How to Use Quantum Theory Locally to Explain EPR-Bell Correlations

Marlan O. Scully, Noam Erez A, Edward S. Fry

Richard Healey

Abstract This essay sketches a pragmatist interpretation of quantum theory and shows how to use it to explain EPR-Bell correlations consistently with relativity. Quantum theory is not a locally causal theory, not because it violates Bell’s local causality condition, but because that condition is simply inapplicable to it. Any agent can use quantum theory to show why EPR-Bell correlations are to be expected. For space-like separated measurements of vertical/horizontal polarization of each photon from a pair in Bell state Φ+, an agent’s explanation of why the distant measurement outcome matches his own appeals neither to a preferred frame nor to any direct connection or influence between these events. Here, as elsewhere, quantum theory helps one explain an initially puzzling phenomenon not by locating it in a causal net but by showing why its occurrence is just what one should have expected in the circumstances.

Jay Lawrence

We present a local unitary theory of a Bell-EPR measurement, starting with the premeasurement filtering of the individual photon polarizations and extending through the detection process involving four photodetectors, two at each receiving station. The essential feature is that decoherence occurs locally and independently with each detector upon its absorption of a photon. Communication between observers after they read their local outcomes confirms the known Bell-EPR correlations. This theory is manifestly local, but there exist other formulations, and interpretations, that are non-local.

Travis Norsen

J.S. Bell's remarkable 1964 theorem showed that any theory sharing the empirical predictions of orthodox quantum mechanics would have to exhibit a surprising – and, from the point of view of relativity theory, very troubling – kind of nonlocality. Unfortunately, even still on this 50th anniversary, many commentators and textbook authors continue to misrepresent Bell's theorem. In particular, one continues to hear the claim that Bell's result leaves open the option of concluding either nonlocality or the failure of some unorthodox “hidden variable” (or “determinism” or “realism”) premise. This mistaken claim is often based on a failure to appreciate the role of the earlier 1935 argument of Einstein, Podolsky, and Rosen in Bell's reasoning. After briefly reviewing this situation, I turn to two alternative versions of quantum theory – the “many worlds” theory of Everett and the quantum Bayesian interpretation of Fuchs, Schack, Caves, and Mermin – that purport to provide actual counterexamples to Bell's claim that nonlocality is required to account for the empirically verified quantum predictions. After analyzing each theory's grounds for claiming to explain the EPR–Bell correlations locally, however, one can see that (despite a number of fundamental differences) the two theories share a common for-all-practical-purposes (FAPP) solipsistic character. This dramatically undermines such theories’ claims to provide a local explanation of the correlations and thus, by concretizing the ridiculous philosophical lengths to which one must go to elude Bell's own conclusion, reinforces the assertion that nonlocality really is required to coherently explain the empirical data. Introduction Fifty years ago, in 1964 [1], John Stewart Bell first proved the theorem that has become widely known as “Bell's Theorem” but that Bell himself instead referred to as the “locality inequality theorem” [2]. In Bell's own view, the theorem showed that the empirical predictions of local theories will be constrained by Bell's inequality (or as Bell himself preferred to call it, the “locality inequality”). Hence, nonlocality is a necessary feature of any theory that shares the empirical predictions of standard quantum mechanics.

Ravi K. Menon, V. K. Thankappan

M. A. B. Whitaker, J.G. Dennison