  For many physical systems which can be approximated by a classical background
field plus small (linearized) quantum fluctuations, a fundamental question
concerns the correct description of the backreaction of the quantum
fluctuations onto the dynamics of the classical background. We investigate this
problem for the example of dilute atomic/molecular Bose-Einstein condensates,
for which the microscopic dynamical behavior is under control. It turns out
that the effective-action technique does not yield the correct result in
general and that the knowledge of the pseudo-energy-momentum tensor ${<\hat
T_{\mu\nu}>}$ is not sufficient to describe quantum backreaction.
