Birkhoff's theorem, many-body response functions, and the ergodic condition.

The ergodic hypothesis is viewed as one that is physically measurable through scattering by an external probe. Linear response theory is used to derive a general ergodic condition on the response functions. It is shown that the same condition is also implied by Birkhoff's theorem. This coincidence allows us to shed light on the abstract terms of the theorem via classical many-body models.