  Recently the Casmir-Polder force felt by an atom near a substrate under
nonequilibrium stationary conditions has been studied theoretically with
macroscopic quantum electrodyanamics (MQED) and verified experimentally with
cold atoms. We give a quantum field theory derivation of the Langevin equation
describing the atom's motion based on the influence functional method valid for
fully nonequilibrium (nonstationary) conditions. The noise associated with the
quantum field derived from first principles is generally colored and nonlocal,
which is at variance with the `local source hypothesis' of MQED's
generalization to nonequilibrium conditions. Precision measurements on the
shape deformation of an atomic gas as a function of its distance from a mirror
would provide a direct check of our predictions based on this Langevin
equation.
