Jerome Romagosa

Do the worlds described by the Many Worlds Interpretation (MWI)—the most prominent Everettian interpretation of quantum mechanics—genuinely ‘branch’? According to the overlap view, the relationship between branching worlds is described in terms of mereological overlap: worlds share parts with one another through some temporal intervals. In recent years, an alternative known as the divergence view has gained prominence. The divergence view eschews ‘splitting’ and instead holds that Everettian worlds are mereologically disjoint parallel worlds whose histories diverge. I introduce a dilemma for the divergence view to demonstrate that it is metaphysically unsuitable for the MWI.

Everettian quantum mechanics tells us that the fundamental dynamics of the universe are deterministic. So what are the `probabilities' that the Born rule describes? One popular answer has been to treat these probabilities as rational credences. A recent alternative, Isaac Wilhelm's centered Everett Interpretation (CEI), takes the Born probabilities to be centered chances: the objective chances that some centered propositions are true. Thus, the CEI challenges the `orthodox assumption’ that fundamental physical laws concern only uncentered facts. I provide three arguments against the centered Everett Interpretation. First, I argue that the CEI is in apparent tension with a significant motivation for adopting Everettian quantum mechanics: rejecting the attribution of special significance to observers or agents in fundamental physics. I suggest the CEI can avoid this tension, but only at the cost of sacrificing its central claim that there are objective chances in an Everettian multiverse. My second argument concerns the CEI’s claim that the centered Born rule is a fundamental physical law. I provide two plausible notions of fundamentality for physical laws, and I argue that the centered Born rule satisfies neither. My final argument is that the CEI’s branch-relative laws cannot explain or constrain an agent’s rational credences in the way that the CEI claims.