The average velocity of a particle will decrease if the particle interacts with a random force. This type of friction originates from the theory of Brownian motion. In 1827, the British botanist R. Brown was the first to report the irregular motion of a pollen in a liquid. A theoretical description of Brownian motion was provided by Einstein and Smoluchowski in the early twentienth century. Later, it turned out that fluctuations and dissipation are closely related. This relationship, known as fluctuation-dissipation theorem, is encountered in many disciplines. Also, dissipation is related to the phenomenon of decoherence encountered, for example, in quantum mechanics.

In our investigations we study the friction experienced by a particle due to its interaction with thermal electromagnetic fields. These fields are generated by matter at finite temperatures. The statistical correlation properties of thermal fields determine the strength of the particle's dissipation. The statistical properties depend on the geometry of the particle's environment as well as the electromagnetic properties of particle and surrounding matter.
Our work is motivated by recent experiments that measured friction acting on nanoscale probes near planar substrates in ultra-high vacuum conditions. Existing theories lead to damping coefficients that are orders of magnitude weaker than those measured in experiments. Therefore, until recently, the physical picture underlying friction in nanoscale systems was not clear. However, our theory predicts that friction originates from electromagnetic interactions between dielectric objects as opposed to metallic objects. Furthermore, these interactions can penetrate through thin metal layers which explains the previous discrepancy between theory and experiment. More details on our theoretical approach can be found in our preprint [PDF, 903K].