A pair of gloves is put in two boxes, one in each. If one is right, the other must be left. That is not strange, because it was decided from the start.

But in the world of tiny particles, a pair can be linked in a stranger way. Their results are not decided until someone measures them. Three scientists tested this with experiments and won the Nobel Prize in 2022.

Quantum entanglement is a link between two particles: even when they are far apart, their measured results match in surprising ways. Einstein thought this was strange. In 1935, he argued that, like a pair of gloves in separate boxes, the results must be fixed in advance by hidden information.

In 1964, John Bell showed how to test this idea: if Einstein's view were correct, the results could only match up to a certain limit. Experiments by John Clauser in 1972 and Alain Aspect in 1982 went beyond that limit, supporting quantum mechanics. For this work, Aspect, Clauser, and Anton Zeilinger won the 2022 Nobel Prize in Physics.

In 1935, Einstein, Podolsky, and Rosen argued that quantum mechanics must be incomplete. If measuring one particle seemed to fix the state of a distant partner instantly, they reasoned, the outcomes must have been determined in advance by hidden variables, much as a glove in a sealed box is already left- or right-handed. For three decades this remained a philosophical dispute, until John Bell showed in 1964 that any such local theory places an upper limit on how strongly results measured at various angles can correlate. Experiments could therefore settle the question. Clauser and Freedman reported a violation of this limit in 1972, and Alain Aspect's team in 1982 switched measurement settings while photons were in flight, reducing the chance that the detectors were somehow coordinating.

Later work, including random setting choices by Anton Zeilinger's group and 2015 experiments that closed the main loopholes simultaneously, confirmed the quantum predictions. Bell's theorem rules out local hidden variables but not every hidden-variable theory, and entanglement cannot transmit information faster than light, because each individual outcome is random. The three physicists shared the 2022 Nobel Prize.