  In this paper, we address the issue of the stability of the thermal
equilibrium of large quantum systems with respect to variations of the thermal
contact between them. We study the Schr\"odinger time evolution of a free
bosonic field in two coupled one-dimensional cavities after a sudden change of
the contact between the cavities. Though the coupling we consider is
thermodynamically small, modifying it has a considerable impact on the
two-point correlation functions of the system. We find that they do not return
to equilibrium but essentially oscillate with a period proportional to the
length of the cavities. We compare this coupled cavities system with the
perfect gas which is described by similar expressions but behaves very
differently.
