Not Just Many Worlds but Many Universes? A Problem for the Many Worlds View of Quantum Mechanics

Abstract The many-worlds view is one of the most discussed “interpretations” of quantum mechanics. As is well known, this view has some very controversial and much discussed aspects. This paper focuses on one particular problem arising from the combination of quantum mechanics with Special Relativity. It turns out that the ontology of the many-worlds view – the account of what there is and what branches of the universe exist – is relative to inertial frames. If one wants to avoid relativizing ontology, one has to argue either that there is an additional source of branching due to Special Relativity and thus additional branches or worlds. Or one has to argue that there are not only many worlds but also many universes (sets of worlds or world-branches); there is thus not only one tree of many world-branches but many frame-specific trees, a “forest” of many world-trees. The main problem here is how one can understand all or any of this.