Remarks on Space-Time and Locality in Everett’s Interpretation

Roland E. Allen

Mark A. Rubin

Mark A. Rubin

Paul A. Benioff

Meir Hemmo, Orly Shenker

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.

Samuel Kuypers, David Deutsch

Everett's relative-state construction in quantum theory has never been satisfactorily expressed in the Heisenberg picture. What one might have expected to be a straightforward process was impeded by conceptual and technical problems that we solve here. The result is a construction which, unlike Everett's one in the Schrödinger picture, makes manifest the locality of Everettian multiplicity, its inherently approximative nature and its origin in certain kinds of entanglement and locally inaccessible information. (By Everettian , we are referring not only to Everett's own work, but also to versions of quantum theory that elaborate and refine his. The notion of relative states first appeared in Everett (Everett 1973 In The many worlds interpretation of quantum mechanics (eds BS DeWitt, N Graham)). We are proposing a formalism for relative states that is more detailed and more illuminating than Everett's.) Our construction also allows us to give a more precise definition of an Everett ‘universe’, under which it is fully quantum, not quasi-classical, and we compare the Everettian decomposition of a quantum state with the foliation of a space–time.