Abstract

Using computer simulations, we investigate the time evolution of the (Boltzmann) entropy of a dense fluid not in local equilibrium. The macrovariables M describing the system are the (empirical) particle density f = {f(x,v)} and the total energy E. We find that S(ft,E) is a monotone increasing in time even when its kinetic part is decreasing. We argue that for isolated Hamiltonian systems monotonicity of S(Mt) = S(MXt) should hold generally for ‘‘typical’’ (the overwhelming majority of) initial microstates (phase points) X0 belonging to the initial macrostate M0, satisfying MX0 = M0. This is a consequence of Liouville's theorem when Mt evolves according to an autonomous deterministic law.