Our day-to-day reality is deterministic; things happen in specific locations and in a well-defined order. But all that nice, orderly behavior seems to go out the window in the quantum world, a fact that has prompted scientists to try to find ways of squeezing determinism back in. But now, one of the leading ideas for doing so—one that has been on shaky ground for years—may have been definitively ruled out.
The history of this issue dates back to when Einstein derided quantum mechanics for allowing "spooky action at a distance." In a paper where he famously argued his objections formally, he described how entanglement, where the states of two particles are inextricably linked, violates what we call "local realism." If we measure the state of one entangled particle, then we know the state of the other, even if it's halfway across the Universe.
Einstein felt this was ludicrous, but, as people started to devise experiments to test the ideas, it turned out to be correct. As far as we can tell, entanglement either means that reality isn't localized, or measurement triggers some event that is either instantaneous regardless of distance, or moves faster than the speed of light.
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