Two couriers leave the same city in opposite directions. An hour later, each opens an envelope: one contains a slip marked “plus,” the other “minus.” How did they know who would get which?
Two particles separated by kilometers change their states in concert, as if there were no distance between them. This is called quantum entanglement, and since its discovery it has baffled physicists: no known carrier can travel faster than light, yet correlations appear instantaneously.