  Here we show that within classical physics, the Planck blackbody spectrum can
be derived directly from the structure of relativistic spacetime. In
noninertial frames, thermal radiation at positive temperature is connected
directly to zero-point radiation whose spectrum follows from the geodesic
structure of the spacetime. The connection between zero-point radiation and
thermal radiation at postive temperature is through a time-dilating conformal
transformation in the noninertial frame. Transferring the spectrum back to
Minskowski spacetime, the Planck spectrum is obtained.
