Bell's Theorem and Locally-Mediated Reformulations of Quantum Mechanics

Bell's Theorem rules out many potential reformulations of quantum mechanics,but within a generalized framework, it does not exclude all "locally-mediated"models. Such models describe the correlations between entangled particles asmediated by intermediate parameters which track the particle world-lines andrespect Lorentz covariance. These locally-mediated models require therelaxation of an arrow-of-time assumption which is typically taken for granted.Specifically, some of the mediating parameters in these models mustfunctionally depend on measurement settings in their future, i.e., on inputparameters associated with later times. This option (often called"retrocausal") has been repeatedly pointed out in the literature, but theexploration of explicit locally-mediated toy-models capable of describingspecific entanglement phenomena has begun only in the past decade. A briefsurvey of such models is included here. These models provide a continuous andconsistent description of events associated with spacetime locations, withaspects that are solved "all-at-once" rather than unfolding from the past tothe future. The tension between quantum mechanics and relativity which isusually associated with Bell's Theorem does not occur here. Unlike conventionalquantum models, the number of parameters needed to specify the state of asystem does not grow exponentially with the number of entangled particles. Thepromise of generalizing such models to account for all quantum phenomena isidentified as a grand challenge.