  Vanishing of the equilibrium Green-Kubo fluctuation expression for the
friction coefficient of a massive particle moving in a finite-volume liquid is
usually interpreted as an unphysical consequence of the finite volume. Here I
show that it is a physical consequence of the finite mass of the rest of the
system, which allows it to be dragged by the moving particle. As a consequence,
it is sufficient to have two infinite masses in the liquid for the friction
coefficient to be finite. In addition, I give the physical interpretation of
different friction coefficients for two infinite-mass particles moving in the
liquid.
