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Locality in Everett: Dynamics vs. Ontology

Christopher G. Timpson, David Wallace

Published Aug 26, 2026• 1 citation
Publisher

Abstract

Abstract This chapter defends the idea that the Everett (many-worlds) interpretation is, given Hamiltonians with only local interactions, dynamically local at both the level of fundamental and emergent ontology. At the level of fundamental ontology because the complete state of a given spacetime region will be fixed by the complete state on a spacelike hypersurface that obstructs the past light cone of that first region. At the level of emergent ontology because the dynamics of the emergent ontology will be grounded in the dynamics of the fundamental ontology. If the world-notion in the many-worlds interpretation is understood in terms of emergent ontology, then it will follow that the dynamical locality holds also at the level of worlds. Our picture pairs dynamical locality with kinematical nonlocality (non-separability). The chapter articulates how the demands of Lorentz covariance are fully satisfied in this picture. In our view, failure of kinematical locality is a harmless, indeed beneficial (if nonclassical), feature and it coexists with dynamical locality and Lorentz covariance. It has sometimes been suggested that—along the lines indicated by Deutsch and Hayden—seeking a kinematically local theory is nonetheless to be preferred or is, perhaps, required. The chapter revisits and restates some of our previous arguments to the effect that reintroducing kinematical locality in this way is both unnecessary and undesirable: the benefits of the move, such as they are, are not worth the costs.

DOI
10.1093/9780197792766.003.0002

Related papers

Snakes on a Hyper-Plane: Dynamical and Ontological Locality in Everettian Quantum Mechanics

Travis Norsen

An apparently emerging consensus holds that Everettian Quantum Mechanics (EQM), in the Spacetime State Realism (SSR) version of Wallace and Timpson, involves ontological non-locality (i.e., non-separability) but is fully compatible with the dynamical notion of locality usually taken to be required by relativity. The claim of dynamical locality is based on the fact that the local beables postulated by the theory (namely, Reduced Density Matrices, RDMs, associated with localized regions) are not affected by interventions outside their past light cones. However, the theory's non-separability means that it posits non-local beables (namely RDMs for non-localized regions) in addition to the local ones. We argue here that the involvement of non-local beables introduces complications for extant formulations of dynamical locality and at least suggests, contrary to the emerging consensus, that SSR EQM should perhaps not be considered compatible with relativistic causality after all.

quant-pharXiv:2609.07860•Sep 9, 2026•0 citations

Should Everettians Aspire to Locality?

Emily Adlam

Abstract One influential argument for the Everett interpretation of quantum mechanics is the idea that the Everettian view allows us to preserve locality. Evidently this argument assumes that preserving locality is something we should all aspire to—but what is the justification for treating locality as a theoretical desideratum? But there is no good reason for Everettians—or indeed anyone else—to aspire to “locality” as a general feature of their theory. It is true that certain specific ways in which nonlocality may be implemented have undesirable properties and should probably be avoided, but that does not mean that nonlocality itself is always undesirable—nonlocal theories should be assessed on their individual merits, rather than dismissed as a general class. Thus the “argument from locality” for the Everett interpretation is not compelling unless it is coupled with some justification for why the specific kind of locality instantiated by the Everettian picture is worth pursuing.

•Aug 26, 2026•0 citations

Local Many Worlds in Spacetime: Deutsch-Hayden Descriptors and Local Wavefunctions

Mordecai Waegell

Bell's theorem establishes that a local and spatially separable physical theory with no superdeterminism or retrocausality must be a theory of many local worlds. The local many worlds formalism of quantum mechanics is a transparent separable physical theory in spacetime with a Lorentz covariant causal structure. Unlike the Everett many worlds interpretation, there is no nonseparable universal wavefunction in configuration space, but like Everett, there is only local unitary evolution which conserves probability (fluid) current, and no wavefunction collapse. Each event in spacetime has its own local wavefunction which is built up using only the local wavefunctions and local unitary evolution operators within its past light cone, and which is entirely disconnected from spacelike separated local wavefunctions. We call this the local Schrödinger picture. Local many worlds can also be described in the Heisenberg picture by ascribing Deutsch-Hayden (DH) descriptors to each individual system, each of which evolves under local unitary operations. Both the local wavefunctions and DH descriptors contain records of past interactions, and while the local wavefunctions provide a clear and intuitive physical narrative, the descriptors contain all of the same information in a much more compact form. Unfortunately, the compact form is also somewhat cryptic, which has made the local Heisenberg picture relatively inaccessible. The purpose of this article is to show that the local wavefunctions for any $N$-qubit system can always be constructed directly from the DH descriptors, allowing us to combine the benefits of both pictures.

quant-pharXiv:2609.33453•Sep 29, 2026•0 citations

Introduction

Alyssa Ney

The aim of this volume is to develop and evaluate the suggestion that if one adopts the interpretation of quantum mechanics initially proposed by Hugh Everett, today frequently referred to as the “many-worlds interpretation,” then one can avoid the kind of nonlocality that is often thought to be a consequence of quantum entanglement according to other approaches, and thus permit the compatibility of these two pillars of modern physics: quantum mechanics and special relativity. Although this is an idea that seems to have been recognized by Everett and is sometimes touted as a reason to prefer his approach to interpreting quantum mechanics over others, the precise way in which Everett’s interpretation achieves this locality, indeed whether it even does achieve locality, has been debated, including among the interpretation’s proponents. The aim of this introductory chapter is to provide readers with a background of this debate concerning the locality of Everettian quantum mechanics. We begin with a brief historical introduction to Everett’s proposed interpretation of quantum mechanics and its later development into the many-worlds interpretation. The following section discusses how the by now well-established phenomenon of quantum entanglement seems to imply what Albert Einstein, in a 1947 letter to Max Born, called “spooky action at a distance,” and a tension with special relativity. We then discuss why one might think that the many-worlds interpretation allows one to avoid nonlocality in this sense. The next sections bring the reader up to date with how the locality of Everettian quantum mechanics has been debated both by critics and proponents of the interpretation. As we will see, the desire to understand Everettian quantum mechanics as an interpretation that avoids nonlocality has opened up some interesting debates concerning both what it means to say an interpretation of quantum mechanics is local and over the metaphysical and mathematical interpretation of Everettian quantum mechanics itself. This introductory chapter concludes with a summary of the contributions to the present volume in the context of this ongoing debate.

•Aug 26, 2026•0 citations

The Emergence of Multiplicity

David Wallace

The aim of this chapter is to explain how the ‘many worlds’ of the Everett interpretation can emerge from the underlying physics of quantum theory, without being put in by hand at the microscopic level. The discussion is conceptually focussed: in the main the chapter is concerned with sketching the way in which higher-level ontology emerges, in general, from lower-level scientific theories. When this general framework is applied to quantum theory in particular, the chapter shows that it leads to a continual branching of physical reality at the emergent, macroscopic level.

Oxford University Press eBooks•May 24, 2012•0 citations

A Prolegomenon to the Ontology of the Everett Interpretation*

David Wallace

According to the Everett interpretation of quantum mechanics, the apparent paradoxes of quantum theory require no change either in our philosophical attitude toward scientific theories or in the formalism of quantum mechanics itself. Rather, the unaltered theory can be understood as a literal description of physical reality. But the physical reality thus described is one of many noninteracting, approximately classical worlds. These worlds enter the theory not at the fundamental level—they are nowhere to be found in the basic formalism—but at a higher, emergent level; in this sense they are (ontologically speaking) on a par with the ontology of the special sciences.

Oxford University Press eBooks•Apr 8, 2013•24 citations
Abstract This chapter defends the idea that the Everett (many-worlds) interpretation is, given Hamiltonians with only local interactions, dynamically local at both the level of fundamental and emergent ontology. At the level of fundamental ontology because the complete state of a given spacetime region will be fixed by the complete state on a spacelike hypersurface that obstructs the past light cone of that first region. At the level of emergent ontology because the dynamics of the emergent ontology will be grounded in the dynamics of the fundamental ontology. If the world-notion in the many-worlds interpretation is understood in terms of emergent ontology, then it will follow that the dynamical locality holds also at the level of worlds. Our picture pairs dynamical locality with kinematical nonlocality (non-separability). The chapter articulates how the demands of Lorentz covariance are fully satisfied in this picture. In our view, failure of kinematical locality is a harmless, indeed beneficial (if nonclassical), feature and it coexists with dynamical locality and Lorentz covariance. It has sometimes been suggested that—along the lines indicated by Deutsch and Hayden—seeking a kinematically local theory is nonetheless to be preferred or is, perhaps, required. The chapter revisits and restates some of our previous arguments to the effect that reintroducing kinematical locality in this way is both unnecessary and undesirable: the benefits of the move, such as they are, are not worth the costs.
An apparently emerging consensus holds that Everettian Quantum Mechanics (EQM), in the Spacetime State Realism (SSR) version of Wallace and Timpson, involves ontological non-locality (i.e., non-separability) but is fully compatible with the dynamical notion of locality usually taken to be required by relativity. The claim of dynamical locality is based on the fact that the local beables postulated by the theory (namely, Reduced Density Matrices, RDMs, associated with localized regions) are not affected by interventions outside their past light cones. However, the theory's non-separability means that it posits non-local beables (namely RDMs for non-localized regions) in addition to the local ones. We argue here that the involvement of non-local beables introduces complications for extant formulations of dynamical locality and at least suggests, contrary to the emerging consensus, that SSR EQM should perhaps not be considered compatible with relativistic causality after all.
Abstract One influential argument for the Everett interpretation of quantum mechanics is the idea that the Everettian view allows us to preserve locality. Evidently this argument assumes that preserving locality is something we should all aspire to—but what is the justification for treating locality as a theoretical desideratum? But there is no good reason for Everettians—or indeed anyone else—to aspire to “locality” as a general feature of their theory. It is true that certain specific ways in which nonlocality may be implemented have undesirable properties and should probably be avoided, but that does not mean that nonlocality itself is always undesirable—nonlocal theories should be assessed on their individual merits, rather than dismissed as a general class. Thus the “argument from locality” for the Everett interpretation is not compelling unless it is coupled with some justification for why the specific kind of locality instantiated by the Everettian picture is worth pursuing.
Bell's theorem establishes that a local and spatially separable physical theory with no superdeterminism or retrocausality must be a theory of many local worlds. The local many worlds formalism of quantum mechanics is a transparent separable physical theory in spacetime with a Lorentz covariant causal structure. Unlike the Everett many worlds interpretation, there is no nonseparable universal wavefunction in configuration space, but like Everett, there is only local unitary evolution which conserves probability (fluid) current, and no wavefunction collapse. Each event in spacetime has its own local wavefunction which is built up using only the local wavefunctions and local unitary evolution operators within its past light cone, and which is entirely disconnected from spacelike separated local wavefunctions. We call this the local Schrödinger picture. Local many worlds can also be described in the Heisenberg picture by ascribing Deutsch-Hayden (DH) descriptors to each individual system, each of which evolves under local unitary operations. Both the local wavefunctions and DH descriptors contain records of past interactions, and while the local wavefunctions provide a clear and intuitive physical narrative, the descriptors contain all of the same information in a much more compact form. Unfortunately, the compact form is also somewhat cryptic, which has made the local Heisenberg picture relatively inaccessible. The purpose of this article is to show that the local wavefunctions for any NNN-qubit system can always be constructed directly from the DH descriptors, allowing us to combine the benefits of both pictures.
The aim of this volume is to develop and evaluate the suggestion that if one adopts the interpretation of quantum mechanics initially proposed by Hugh Everett, today frequently referred to as the “many-worlds interpretation,” then one can avoid the kind of nonlocality that is often thought to be a consequence of quantum entanglement according to other approaches, and thus permit the compatibility of these two pillars of modern physics: quantum mechanics and special relativity. Although this is an idea that seems to have been recognized by Everett and is sometimes touted as a reason to prefer his approach to interpreting quantum mechanics over others, the precise way in which Everett’s interpretation achieves this locality, indeed whether it even does achieve locality, has been debated, including among the interpretation’s proponents. The aim of this introductory chapter is to provide readers with a background of this debate concerning the locality of Everettian quantum mechanics. We begin with a brief historical introduction to Everett’s proposed interpretation of quantum mechanics and its later development into the many-worlds interpretation. The following section discusses how the by now well-established phenomenon of quantum entanglement seems to imply what Albert Einstein, in a 1947 letter to Max Born, called “spooky action at a distance,” and a tension with special relativity. We then discuss why one might think that the many-worlds interpretation allows one to avoid nonlocality in this sense. The next sections bring the reader up to date with how the locality of Everettian quantum mechanics has been debated both by critics and proponents of the interpretation. As we will see, the desire to understand Everettian quantum mechanics as an interpretation that avoids nonlocality has opened up some interesting debates concerning both what it means to say an interpretation of quantum mechanics is local and over the metaphysical and mathematical interpretation of Everettian quantum mechanics itself. This introductory chapter concludes with a summary of the contributions to the present volume in the context of this ongoing debate.
The aim of this chapter is to explain how the ‘many worlds’ of the Everett interpretation can emerge from the underlying physics of quantum theory, without being put in by hand at the microscopic level. The discussion is conceptually focussed: in the main the chapter is concerned with sketching the way in which higher-level ontology emerges, in general, from lower-level scientific theories. When this general framework is applied to quantum theory in particular, the chapter shows that it leads to a continual branching of physical reality at the emergent, macroscopic level.
According to the Everett interpretation of quantum mechanics, the apparent paradoxes of quantum theory require no change either in our philosophical attitude toward scientific theories or in the formalism of quantum mechanics itself. Rather, the unaltered theory can be understood as a literal description of physical reality. But the physical reality thus described is one of many noninteracting, approximately classical worlds. These worlds enter the theory not at the fundamental level—they are nowhere to be found in the basic formalism—but at a higher, emergent level; in this sense they are (ontologically speaking) on a par with the ontology of the special sciences.