Bell test experiment
A Bell test is a real experiment that pits quantum mechanics against the whole family of local hidden-variable theories. The recipe is to make many entangled pairs, send one particle to each of two distant stations, and at each station randomly choose between two measurement settings. By collecting the matched outcomes over many runs and tallying how often they agree, experimenters compute a correlation number and check whether it stays under the Bell bound or breaks it.
What makes such tests so striking is that the answer comes out on the side of quantum mechanics: the measured correlations exceed what any local, predetermined theory could allow, and they do so by margins of many standard deviations. The world simply does not behave as if each particle quietly carried a full set of answers fixed in advance and untouched by distant choices.
Bell tests have grown steadily more careful since the 1970s, using photon polarisation, trapped atoms, and superconducting circuits, and progressively closing the 'loopholes' by which a clever local theory might have slipped through. They are now among the most thoroughly confirmed results in physics, and they underpin practical technologies such as device-independent quantum cryptography, where security rests on a genuine violation of a Bell inequality.
A typical result: the correlation sits comfortably past the Bell bound, favouring quantum mechanics.
Early Bell tests left open 'loopholes' — gaps in detection or in how settings were chosen — that a determined local theory might have exploited. Closing them, especially all at once, was the achievement of the loophole-free experiments of 2015.