Quantum entanglement: spooky action explained

Measure one particle here, and its partner — on the other side of the galaxy — answers the question in the same instant. No signal travels between them. Nothing crosses the gap. The correlation is simply there.

Einstein hated this. He called it "spooky action at a distance" and spent the last decades of his life insisting the universe could not work that way. There had to be, he argued, a hidden variable: some property the two particles secretly carried with them since their shared origin, predetermining how they would respond.

In 1964, John Bell devised a mathematical test that could tell the difference between hidden variables and genuine quantum weirdness. Experiments since the 1970s — culminating in the work that won the 2022 Nobel Prize in Physics — have ruled the hidden variables out. The particles are not carrying secret answers. The correlation is real, instantaneous, and impossible to use for sending a message.

Entanglement does not let anything travel faster than light. It is stranger than that: two particles that are, in some deep sense, no longer separate things at all.