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Splitting worlds

Phil Pickering

Published Mar 12, 2026• 0 citations
Publisher

Abstract

Abstract Inspired by David Lewis’s work on possible worlds, there has been some discussion about whether the fission of worlds in the quantum many-worlds theory should be considered branching (where fission is like a branching path) or diverging (where the worlds are separate but qualitatively identical until some point of divergence). Several authors favour the latter on the basis that we can delineate worlds that are separate from each other prior to fission using a consistent histories approach. However, these worlds are not entirely independent of each other prior to fission, rendering neither of Lewis’s options fully compelling. I propose that the process is better conceptualised as splitting , analogous to the splitting of classical waves. When an entity splits, it separates into its component parts, each reduced somehow from the original, and so the original entity only partially persists as each of its successors. I propose that understanding many-worlds fission as one world splitting into its component parts aligns more closely with the physics and provides a more natural and intuitive understanding of the fission process.

DOI
10.1007/s11229-026-05515-y

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•Feb 11, 2020•0 citations
Abstract Inspired by David Lewis’s work on possible worlds, there has been some discussion about whether the fission of worlds in the quantum many-worlds theory should be considered branching (where fission is like a branching path) or diverging (where the worlds are separate but qualitatively identical until some point of divergence). Several authors favour the latter on the basis that we can delineate worlds that are separate from each other prior to fission using a consistent histories approach. However, these worlds are not entirely independent of each other prior to fission, rendering neither of Lewis’s options fully compelling. I propose that the process is better conceptualised as splitting , analogous to the splitting of classical waves. When an entity splits, it separates into its component parts, each reduced somehow from the original, and so the original entity only partially persists as each of its successors. I propose that understanding many-worlds fission as one world splitting into its component parts aligns more closely with the physics and provides a more natural and intuitive understanding of the fission process.
The Many Worlds Interpretation of quantum mechanics - arguably our most powerfully predictive scientific theory to date - describes a branching Universe composed of an infinite number of quasi-classical macroscopic physical worlds. Though elegant in its straightforward rendering of the mechanics, the Many Worlds Interpretation presents a challenge for understanding identity over time. If we wish to preserve the notion of strict numerical identity, we are faced with the choice between: denying the transitivity of identity; very short-lived lives with near constant death; or accepting that the world is filled with many more individuals than we previously dreamed. In adopting a perdurantist account of identity over time, I argue for this last option. But questions remain about the relationship that branching individuals have to those from whom they’ve split. In this dissertation, I develop a novel account that I call Many Worlds Counterparts. This theory takes its inspiration from Lewis’s Counterpart Theory in offering a modal analysis of de re possibility, but avoids the major challenges that Lewis’s theory faces.
The 2022 Tel Aviv conference on the many-worlds interpretation of quantum mechanics highlighted many differences between theorists. A very significant dichotomy is between Everettian fission (splitting) and Saunders–Wallace–Wilson divergence. For fission, an observer may have multiple futures, whereas for divergence they always have a single future. Divergence was explicitly introduced to resolve the problem of pre-measurement uncertainty for Everettian theory, which is universally believed to be absent for fission. Here I maintain that there is indeed pre-measurement uncertainty prior to fission, so long as objective probability is a property of Everettian branches. This is made possible if the universe is a set and branches are subsets with a probability measure. A universe that is a set of universes that are macroscopically isomorphic and span all possible configurations of local beäbles fulfills that role. If objective probability is a property of branches, then a successful Deutsch–Wallace decision-theoretic argument would justify the Principal Principle and be part of probability theory rather than specific to many-worlds theory. Any macroscopic object in our environment becomes a set of isomorphs with different microscopic configurations, each in an elemental universe (elemental in the set-theoretic sense). This is similar to the many-interacting-worlds theory, but the observer inhabits the set of worlds, not an individual world. An observer has many elemental bodies.
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.
Everett ( 1957a , b , 1973 ) relative-state formulation of quantum mechanics has often been taken to involve a metaphysical commitment to the existence of many splitting worlds each containing physical copies of observers and the objects they observe. While there was earlier talk of splitting worlds in connection with Everett, this is largely due to DeWitt’s (Phys Today 23:30–35, 1970 ) popular presentation of the theory. While the thought of splitting worlds or parallel universes has captured the popular imagination, Everett himself favored the language of elements, branches, or relative states in describing his theory. The result is that there is no mention of splitting worlds or parallel universes in any of Everett’s published work. Everett, however, did write of splitting observers and was willing to adopt the language of many worlds in conversation with people who were themselves using such language. While there is evidence that Everett was not entirely comfortable with talk of many worlds, it does not seem to have mattered much to him what language one used to describe pure wave mechanics. This was in part a result of Everett’s empirical understanding of the cognitive status of his theory.
A completely different interpretation of the measurement problem, one which many professional scientists have found attractive if only because of its mathematical elegance, was first suggested by Hugh Everett III in 1957 and is known variously as the ‘relative state’, ‘many-worlds’ or ‘branching-universe’ interpretation. This viewpoint gives no special role to the conscious mind and to this extent the theory is completely objective, but we shall see that many of its other consequences are in their own way just as revolutionary as those discussed in the previous chapter. The essence of the many-worlds interpretation can be illustrated by again considering the example of the 45° polarised photon approaching the H/V detector. Remember what we demonstrated in Chapters 2 and 4: from the wave point of view a 45° polarised light wave is equivalent to a superposition of a horizontally polarised wave and a vertically polarised wave. If we were able to think purely in terms of waves, the effect of the H/V polariser on the 45° polarised wave would be simply to split the wave into these two components. These would then travel through the H and V channels respectively, half the original intensity being detected in each. In contrast, photons cannot be split, but they can be considered to be in a superposition state until a measurement ‘collapses’ the system into one or other of its possible outcomes.
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.