Why the Many Worlds Interpretation of Quantum Mechanics Needs More Than Hilbert Space Structure

n their contribution to this book, McQueen and Vaidman argue that common sense requires that explanations in physics be not only causal1 but also local. Their main claim is that the Many Worlds Interpretation (MWI) of quantum mechanics (originally due to Everett) provides local explanations of the outcomes of experiments that in other interpretations of quantum mechanics seem to require (some sort of) nonlocality. Particles in the MWI leave traces both in the world where they exist but also in other worlds in which they do not exist. So the explanation is said to be local because spacetime splits together with the particles so that there is no influence at space-like separation within a world, but the causal influence literally travels across ‘parallel’ worlds. This seems to stretch the concept of locality beyond common sense, if not beyond breaking point, even if one understands causation weakly in terms of counterfactual dependence rather than by straightforward physical interaction.