John Stuart Bell in 1964 devised a statistical test that could distinguish between the world views of quantum mechanics and objective realism. Subsequent experiments using correlated photons, notably those of Clauser, Horne, Shimony, and Holt and, most definitively, by Alain Aspect and coworkers, demonstrate convincingly that quantum mechanics is correct. This result can be summarized as Bell's theorem: No physical theory of local hidden variables can ever reproduce all the predictions of quantum mechanics. Henry Stapp regards Bell's theorem as the most significant development in science (not just physics) of the twentieth century.

This Demonstration describes a simplified version of Aspect's experiment. The little cube in the center emits pairs of correlated photons in opposite directions toward two polarizers, oriented at angles and . A coincidence counter at the bottom monitors the number of photon pairs that get through both polarizers and are detected. The counter tabulates the coincidence fraction for several hundred or several thousand emitted photon pairs for a given setting of the polarizers and displays this result. The upper part of the diagram shows the actual experimental results while the lower part, enclosed in a gray philosophical haze, shows the expectations of objective reality. Except for a few special orientations, these results differ.