The role of the number of degrees of freedom and chaos in macroscopic irreversibility

This article aims at revisiting, with the aid of simple and neat numerical examples, some of the basic features of macroscopic irreversibility, and, thus, of the mechanical foundation of the second principle of thermodynamics as drawn by Boltzmann. Emphasis will be put on the fact that, in systems characterized by a very large number of degrees of freedom, irreversibility is already manifested at a single-trajectory level for the vast majority of the far-from-equilibrium initial conditions-a property often referred to as typicality. We also discuss the importance of the interaction among the microscopic constituents of the system and the irrelevance of chaos to irreversibility, showing that the same irreversible behaviors can be observed both in chaotic and non-chaotic systems.