  The law of action-reaction, considered by Ernst Mach the cornerstone of
physics, is thoroughly used to derive the conservation laws of linear and
angular momentum. However, the conflict between momentum conservation law and
Newton's third law, on experimental and theoretical grounds, call for more
attention. We give 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. Then, in the framework of statistical
mechanics, using a maximizing entropy procedure, we obtain an expression for
the general linear momentum of a body-particle. The new approach presented here
shows 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.
