The Ironic Many-Worlds Interpretation of Quantum Theory

Recent results, such as the Pusey-Barrett-Rudolph theorem, imply that a realist interpretation of quantum theory needs to include an objectively real quantum state if it is to fit into the standard ontological models framework. Adopting a more exotic ontology allows one to evade the conclusions of these results and possibly construct a realist interpretation in which the quantum state just represents knowledge or information (known as a ψ-epistemic interpretation). In this talk, I will describe such an interpretation based on modifying the Everett/many-worlds interpretation. The basic idea is that the branching structure of "worlds" in Everett does not depend on the amplitudes assigned to the branches, so these can be epistemic while retaining the usual decoherence account of the emergence of worlds. One can do this in such a way that there is still enough structure left to derive the Born rule, now interpreted in a subjective Bayesian way similar to how it is viewed in QBism. In addition to providing a concrete example of a realist ψ-epistemic interpretation, this solves a conceptual puzzle about probability in Everett due to Adrian Kent, known as the "numbers in the sky" objection.