What Would Count as Evidence for Many Worlds? Operational Equivalence and the Evidential Architecture of Everettian Quantum Mechanics

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Abstract

Claims that experiments confirm the many-worlds interpretation often conflate evidence for quantum coherence, universal unitarity, the absence of objective collapse, decoherence-defined branches, and the reality of every branch. This paper develops a contrast-relative architecture that separates those targets. A complete theory package includes dynamics, ontology, state and parameter assumptions, and a probability or typicality rule that generates empirical likelihoods. Four formal results follow. Operationally equivalent theories have a Bayes factor of one for every fixed or adaptive protocol in the domain of equivalence. Evidence can redistribute probability toward a theoretical family while leaving the odds among equivalent members unchanged. An exact decomposition identifies the bridge factors required when support is inherited from shared dynamics by stronger structural or ontological claims. Finally, an inheritance-attenuation formula quantifies how evidence for unitarity weakens when Everett is compared with a complement containing other unitary theories. Applications include massive matter-wave interference, XENONnT collapse constraints, experimental quantum Darwinism, Local Friendliness tests, and proposed interbranch communication. Current results extend the tested domain of coherent quantum dynamics, constrain specified collapse models, and support mechanisms for stable, redundant records. They do not supply an outcome distribution favoring Everett over all operationally equivalent unitary rivals or directly establish the reality of every decohered component. Everett can therefore receive indirect, family-mediated confirmation while its distinctive ontology remains underdetermined within a predictive equivalence class.