Erwin Schrodinger called it the "defining trait" of quantum theory. Einstein could not bring himself to believe in it at all, thinking it proof that quantum theory was seriously buggy. It is entanglement: the idea that particles can be linked in such a way that changing the quantum state of one instantaneously affects the other, even if they are light years apart.
This "spooky action at a distance", in Einstein's words, is a serious blow to our conception of how the world works. In 1964, physicist John Bell of the European Organization for Nuclear Research (CERN) in Geneva, Switzerland, showed just how serious. He calculated a mathematical inequality that encapsulated the maximum correlation between the states of remote particles in experiments in which three "reasonable" conditions hold: that experimenters have free will in setting things up as they want; that the particle properties being measured are real and pre-existing, not just popping up at the time of measurement; and that no influence travels faster than the speed of light, the cosmic speed limit.
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