On Newton's Third Law

TL;DR

This paper explores the foundational aspects of Newton's Third Law, examining its validity in different physical contexts, especially out of equilibrium, and highlights the role of the physical vacuum and entropy in potential violations.

Contribution

It provides a comprehensive survey of questions related to the action-reaction law and introduces a framework involving the physical vacuum to understand its violations.

Findings

Newton's third law may not hold out of equilibrium due to entropic effects.

The physical vacuum can clarify conditions for action-reaction law violations.

An expression for particle momentum in statistical mechanics is derived.

Abstract

The law of action-reaction is thoroughly used in textbooks to derive the conservation laws of linear and angular momentum, and it was considered by Ernst Mach the the cornerstone of physics. We give here a background survey of several questions raised by the action-reaction law, and in particular, the role of the physical vacuum is shown to provide an appropriate framework to clarify the occurrence of possible violations of the action-reaction law. It is also obtained an expression for the general linear momentum of a body-particle in the context of statistical mechanics. It is shown that Newton's third law is not verified in systems out of equilibrium due to an additional entropic gradient term present in the particle's momentum.