  The standard argument for the Lorentz invariance of the thermodynamic entropy
in equilibrium is based on the assumption that it is possible to perform an
adiabatic transformation whose only outcome is to accelerate a macroscopic
body, keeping its rest mass unchanged. The validity of this assumption
constitutes the very foundation of relativistic thermodynamics and needs to be
tested in greater detail. We show that, indeed, such a transformation is always
possible, at least in principle. The only two assumptions invoked in the proof
are that there is at least one inertial reference frame in which the second law
of thermodynamics is valid and that the microscopic theory describing the
internal dynamics of the body is a field theory, with Lorentz invariant
Lagrangian density. The proof makes no reference to the connection between
entropy and probabilities and is valid both within classical and quantum
physics. To avoid any risk of circular reasoning, we do not postulate that the
laws of thermodynamics are the same in every reference frame, but we obtain
this fact as a direct consequence of the Lorentz invariance of the entropy.
