Friction force at the motion of a small relativistic neutral particle with respect to blackbody radiation

The friction force at the motion of a small relativistic neutral particle with respect to blackbody radiation has been calculated within fluctuation electrodynamics. It has been shown that the acceleration of the particle is determined by the friction force in the rest reference frame of the particle ($K'$ reference frame), which is not generally equal to the friction force in the reference frame specified by blackbody radiation ($K$ reference frame). The difference between friction forces in different reference frames is due to a change in the rest mass of the particle owing to the absorption and emission of radiation by the particle. The friction force in the $K'$ reference frame is determined only by the interaction of the particle with blackbody radiation. Consequently, it depends only on the temperature of this radiation and is independent of the temperature of the particle. The interaction of the particle with its own thermal radiation in the $K$ reference frame also contributes to the friction force. At a constant temperature of the particle, the friction forces in the $K'$ and $K$ reference frames are equal to each other. The friction force of blackbody radiation for an atom is determined by the radiative broadening of an electron line of the atom, which is calculated by taking into account the interaction of the atom with its radiation. In the ultrarelativistic case ($1-\beta\to0$), the friction force for the atom diverges as $(1-\beta)^{-3}$ and the (average) temperature of the atom is $T_2\approx(1-\beta)^{-3/8}T_1$, where $T_1$ is the temperature of blackbody radiation and $\beta=V/c$. Discrepancies in the theory of the friction force caused by blackbody radiation have been discussed.