  The thermal friction force acting on an atom moving relative to a thermal
photon bath has recently been calculated on the basis of the
fluctuation-dissipation theorem. The thermal fluctuations of the
electromagnetic field give rise to a drag force on an atom provided one allows
for dissipation of the field energy via spontaneous emission. The drag force
exists if the atomic polarizability has a nonvanishing imaginary part. Here, we
explore alternative derivations. The damping of the motion of a simple harmonic
oscillator is described by radiative reaction theory (result of Einstein and
Hopf), taking into account the known stochastic fluctuations of the
electromagnetic field. Describing the excitations of the atom as an ensemble of
damped harmonic oscillators, we identify the previously found expressions as
generalizations of the Einstein-Hopf result. In addition, we present a simple
explanation for blackbody friction in terms of a Doppler shift of the thermal
radiation in the inertial frame of the moving atom: The atom absorbs
blue-shifted photons from the front and radiates off energy in all directions,
thereby losing energy. The original plus the two alternative derivations
provide for additional confirmation of an intriguing physical effect, and leave
no doubt regarding its existence.
