Bell's theorem shows that no local hidden-variable theory can reproduce every quantum prediction.
John Stewart Bell published the result in 1964. He derived inequalities that local hidden-variable theories must satisfy when measurements are made on entangled systems. Quantum mechanics predicts—and experiments observe—correlations that can violate those inequalities. The result sharply separates quantum entanglement from the idea that particles merely carry pre-existing local instructions.
Bell's theorem does not allow usable faster-than-light messaging. Although entangled measurement results show strong correlations, the individual outcomes remain unpredictable, so they cannot be controlled to transmit a chosen signal. Experiments have progressively closed important loopholes, including major tests associated with Alain Aspect, John Clauser, and Anton Zeilinger, who received the 2022 Nobel Prize in Physics. The theorem concerns locality and hidden variables, not uncertainty alone.