Here we discuss blackbody radiation within the context of classical theory. We note that
nonrelativistic classical mechanics and relativistic classical electrodynamics have contrasting
scaling symmetries which influence the scattering of radiation. Also, nonrelativistic mechanical
systems can be accurately combined with relativistic electromagnetic radiation only provided the
nonrelativistic mechanical systems are the low-velocity limits of fully relativistic systems.
Application of the no-interaction theorem for relativistic systems limits the scattering mechanical
systems for thermal radiation to relativistic classical electrodynamic systems, which involve the
Coulomb potential. Whereas the naive use of nonrelativistic scatterers or nonrelativistic classical
statistical mechanics leads to the Rayleigh–Jeans spectrum, the use of fully relativistic scatterers
leads to the Planck spectrum for blackbody radiation within classical physics.