A Newtonian approach to the study of irreversibility in many-body systems

TL;DR

This paper explores how the Newtonian dynamics of a system's center of mass can explain the emergence of irreversibility in macroscopic systems, despite microscopic time symmetry, addressing Loschmidt's paradox.

Contribution

It introduces a Newtonian perspective focusing on the center of mass to explain macroscopic irreversibility, independent of internal interactions and equilibrium distance.

Findings

External conditions induce unidirectional time evolution.

Microscopic time symmetry does not prevent macroscopic irreversibility.

Center of mass dynamics can predict the approach to equilibrium.

Abstract

Irreversibility remains one of the least understood concepts in physics. One of the main reasons is the fact that the fundamental laws of classical and quantum physics are time symmetric, whereas macroscopic processes evolve in a preferred time direction. This long-standing conflict of ideas goes to the heart of the so-called Loschmidt's paradox. Here, we address this dichotomy from a Newtonian perspective of the dynamics of the center of mass of a many-body system, which was found to impose constraints on the time evolution. We demonstrate that despite the time symmetric behavior of the microscopic constituents of the system, in many typical cases the external conditions lead naturally to the unidirectional macroscopic time evolution, independent of the internal interactions and how far the system is from equilibrium. We also illustrate our findings by direct calculation of the center of mass dynamics for a nonequilibrium steady state system partially free from external influence and for a system following the route to equilibrium.