Force autocorrelation function in linear response theory and the origin of friction

Vanishing of the equilibrium fluctuation expression for the friction coefficient of a massive particle in a finite-volume liquid has been well documented and discussed in literature. This paper investigates the decay of the friction force in the corresponding nonequilibrium situation, when the massive particle moves through a finite volume at a constant velocity. The friction force ultimately vanishes (with the decay form as predicted by the equilibrium integral) because of the finite mass of the rest of the system, which allows it to be dragged by the moving particle. However, it is sufficient to have two infinite masses moving relative to each other in a finite liquid volume for the friction force to be finite at all times. (C) 2008 American Institute of Physics.