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 molecula
r dissociation process), and Experiment 2, which is similar but refers to t
wo bare protons. The crucial question to be asked is whether the spin coupl
ing will be conserved at all distances: if it is, then the system exhibits
"nonlocality" (the two particles stay "correlated" in some way, even at inf
inite distance) and thus violates Einstein's principle of locality-which de
nies that possibility. A recent discussion of Experiment 1 shows that nonlo
cality 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 de
scribed by a wave function but instead by a statistical ensemble. When ense
mble 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 pa
rticles 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 simila
r: The protons are described by appropriate wave packets, with an initial o
verlap sufficient to give a substantial singlet-triplet separation Delta E,
and, again, the spin coupling is broken when the overlap (and, consequentl
y, Delta E) decreases to a sufficiently small value. (C) 1999 John Wiley gr
Sons, Inc.