Locality and nonlocality in quantum mechanics: A two‐proton EPR experiment

Two “thought experiments” are central to most discussions of the famous EPR paradox: Experiment 1, in which two electrons with spins initially coupled to total spin S are carried apart to a great distance (e.g., in a molecular dissociation process), and Experiment 2, which is similar but refers to two bare protons. The crucial question to be asked is whether the spin coupling will be conserved at all distances: if it is, then the system exhibits “nonlocality” (the two particles stay “correlated” in some way, even at infinite distance) and thus violates Einstein's principle of locality—which denies that possibility. A recent discussion of Experiment 1 shows that nonlocality is the rule only up to a point at which the singlet–triplet interval is small enough to be bridged by weak interaction with the “heat bath” in which the system is embedded: Beyond that point, the system is no longer described by a wave function but instead by a statistical ensemble. When ensemble averaging is admitted, the spin‐correlation function Q c ( r 1 ,  r 2 ) decreases to zero at all points in space and the coupling is broken; the particles are then independent and neither can be influenced by its previous interaction with the other. In the present work, the same approach is used to discuss the two‐proton system (Experiment 2). The conclusions are similar: The protons are described by appropriate wave packets, with an initial overlap sufficient to give a substantial singlet–triplet separation Δ E , and, again, the spin coupling is broken when the overlap (and, consequently, Δ E ) decreases to a sufficiently small value. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 74: 573–584, 1999