Einstein called it 'spooky action at a distance' — and tried to prove quantum theory wrong
In 1935, Einstein, Boris Podolsky, and Nathan Rosen described a thought experiment with two particles that stay correlated no matter how far apart they travel; measuring one instantly seems to affect the other. Einstein argued this 'spooky action' proved quantum mechanics was incomplete. Decades later, experiments by physicists including Alain Aspect confirmed quantum mechanics was right all along, and the correlation is real, not a measurement trick.