Why I Am Not an Everettian

Abstract

Everettian quantum mechanics (EQM) results in “multiple, emergent, branching quasi-classical realities” (Wallace (2009)). The possible outcomes of measurement as per ‘orthodox’ quantum mechanics, are, in EQM, all instantiated. Given this metaphysics, Everettians face the ‘probability problem’ – how to make sense of probabilities and recover the Born Rule. To solve the probability problem, Everettians have derived a quantum representation theorem. There is a notable argument against the soundness of the representation theorem based on so-called ‘branch counting’. Everettians have sought to undercut this argument by claiming that there is no such thing as the number of branches. In what sense is it both true that there is no such thing as the number of branches and that there are multiple branches? Various answers to this question have been given. These can be categorised into two kinds: that there are ‘indeterminately-many’ branches or that there are ‘indeterminably-many’ branches. I argue that neither suffices to undercut the argument against the quantum representation theorem. I conclude that the quantum representation theorem is unsound and that the probability problem facing EQM remains unsolved.