The Source of Irreversibility in Macroscopic Dynamics

The dynamics of irreversible relaxation of non-equilibrium macroscopic systems is discussed. Arguments are developed showing that the general process is supported by two independent successive mechanisms. One is mixing and it follows pure conservative dynamics. It does not lead to thermodynamic equilibrium. The second step involves dissipation to the outside world of the information contained in the initial constraints. This step implies interactions with the surroundings. Strict isolation is excluded. The first step is only apparently irreversible. Strict irreversibility concerns only the second one. The first step is isentropic. Entropy change implies the second step. Strictly isolated non-equilibrium systems do not relax towards a new thermodynamic equilibrium.