According to the Second Law of Thermodynamics, cycles applied tothermodynamic equilibrium states cannot perform work (passivity property ofthermodyamic equilibrium states). In the presence of matter this can hold onlyin the rest frame of the matter, as moving matter drives, e.g., windmills andturbines. If, however, a homogeneous and stationary state has the property thatno cycle can perform more work than an ideal windmill, then it can be shownthat there is some inertial frame where the state is a thermodynamicequilibrium state. This provides a covariant characterization of thermodynamicequilibrium states. In the absence of matter, cycles should perform work only when driven bynonstationary inertial forces caused by the observer's motion. If a pure stateof a relativistic quantum field theory behaves this way, it satisfies thespectrum condition and exhibits the Unruh effect.
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