Comment on "Universal Thermal Radiation Drag on Neutral Objects"

In a recent letter [Phys. Rev. Lett. 91, 220801 (2003)], V. Mkrtchian and co-workers calculated the radiation pressure force on a moving body, assuming the electromagnetic field to be at temperature $T,$ and the velocity to be much smaller than $c.$ They considered both dielectrics and conductors, and related the effect to Casimir dissipative forces. Here we claim that their approach may only apply in the Rayleigh-Ganz scattering regime, which corresponds to very small particles and/or electromagnetically rarefied media. Moreover, we argue that their interpretation in terms of the Casimir effect is misleading, since vacuum fluctuations do not contribute in the (implicitly assumed) regime of uniform motion.