Local Everettianism and Asymmetric Relative Statehood

This article disputes the consistency of Everettian quantum mechanics with locality in the context of certain entangled measurements of spin. It considers the ontological conception of locality provided by Timpson and Brown, whose prominent explanatory framework aims to show that the conditions of this definition are met by their Everettian reading of a canonical EPR-style setup. It then argues that this reading fails to generalise to more complex entangled spin-measurements. It presents by way of a counter-example a thought-experiment in which the definiteness of the relative state of a given subsystem is influenced by measurements performed at space-like separation. These measurements qualify the theory as nonlocal, except at exorbitant conceptual costs, such as the repudiation of realism. Four arguments are provided by way of justification. This result undermines the claim that Everettian quantum mechanics is compatible with a substantive notion of locality provided by Timpson, Brown, Wallace and other authors.