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If a pair of photons or electrons becomes entangled, you could send them to opposite ends of the Galaxy, and measuring the properties of one will still give you information about the properties of the other. For example, if the pair conserves a quantum mechanical property called spin, when you measure one to have clockwise spin, you instantly know the other has counterclockwise spin.
Albert Einstein famously objected to quantum entanglement, framing the phenomenon as “spooky action at a distance.” Einstein, along with the scientists Boris Podolsky and Nathan Rosen, cited the phenomenon as evidence that quantum mechanics was an incomplete theory for explaining reality. They proposed that entangled particles must either communicate with each other faster than light, which is impossible according to Einstein’s Theory of Special Relativity, or have some kind of hidden, underlying properties that physicists have yet to find that explain their long-range connection.
Over the subsequent decades, quantum physicists like Aspect, Clauser, and Zeilinger have produced experimental evidence that challenges the assumptions underlying the Einstein-Podolsky-Rosen paradox. They’ve shown that both conditions are false. Somehow, in a way totally unlike ordinary objects, entangled particles must exhibit corresponding behaviors without communicating with each other or sharing hidden properties that exist before they are observed. Spooky indeed.
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