Origin of the violation of the fluctuation–dissipation theorem in systems with activated dynamics

We analyze the validity of the fluctuation–dissipation theorem for slow relaxation systems in the context of mesoscopic nonequilibrium thermodynamics. We demonstrate that the violation arises as a natural consequence of the elimination of fast variables in the description of a glassy system, and it is intrinsically related to the underlying activated nature of slow relaxation. In addition, we show that the concept of effective temperature, introduced to characterize the magnitude of the violation, is not robust since it is observable-dependent, can diverge, or even be negative.