  We discuss the backreaction force exerted by quantum fluctuations in dilute
Bose-Einstein condensates onto the motion of the classical background, derived
by an ab initio approach from microscopic physics. It is shown that the
effective-action method, widely employed in semiclassical quantum gravity,
fails to give the full backreaction force. The failure of the effective-action
method is traced back, inter alia, to the problem of the correct choice of the
fundamental variables and the related operator ordering issues.
