Abstract

Einstein's famous relation between mass and energy is interpreted in terms of the equivalence of the rate of heating of a body and the rate of increase of its inertial mass. In an adiabatic process, where the proper mass remains constant, it is the heat content, and not the energy, which is conserved because the pressure, and not the volume, is Lorentz-invariant. There are two categories of relativistic quantities: inertial and thermodynamic ones, which are transformed into one another by the work necessary to keep the inertial state in motion. In a non-adiabatic process, the rate of heating is Lorentz-invariant, which must always be greater than the power that it generates.