What did Einstein call "spooky action at a distance"?

Einstein's 'spooky action at a distance' was his mocking name for quantum entanglement. Measure one of a pair of entangled particles, and the other's result lines up, however far away it is. Make two particles with a total spin of zero. If one is found spinning clockwise on some axis, the other is always found spinning counterclockwise on that axis, even a continent away.

In 1935 Einstein, Boris Podolsky and Nathan Rosen argued that this was impossible. So, they said, quantum mechanics must be an incomplete theory. Erwin Schrödinger, who coined the word 'entanglement', was uneasy too. Yet he called it the characteristic trait of quantum mechanics, the one that breaks with classical thinking.

For decades the argument stayed philosophical. Then, in 1964, John Bell found a way to test it. He worked out a limit, now called Bell's inequality. It caps how strongly two particles can be correlated if each one secretly carries its own answers from the start. Quantum mechanics predicted entangled particles would break that limit. Experiments by Stuart Freedman and John Clauser in 1972, and by Alain Aspect in 1982, showed that they do. Clauser, Aspect and Anton Zeilinger shared the 2022 Nobel Prize in Physics for this work.

None of this lets anyone send a signal faster than light. Each measurement on an entangled particle still comes out random. You can't choose what you'll see. You can only compare notes later, by ordinary means, and find the results matched. The correlation is real, but it can't carry a message.