Indeterminism, Non-Maximality, and Discontinuity in Everettian Quantum Mechanics

Whilst several of the merits of Everettian Quantum Mechanics (EQM) have been subject to dispute, its status as a deterministic theory has been granted as more or less axiomatic. Accordingly, literature in the field of quantum foundations is replete with strident professions of EQM’s deterministic credentials. This is surprising in view of the scant independent justification for them, as well as the diversity of characterisations of determinism, filling out a spectrum which might more typically elicit scrutiny of claims of compliance. This article distinguishes and analyses three separate characterisations of determinism, including a Laplacian variant, a branching-style variant (DBRN) inspired by recent commentary, and a variant couched in terms of the continuity of subsystem identity. Applying these to the case of EQM shows that the latter qualifies as deterministic only under the weakest of these definitions. It is concluded that EQM’s violation of these stronger standards constitutes a meaningful limitation on the sense in which the theory can be considered genuinely deterministic, and that claims of this nature must be qualified carefully.