Gao, Shan (2025) Is Branching Truly Global? From Cambridge Changes to Oxford Changes in the Many-Worlds Interpretation. [Preprint]

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Abstract

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.

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Item Type:	Preprint
Creators:	Creators Email ORCID Gao, Shan sgao7319@uni.sydney.edu.au
Keywords:	Many-Worlds Interpretation; global branching; nonlocal branching; locality; Cambridge change; Oxford change
Subjects:	Specific Sciences > Physics > Cosmology Specific Sciences > Physics > Quantum Mechanics Specific Sciences > Physics > Relativity Theory
Depositing User:	Prof. Shan Gao
Date Deposited:	23 Dec 2025 21:36
Last Modified:	23 Dec 2025 21:36
Item ID:	27618
Subjects:	Specific Sciences > Physics > Cosmology Specific Sciences > Physics > Quantum Mechanics Specific Sciences > Physics > Relativity Theory
Date:	23 December 2025
URI:	https://philsci-archive.pitt.edu/id/eprint/27618

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