The third law is just about the conservation of the momentum. It follows from the translational symmetry of the laws of physics and it holds for any description, whether or not it contains fields. The description in terms of a force, action at a distance, has the same force with the opposite sign acting on both objects. If there are fields, the fields may carry some momentum, too. In quantum field theory, the force comes from virtual particles – virtual quanta of the fields – and they again carry the right momentum so that it is always conserved. So the law always holds. Does it answer “How”?

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— Physics Stack Exchange

— Apr 6 ’11 at 12:17

— Lubos Motl

2013.08.23 Friday ACHK

The ergodicity requirement is that the ensemble average coincide with the time average. A sufficient condition for ergodicity is that the time evolution of the system is a mixing. (See also ergodic hypothesis.) Not all systems are ergodic. For instance, it is unknown at this time whether classical mechanical flows on a constant energy surface are ergodic in general.

Physically, when a system fails to be ergodic, we may infer that there is more macroscopically discoverable information available about the microscopic state of the system than what we first thought. In turn this may be used to create a better-conditioned ensemble.

— Wikipedia on Statistical ensemble (mathematical physics)

2013.08.22 Thursday ACHK