Physics
Bell Inequalities
Quick fact
John Bell derived his famous inequality in 1964, but it took nearly two decades before experimental violations were conclusively observed, earning the 2022 Nobel Prize in Physics for Aspect, Clauser, and Zeilinger.
Why this is interesting
Imagine two dice that always show opposite numbers, no matter how far apart they are—could they be secretly coordinated? This gut feeling is what Bell inequalities challenge, revealing a weirdness at the heart of reality.
Read the full explanation
Understanding Bell Inequalities
Bell inequalities are rules that any theory based on two commonsense ideas must obey: 'locality' (objects are only directly influenced by their immediate surroundings) and 'realism' (objects have definite properties before measurement). Imagine you and a friend each have a box that flashes either red or blue when you press a button. If the boxes are 'local realistic,' the probabilities of seeing certain combos follow a limit. But quantum mechanics predicts—and experiments prove—that entangled particles can exceed this limit, forcing us to abandon at least one of those intuitive assumptions.
A deeper explanation
Bell's theorem shows mathematically that if local realism were true, the correlations between measurements on entangled particles would be bounded by a specific inequality. Quantum mechanics, however, predicts stronger correlations that violate this bound. Experiments with photons, ions, and other systems consistently violate Bell inequalities, confirming quantum predictions. The mechanism is quantum entanglement: when two particles are entangled, their states are linked such that measuring one instantly determines the outcome of the other, regardless of distance. This 'nonlocal' correlation has no classical analogue and is harnessed in quantum technologies. The violation of Bell inequalities proves that the universe is fundamentally nonlocal—spooky action at a distance is real.