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Hidden variables and non-locality
The critical advance in theory came from Irish physicist, John Bell, whose 1965 theorem, among other important predictions, showed that in order to be compatible with quantum theory, hidden variables must be non-local. This was contrary to the criticism of quantum theory by Einstein who insisted that the theory was incomplete, that there must be undiscovered "hidden variables" that would complete the theory and make its extraordinary results rational. To his dying day, Einstein would not accept the concept of non-local signals. For him, all had to be predictable, determinist, and within the boundaries set by local signaling, the speed of light being the upper limit.
Einstein was long dead before technology advanced sufficiently to permit the concept of non-locality to be put to empirical testing.
Though Bell's theorem had been scrutinized in the laboratory and had given some positive results, it was only in the year 1982 that incontrovertible evidence, acceptable to peer scrutiny, became available through the work of a French group of physicists led by Alain Aspect.
This French group took advantage of the fact that a radioactive isotope of calcium emitted twin pairs of correlated photons in opposite directions. Being correlated means that they share certain properties such that if the magnitude of such a property for one of the twins is known, that for the other can also be determined.
The result of their experiments was to show that whatever happened to one of the correlated photons affected its twin even though no signal at light speed or less could pass between them-- implying that instantaneous communication somehow occurred and would still do so even if the photons were at opposite ends of the universe..
Thus the criticism by Einstein and co-workers, Podolsky and Rosen, regarding hidden variables and correlated properties was shown by Aspect's group to be entirely wrong. It also established the reality beyond doubt of the phenomenon of non-locality as well as demonstrating that if hidden variables existed they must be non-local--that is in a transcendent dimension outside of our space-time..
The Aspect experiment has since been confirmed by independent workers, one such group being in Switzerland where the optical fiber system between two villages separated by a high mountain was utilized. The distance between them was about 15 kilometers. But that was some time back. The record is probably very much larger at the present.
Among the many confirmatory experiments demonstrating the reality of non-local effects is a group of optical tests that "raced" twin photons to a target, one of which had to tunnel through a barrier placed in its path. Curiously, the photon tunneling through the barrier arrived at the target before its twin (which traveled at the speed of light.) For the twin that tunneled through the barrier, the average tunneling velocity was 1.7 times that of light, so a "non-local" effect. A second curiosity was that the twin doing the tunneling was able to "sense" the far side of the barrier and cross it in the same amount of time, no matter how thick the barrier was made. (see Chiao et al. 1993)
Query: how does a mere photon "sense" the thickness of a barrier?
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