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A Dilemma for Divergence in the Many Worlds Interpretation

Jerome Romagosa

Published Jan 1, 2026• 0 citations
OpenAlex

Abstract

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.

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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.
Abstract This chapter sets out the version of quantum mechanics—diverging EQM—that constitutes the physics component of quantum modal realism. After a potted history of Everettian quantum mechanics, and an introduction to recent decoherence-based versions of the approach, I sketch a distinctive metaphysical understanding of Everettian quantum mechanics that involves diverging ‘parallel worlds’ instead of overlapping ‘splitting worlds’. I argue that the choice between divergence and overlap is underdetermined by the physics itself; I then adapt an argument of David Lewis’s that supports divergence over overlap, emphasizing the need to make room for objective probabilities in the Everettian picture. The chapter concludes by rejecting some unpersuasive motivations for overlap.
The nature of branching in the many-worlds interpretation (MWI) of quantum mechanics remains an open question, particularly regarding its locality and compatibility with special relativity. This paper challenges the conventional view that branching is either global or local, demonstrating instead that it is nonlocal for entangled systems. Through a new analysis of the EPR-Bohm experiment, I argue that global branching has several potential issues and can hardly be justified. At the same time, I argue that branching cannot be entirely local, as entangled particles exhibit simultaneous, spacelike-separated branching, manifesting an apparent action at a distance within individual worlds. However, while nonlocal branching suggests the emergence of a preferred Lorentz frame within each world, the multiverse as a whole retains full Lorentz invariance, ensuring no superluminal signaling. By refining the ontology of branching and resolving tensions between MWI and relativistic constraints, this analysis may help advance our understanding of quantum nonlocality and also strengthen MWI's standing as a viable interpretation of quantum mechanics.
I continue to maintain that David Lewis’s concept of overlapping persons cannot yield pre-measurement uncertainty in the Everett interpretation of quantum mechanics in the way that Simon Saunders and David Wallace originally seemed to suggest. However, I argue that in their reply to me they make it clear that they do not wish to invoke overlap of persons after all. That makes it mysterious why they defended their interpretation of personal overlap in the first place and questionable what role overlap has to play in their proposal. If Everettian branching can be understood to involve the divergence of distinct, non-overlapping worlds a concept of pre-measurement uncertainty is available. That idea was first proposed by David Deutsch but required an ad hoc postulate. Saunders has recently suggested that a similar scheme arises naturally out of the physics. If correct, that is important as it offers escape from some bizarre consequences of current alternative ways of understanding probability in the Everett interpretation
If the most familiar overlapping (branching universe) interpretation of Everettian quantum mechanics (EQM) is correct, then each of us is constantly splitting into multiple people. This consequence gives rise to the quantum doomsday argument, which threatens to draw crippling epistemic consequences from EQM. However, a diverging (parallel universe) interpretation of EQM undermines the quantum doomsday argument completely. This appears to tell in favour of the diverging interpretation. But it is surprising that a metaphysical question that is apparently underdetermined by the physics should be settled by purely epistemological considerations; and I argue that the positive case for divergence based on the quantum doomsday effect is ultimately unsuccessful. I discuss how some influential treatments of Everettian confirmation handle the quantum doomsday puzzle, and suggest that it can most satisfyingly be resolved via a naturalistic approach to the metaphysics of modality. 1. Introduction2. The Doomsday Argument3. The Quantum Doomsday Argument4. Doomsday and Divergence5. Solutions to the Quantitative Problem6. Many Worlds and Modal Realism7. Conclusion
Since the 1970s, the Everett-Wheeler many-worlds interpretation (MWI) of Quantum Mechanics (1955) has been much in the news. One wonders about the worlds in question, their branches, their splittings, their number. It is most often ignored that this language is not that of Everett, whom Wheeler very quickly stopped supporting. Moreover, for some interpreters, the real meaning of Everett ideas is not the coexistence of many worlds, but the existence of a single quantum one. In this context, what should we think of attempts to verify Everett thesis? What about the connexion between MWI and the cosmological multiverse? How to understand the links between the quantum world and the classical one? This article tries to answer some of these questions.
Abstract The Many-Worlds Interpretation (MWI) is a theory in physics which proposes that, rather than quantum-level events being resolved randomly as according to the Copenhagen Interpretation, the universe constantly divides into different versions or worlds. All physically possible worlds occur, though some outcomes are more likely than others, and therefore all possible histories exist. This paper explores some implications of this for history, especially concerning causation. Unlike counterfactuals, which concern different starting conditions, MWI concerns different outcomes of the same starting conditions. It is argued that analysis of causation needs to take into account the divergence of outcomes and the possibility that we inhabit a less probable world. Another implication of MWI is convergent history: for any given world there will be similar worlds which are the result of different pasts which are, however, more or less probable. MWI can assist in thinking about historical causation and indicates the importance of probabilistic causation.