Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Does the Universe Split Everywhere at Once? Rethinking Branching and Nonlocality in the Many-Worlds Interpretation of Quantum Mechanics

Gao, Shan

Published Feb 17, 2025• 0 citations
OpenAlex

Abstract

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.

Related papers

Is Branching Truly Global? From Cambridge Changes to Oxford Changes in the Many-Worlds Interpretation

Shan Gao

Ney (2025) offers a compelling defense of the Many-Worlds Interpretation (MWI) of quantum mechanics as uniquely local through global branching: a local measurement instantaneously splits the entire universe, with distant effects manifesting solely as Cambridge changes—purely relational shifts devoid of intrinsic physical alteration. This paper critically examines Ney’s analysis in the context of an EPR-Bohm setup, focusing on the branching of a distant observer (Bob) and the remote entangled particle ($b$). I argue that global branching of unentangled systems like Bob is problematic, as it undermines the Born rule through loss of local amplitudes, restricts the possibility of measuring entangled superpositions, and implies superluminal influences. For particle $b$, while its reduced density matrix remains invariant, its acquisition of definiteness within each branch carries greater ontological weight than a classical Cambridge change, reflecting a physical, branch-relative transformation. To capture this distinction, I introduce a three-way taxonomy of changes in MWI: intrinsic physical changes (local and dynamical), classical Cambridge changes (purely relational), and the novel “Oxford change”—a hybrid that is relational from the multiverse perspective yet physically real and ontic within individual branches. Finally, I sketch a preliminary alternative: nonlocal branching that propagates selectively along entanglement relations via Oxford changes, leaving unentangled observers unbranched. This approach appears to better preserve the Born rule, measurement flexibility, and relativistic locality, though further development is needed. Overall, while Ney’s global model advances the debate on Everettian locality, the considerations raised here highlight challenges and suggest pathways toward a more refined branching ontology.

•Dec 23, 2025•0 citations

Einstein's Electron and Local Branching: Unitarity without Many-Worlds --Local Hilbert spaces, boundaries, and quantum nonlocality

Xing M. Wang

Traditional interpretations of quantum mechanics often present a dichotomy: either the wavefunction collapses upon measurement (Copenhagen), violating unitarity, or the entire universe branches into countless parallel worlds (Many-Worlds), with significant ontological proliferation. The Branched Hilbert Subspace Interpretation (BHSI) resolves this tension by introducing branching strictly within local Hilbert spaces. This framework reinterprets scenarios such as Einstein’s 1927 electron-diffraction thought experiment, in which all quantum events are confined to a local Hilbert space, allowing the Born rule to emerge naturally from branch weights. Crucially, BHSI treats branching as a dynamical process tied to information recording. This leads to a testable proposal: a dual-layer experiment in which the particle transit time between layers is shorter than the sensor response time, enabling a direct probe of measurement timing and “mismatched” or “uncommitted” outcomes. We argue that a quantum system behaves as a unified whole—an “island of coherence”—within which unitary branching is confined to the system’s boundary, without observable correlations with distant, unentangled systems. Finally, we show that quantum nonlocality (e.g., in Bell tests or tunneling) arises naturally from the intrinsic vector-space structure of local Hilbert spaces, rather than from superluminal signaling.

•Jan 22, 2026•0 citations

Branching (Almost) Everywhere And All at Once

Alyssa Ney

It is sometimes said that a reason to prefer Everettian quantum mechanics is that it allows one to avoid the kind of spooky action at a distance that is a consequence of quantum entanglement according to other approaches. There is a straightforward argument to show EQM has this virtue, however some advocates argue that this requires we see branching as a local causal process. I explain this reasoning and then show why, given the way Everettians standardly conceive of worlds and branching as emergent phenomena, branching need not be viewed as a causal process for EQM to be relativistically local. Forthcoming in Local Quantum Mechanics, OUP 2026.

quant-pharXiv:2608.18644•Aug 20, 2026

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

Peter Baumann

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.

Metaphysica•Aug 3, 2022•0 citations

Why the Many-Worlds Interpretation?

Lev Vaidman A

A brief (subjective) description of the state of the art of the many-worlds interpretation of quantum mechanics (MWI) is presented. It is argued that the MWI is the only interpretation which removes action at a distance and randomness from quantum theory. Limitations of the MWI regarding questions of probability which can be legitimately asked are specified. The ontological picture of the MWI as a theory of the universal wave function decomposed into a superposition of world wave functions, the important parts of which are defined in three-dimensional space, is presented from the point of view of our particular branch. Some speculations about misconceptions, which apparently prevent the MWI from being in the consensus, are mentioned.

arXiv:2208.04618•Quantum Reports•Aug 4, 2022•22 citations

The Many-Worlds Interpretation of Quantum Mechanics

Lev Vaidman

Next year, there will be a 100-year celebration of quantum mechanics, but there is no consensus on the interpretation of this theory. This reprint presents recent works on one of the most intriguing interpretations, the many-worlds interpretation, presented at a workshop in October 2022 at Tel Aviv University. The many-worlds interpretation solves the measurement problem, avoids action at a distance and indeterminism, and does not contradict empirical evidence. The main question discussed in the workshop was as follows: why is it not in the consensus? Major issues such as preferred basis, the meaning of probability, non-locality, and alternative approaches were discussed, which are increasingly drawing attention in physics and philosophy. The contributions demonstrated a striking diversity in understanding the basic concepts. What are the worlds in the many-worlds interpretation? Do the worlds diverge or split? Is probability objective or subjective, or is it just an illusion? Can we avoid the non-separability of quantum mechanics? Many questions were raised and significant progress in understanding was achieved, but the papers showed that consensus is still far away. This reprint describes the current status of the research and will be invaluable for physicists and philosophers who want to resolve the long-standing problem of the interpretation of quantum mechanics.

MDPI eBooks•May 11, 2024•14 citations
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.
Ney (2025) offers a compelling defense of the Many-Worlds Interpretation (MWI) of quantum mechanics as uniquely local through global branching: a local measurement instantaneously splits the entire universe, with distant effects manifesting solely as Cambridge changes—purely relational shifts devoid of intrinsic physical alteration. This paper critically examines Ney’s analysis in the context of an EPR-Bohm setup, focusing on the branching of a distant observer (Bob) and the remote entangled particle (bbb). I argue that global branching of unentangled systems like Bob is problematic, as it undermines the Born rule through loss of local amplitudes, restricts the possibility of measuring entangled superpositions, and implies superluminal influences. For particle bbb, while its reduced density matrix remains invariant, its acquisition of definiteness within each branch carries greater ontological weight than a classical Cambridge change, reflecting a physical, branch-relative transformation. To capture this distinction, I introduce a three-way taxonomy of changes in MWI: intrinsic physical changes (local and dynamical), classical Cambridge changes (purely relational), and the novel “Oxford change”—a hybrid that is relational from the multiverse perspective yet physically real and ontic within individual branches. Finally, I sketch a preliminary alternative: nonlocal branching that propagates selectively along entanglement relations via Oxford changes, leaving unentangled observers unbranched. This approach appears to better preserve the Born rule, measurement flexibility, and relativistic locality, though further development is needed. Overall, while Ney’s global model advances the debate on Everettian locality, the considerations raised here highlight challenges and suggest pathways toward a more refined branching ontology.
Traditional interpretations of quantum mechanics often present a dichotomy: either the wavefunction collapses upon measurement (Copenhagen), violating unitarity, or the entire universe branches into countless parallel worlds (Many-Worlds), with significant ontological proliferation. The Branched Hilbert Subspace Interpretation (BHSI) resolves this tension by introducing branching strictly within local Hilbert spaces. This framework reinterprets scenarios such as Einstein’s 1927 electron-diffraction thought experiment, in which all quantum events are confined to a local Hilbert space, allowing the Born rule to emerge naturally from branch weights. Crucially, BHSI treats branching as a dynamical process tied to information recording. This leads to a testable proposal: a dual-layer experiment in which the particle transit time between layers is shorter than the sensor response time, enabling a direct probe of measurement timing and “mismatched” or “uncommitted” outcomes. We argue that a quantum system behaves as a unified whole—an “island of coherence”—within which unitary branching is confined to the system’s boundary, without observable correlations with distant, unentangled systems. Finally, we show that quantum nonlocality (e.g., in Bell tests or tunneling) arises naturally from the intrinsic vector-space structure of local Hilbert spaces, rather than from superluminal signaling.
It is sometimes said that a reason to prefer Everettian quantum mechanics is that it allows one to avoid the kind of spooky action at a distance that is a consequence of quantum entanglement according to other approaches. There is a straightforward argument to show EQM has this virtue, however some advocates argue that this requires we see branching as a local causal process. I explain this reasoning and then show why, given the way Everettians standardly conceive of worlds and branching as emergent phenomena, branching need not be viewed as a causal process for EQM to be relativistically local. Forthcoming in Local Quantum Mechanics, OUP 2026.
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.
A brief (subjective) description of the state of the art of the many-worlds interpretation of quantum mechanics (MWI) is presented. It is argued that the MWI is the only interpretation which removes action at a distance and randomness from quantum theory. Limitations of the MWI regarding questions of probability which can be legitimately asked are specified. The ontological picture of the MWI as a theory of the universal wave function decomposed into a superposition of world wave functions, the important parts of which are defined in three-dimensional space, is presented from the point of view of our particular branch. Some speculations about misconceptions, which apparently prevent the MWI from being in the consensus, are mentioned.
Next year, there will be a 100-year celebration of quantum mechanics, but there is no consensus on the interpretation of this theory. This reprint presents recent works on one of the most intriguing interpretations, the many-worlds interpretation, presented at a workshop in October 2022 at Tel Aviv University. The many-worlds interpretation solves the measurement problem, avoids action at a distance and indeterminism, and does not contradict empirical evidence. The main question discussed in the workshop was as follows: why is it not in the consensus? Major issues such as preferred basis, the meaning of probability, non-locality, and alternative approaches were discussed, which are increasingly drawing attention in physics and philosophy. The contributions demonstrated a striking diversity in understanding the basic concepts. What are the worlds in the many-worlds interpretation? Do the worlds diverge or split? Is probability objective or subjective, or is it just an illusion? Can we avoid the non-separability of quantum mechanics? Many questions were raised and significant progress in understanding was achieved, but the papers showed that consensus is still far away. This reprint describes the current status of the research and will be invaluable for physicists and philosophers who want to resolve the long-standing problem of the interpretation of quantum mechanics.