Abstract: Despite DeWitt’s claim that Everettian quantum mechanics (EQM) offers its own interpretation, there is no consensus regarding EQM’s best ontology. I organize the possible ontologies according to the two following choice dimensions: one versus many concrete histories, and overlapping versus diverging histories. Accounts such as Conroy’s (2018) Everettian actualism, according to which there is one concrete world among many possible worlds that together form the branching structure, fail to account for precisely how our world earns the honor of being concrete. Regardless of whether the worlds themselves are diverging or overlapping, such an account requires the introduction of stochastic processes to EQM which would deprive EQM of its appealing deterministic nature. On the other hand, metaphysical accounts of EQM that posit many concrete worlds, such as Wilson’s (2020) quantum modal realism, face the serious objection over ontological extravagance, for they demand an infinite number of concrete worlds with identical initial conditions that just happen to together be governed by the global wave function. If one desires a parsimonious and genuinely deterministic account of EQM, one must entertain alternative options.
Fortunately, in my taxonomy, one type of ontology remains: many overlapping concrete worlds. This resembles early metaphysical readings of EQM, such as that of Dewitt (1970) and what Wallace (2014) calls the hydra view. Initial versions of the hydra view, however, cannot withstand the identity problem: post-branching, if there are now multiple agents where there once was one, which agent is really the original agent? How can one possibly make sense of the identity of objects on such an account? To dissolve this issue, I argue for one dendritic world. To any observer within the branching structure, it appears that measurements yield determinate outcomes, and this novel approach demands a new mereological account of the universe and the objects within it. In addition to material parts and temporal parts, I propose branch-like parts (BLiP). Each object in this singular branching world is the mereological sum of its BLiPs; when an agent-BLiP undergoes decoherent branching, it is simply the case that they split into additional agent-BLiPs. This new mereological relation allows one to avoid common objections against one world metaphysics of EQM by increasing the structure of our world, while simultaneously decreasing the number of worlds that we must posit from an infinite number to only one.
The PoP-grunch (Philosophy of Physics Graduate Lunch) is a weekly informal seminar in which graduate students in Philosophy of Physics present their work in progress.
Philosophy of Physics Graduate Lunch Seminar Convenor: Eleanor March