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Published online by Cambridge University Press: 31 January 2023
Bell (1964) demonstrated that if two restrictions are imposed on the hypothetical hidden variables supposed to underlie quantum mechanical states, then it is possible to derive an inequality that is violated by certain predictions of QM (Quantum Mechanics); the predictions concern pairs of systems whose states are strongly correlated. The two restrictions are denoted herein as SL (Strong Locality) and HA (Hidden Autonomy)1, and the inequality as BI (the Bell Inequality). Since SL and HA together entail BI, and QM violates BI, it follows that any HVT (Hidden-Variables Theory) for QM must violate either SL or HA.
It is well known that a condition known as PA (Perfect Anti-correlation) was also used by Bell to derive the inequality. PA says that parallel experiments (experiments with parallel spin-analyzer orientations) never yield the same results.