Thermal Radiation Drag on Neutral Objects

We compute the force on a small neutral polarizable object moving at velocity ~v relative to a photon gas equilibrated at a temperature T We find a drag force linear in ~v. Its physical basis is identical to that in recent formulations of the dissipative component of the Casimir force. We estimate the strength of this universal Casimir drag force for different dielectric response functions and comment on its relevance in various contexts. PACS numbers: 72.20.-i, 72.20 Jv, 72.40.+w, 73.50.Pz The residual drag force on an AFM tip close to, but not in direct contact with a substrate in vacuo raises an important and fundamental question on the origin of non-contact friction [1]. Other experimental techniques also are sensitive to non-contact friction [2]. Since it became clear that the Casimir effect, being par excellence a non-contact phenomenon, can lead to a dissipative drag (see [4] and references therein), it has become a primary focus of theoretical research [3]. Such Casimir dissipative drag occurs when electromagnetic field fluctuations equilibrate in a specific reference frame, relative to which another system (e.g. a dielectric or a conducting body) is in uniform motion [4, 5, 6]. The difference between the frames of reference in the dissipative Casimir effect can be due to relative motion of different bodies, as for two conducting plates with relative motion in the parallel direction, or for a neutral body moving relative to a conducting plate [7, 8, 9, 10, 4, 11]. In these cases the radiation equilibrates in one of the plates, and the friction depends upon the proximity of the other one. We show here that such friction can also come about when a single body moves relative to a thermal bath of the electromagnetic field excitations, such as those between the walls of an oven or in the cosmic microwave background. The friction has no position 1 dependence, i.e. it is spatially homogeneous. The consequence is a universal dissipative drag acting on all matter in relative motion with respect to a thermalized photon gas. To estimate the magnitude of this universal drag we evaluate it as a function of the dominant frequency of the electromagnetic response of the body for dielectrics and conductors. Consider the Lorentz force ~ F = ∫