Phase-Relative Branch Multiplicity and the Illusion of Parallel Worlds

Quantum mechanics does not require parallel universes — only phase-relative structure within a shared spacetime manifold. This preprint presents an interpretive framework for quantum multiplicity that avoids spatial “parallel universe” metaphors. Building on standard quantum field theory, Everettian quantum mechanics, and decoherence theory, it argues that apparent multiplicity can be understood as phase-relative branch structure within a single shared spacetime manifold, rather than as spatially separate worlds. The work introduces no new dynamics, equations, or empirical predictions. Its contribution is conceptual: clarifying how mutually exclusive classical outcomes arise through branch-relative accessibility and loss of correlation, while preserving all governing invariants, including conservation laws and universal gravitational coupling in the semiclassical regime. A companion supplementary document presents the standard mathematical formalism—Hilbert space structure, density matrices, decoherence, and expectation-value stress–energy—to demonstrate that the interpretation remains fully consistent with established physics. In approaches to quantum gravity where spacetime is emergent—such as loop quantum gravity or holographic dualities—the framework would naturally reformulate in relational or entanglement-theoretic terms. In that context, branch-relative multiplicity corresponds not to multiple times or geometries, but to multiple decohered relational histories or bulk reconstructions supported by a single underlying quantum state. This paper is intended as a foundations-level reinterpretation for physicists and philosophers of physics interested in quantum ontology, decoherence, and the conceptual structure of spacetime.