One of the phrases of Albert Einstein, a quotable physicist, that has leaked into the popular consciousness is “spooky action at a distance”. The derisive quotelet arose during the early days of quantum mechanics, a theory that powered a revolution in science that is still playing out. None of today’s gadgets, for example, could have been made without a deep understanding and exploitation of the theory’s basic tenets. Yet those ground rules come with other predictions so counterintuitive that Einstein came to think that the theory was missing something: what seemed odd was, he argued, just a reflection of a lack of knowledge. He railed in particular at the notion of “entanglement”, whereby two particles seem intimately linked (pictured, conceptually). But what is it, and what made him reckon it was spooky?
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