Next weirdness.[*] Two particles (say, two electrons in different shells of an atom) can become “entangled,” where their properties (such as their direction of spin) are linked and perfectly correlated. The correlation is always negative—if one electron spins in one direction, its coupled partner spins the opposite way. Fred Astaire steps forward with his left leg; Ginger Rogers steps back with her right. But it’s stranger than that. For starters, the two electrons don’t have to be in the same atom. They can be a few atoms apart. Okay, sure. Or, it turns out, they can be even farther apart.
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