Bell's theorem

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In theoretical physics, Bell's theorem (AKA Bell's inequality) is a no-go theorem, loosely stating that:

It is the most famous legacy of the late physicist John S. Bell.

Bell's theorem has important implications for physics and the philosophy of science as it indicates that every quantum theory must violate either locality or counterfactual definiteness. In conjunction with the experiments verifying the quantum mechanical predictions of Bell-type systems, Bell's theorem demonstrates that certain quantum effects travel faster than light and therefore restricts the class of tenable hidden variable theories to the nonlocal variety.



Bell’s theorem shows that the local hidden variable interpretation of quantum mechanics, also known as local realism, necessitates certain conditions which are violated by measurements performed on entangled quantum systems. A variety of physical experiments have verified the quantum mechanical predictions, thus ruling out the local hidden variable theories while also proving the existence of superluminal effects. The theorem has several variations, but the most common involve systems of 2 Spin-½ particles (or equivalently, any pair of 2-state quantum systems or qubits), which were used in the original proof and will be the primary focus of this article.

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