, a measurement on the other particle will be *instantly* correlated, even if it's billions of light years away.

Hold on, said Einstein in 1935, this is nonsense, this correlation happens faster than light, which is a no-no. Einstein called it "spooky action at a distance" and, along with Boris Podolsky and Nathan Rosen, two other physicists, he released a paper that pointed out how ludicrous this was. It's referred to as the EPR paper, after the initials of the three physicists.

Einstein suggested that there must be hidden variables involved, and that these hidden variables in a sense travel with the entangled particles. Nothing travels faster than light. Instead the particles had other, hidden properties that are set at the moment of entanglement and that, when the particles are separated by billions of light years, they correlate because they've been correlated from the moment of entanglement. He therefore suggested that quantum mechanics was an incomplete theory, and it would remain so until they discovered and described these hidden variables.

Einstein favoured what is called the *principle of locality*, which holds that one thing affects another thing via some intermediary field that travels at a finite speed, and that the aforementioned 'spookiness' couldn't happen.

The quantum physicists in the 1930s said Einstein was wrong, that quantum mechanics was a probabilistic theory and that entangled particles had no predetermined properties until they were measured. This being the case, the correlation upon measurement must be instantaneous.

There was a stand-off. The quantum physicists were sure they were right, but struggled to prove it.

Then in 1964 along came John Stewart Bell. Bell was a big fan of Einstein, saying:

I felt that Einstein’s intellectual superiority over Bohr, in this instance, was enormous; a vast gulf between the man who saw clearly what was needed, and the obscurantist.

He would have preferred to prove Einstein correct, but his 1964 paper '*On the Einstein-Podolsky-Rosen Paradox*' proved Einstein wrong. Bell made use of some clever statistics to show that if the hidden variables idea is true it would not match what is seen in quantum mechanics. What we observe could not be due to hidden variables.

Bell's theory, later refined by other physicists, has since been proven experimentally. There are no hidden variables and quantum mechanics is indeed non-local. The article I link to explains it in more detail.