  How do indistinguishable identical bosons manage to obey Bose-Einstein
statistics---and hence be correlated---even when they do not interact with each
other? Part of the answer is that the bosons have to interact indirectly with
each other by interacting with the same environment. A joint measurement
interaction provides a good example. Thermalization occurs whenever there are
two competing processes, one diagonal in the energy basis (namely, reversible
Hamiltonian evolution), the other irreversible and diagonal in a complementary
basis (for example, a measurement in a spatially localized basis). Correlations
arise only from initial states in which the bosons start in different
(orthogonal) states.
