  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.
