Blackbody radiation
= Blackbody radiation
{wiki=Black-body_radiation}
Blackbody radiation is a thermal equilibrium gas of <photon>[photons]. In units with $k_B=1$, its energy density is
$$
u(T)=\frac{\pi^2T^4}{15\hbar^3c^3},
$$
and its isotropic <radiation pressure> is $p=u/3$.