  A general expression is derived for the momentum diffusion constant of a
small polarizable particle in blackbody radiation, and is shown to be closely
related to the long-wavelength collisional decoherence rate for such a particle
in a thermal environment. We show how this diffusion constant appears in the
steady-state photon emission rate of two dipoles induced by blackbody
radiation. We consider in addition the Einstein--Hopf drag force on a small
polarizable particle moving in a blackbody field, and derive its fully
relativistic form from the Lorentz transformation of forces.
