  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.
