  In this paper we give a first principles microphysics derivation of the
nonequilibrium forces between an atom, treated as a three dimensional harmonic
oscillator, and a bulk dielectric medium modeled as a continuous lattice of
oscillators coupled to a reservoir. We assume no direct interaction between the
atom and the medium but there exist mutual influences transmitted via a common
electromagnetic field. By employing concepts and techniques of open quantum
systems we introduce coarse-graining to the physical variables - the medium,
the quantum field and the atom's internal degrees of freedom, in that order -
to extract their averaged effects from the lowest tier progressively to the top
tier. The first tier of coarse-graining provides the averaged effect of the
medium upon the field, quantified by a complex permittivity (in the frequency
domain) describing the response of the dielectric to the field in addition to
its back action on the field through a stochastic forcing term. The last tier
of coarse- graining over the atom's internal degrees of freedom results in an
equation of motion for the atom's center of mass from which we can derive the
force on the atom. Our nonequilibrium formulation provides a fully dynamical
description of the atom's motion including back action effects from all other
relevant variables concerned. In the long-time limit we recover the known
results for the atom-dielectric force when the combined system is in
equilibrium or in a nonequilibrium stationary state.
