Einstein's Electron and Local Branching: Unitarity Does not Require Many-Worlds

Abstract: We revisit the 1927 thought experiment of Einstein on electron diffraction, using a single-electron source and an opaque hemispheric detector array, now achievable with modern sensors. In this fully enclosed system, where no signals escape the hemisphere, we provide a direct empirical comparison of the Many-Worlds Interpretation (MWI) and the Branched Hilbert Subspace Interpretation (BHSI). Both maintain unitarity without invoking wavefunction collapse, as in the Copenhagen Interpretation (CI), but differ ontologically: MWI proposes irreversible global branching into parallel worlds, while BHSI describes local, potentially reversible branching into decohered subspaces. In this setup, all quantum events (branching, engagement, disengagement, and relocation) occur entirely within the local system, and the Born rule, naturally emerging through branch weights, can be observed in detector statistics. To explore branching dynamics more thoroughly, we suggest an enhanced dual-layer experimental setup with an inner transparent detector. Because the electron transit time between layers is shorter than the average response times of the inner sensors, this allows for a crucial test of measurement timing and potential anomalies, such as delayed or uncommitted choices. Our analysis challenges the notion that unitarity necessitates parallel worlds, instead advocating for a simpler view: local, unitary branching without collapse or global splitting.