According to the Second Law of Thermodynamics, cycles applied to thermodynamic equilibrium states cannot perform work (passivity property of thermodyamic equilibrium states). In the presence of matter this can hold only in the rest frame of the matter, as moving matter drives, e.g., windmills and turbines. If, however, a homogeneous and stationary state has the property that no cycle can perform more work than an ideal windmill, then it can be shown that there is some inertial frame where the state is a thermodynamic equilibrium state. This provides a covariant characterization of thermodynamic equilibrium states. In the absence of matter, cycles should perform work only when driven by nonstationary inertial forces caused by the observer's motion. If a pure state of a relativistic quantum field theory behaves this way, it satisfies the spectrum condition and exhibits the Unruh effect.