2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/89672Entanglement is perhaps the most important new feature of the quantum world. It is expressed in quantum theory by the joint measurement formula. We prove the formula for self-adjoint observables from a plausible assumption, which for spacelike separated measurements is an expression of relativistic causality. State reduction is simply a way to express the JMF after one measurement has been made, and its result known.New material. Reformatted for journal submissionQuantum Physicstext