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Locally Causal Quantum Theory and the Collapse Locality Loophole
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Abstract: Locally causal quantum theory is an umbrella term for ordinary quantum theory modified by two hypotheses: state vector reduction is a well-defined process, and strict local causality applies. The first of these holds in some versions of Copenhagen quantum theory and need not necessarily imply practically testable deviations from ordinary quantum theory. The second implies that measurement events which are spacelike separated have no non-local correlations. To test this prediction, which sharply differs from standard quantum theory, requires a precise definition of state vector reduction. Formally speaking, any precise version of locally causal quantum theory defines a local hidden variable theory. However, locally causal quantum theory is most naturally seen as a variant of standard quantum theory. For that reason it seems a more serious rival to standard quantum theory than local hidden variable models relying on the locality or detector efficiency loopholes. Some plausible versions of locally causal quantum theory are not refuted by any Bell experiments to date, nor is it evident that they are inconsistent with other experiments. They evade refutation via a neglected loophole in Bell experiments - the collapse locality loophole - which exists because of the possible time lag between a particle entering a measurement device and a collapse taking place. Fairly definitive tests of locally causal versus standard quantum theory could be made by observing entangled particles separated by 0.1 light seconds.
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