  A derivation of the Planck spectrum for thermal radiation is given based upon
wave fluctuations within relativistic classical physics. The derivation depends
crucially on thermal fluctuations existing above the fundamental
inertial-frame-independent fluctuations of classical zero-point radiation. Such
frame-independent zero-point fluctuations exist only in a relativistic wave
theory and cannot exist in a nonrelativistic wave theory. Thus such a classical
derivation of the Planck spectrum exists in a Lorentz-covariant classical
theory, such as classical electrodynamics, but not in a Galilean-covariant
theory where all waves are based upon material media. Classical zero-point
radiation provides a purely classical alternative to quanta in the analysis of
the Planck spectrum.
