Ballistic Diffusion of a Charged Particle in a Blackbody Radiation Field

The generalized Langevin equation is used to describe the motion of a charged particle interacting with a blackbody radiation field via dipole coupling. The exact expressions for the mean-square displacement and velocity of such a particle are obtained, which show a ballistic diffusion and a modified Kubo fluctuation--dissipation relation. In particular, a velocity-dependent coupling or force can induce this novel phenomenon.