Macroscopic effects of athermal noise

In equilibrium systems, the nature of fluctuations around deterministic behaviour is restricted by the fluctuation-dissipation theorem. However, the vast majority of systems are not at equilibrium, but driven out of equilibrium by the environment or (in the case of living systems) though internal processes. As such, fluctuations may arise from a source other than a heat bath and are not constrained by the fluctuation-dissipation theorem. By examining the case of birth-death dynamics in populations - which exhibit highly athermal “demographic” fluctuations - we find that the same deterministic system subjected to noise of two different amplitudes can exhibit vastly different behaviour at the macroscopic scale. We reflect on the consequences of this finding for a variety of physical and not-so-physical systems.