Stefan-Boltzmann law revisited

Abstract

The Stefan-Boltzmann (SB) law relates the emissivity $q$, given in $Wm^{-2}$, of an ideal black-body cavity at thermal equilibrium to the fourth power of the absolute temperature $T$ as $q=σT^4$, with $σ= 5.67 \times 10^{-8} \ W m^{-2} K^{-4}$ the SB constant, firstly estimated by Stefan to within $11$ per cent of the actual value. The law is a pillar of modern physics since its microscopic derivation implies the quantization of the energy related to the electromagnetic field. Somewhat astonishing, Boltzmann presented his derivation in 1878 making use only of electrodynamic and thermodynamic classical concepts, apparently without introducing any quantum hypothesis (here called first Boltzmann paradox). By using Planck (1901) quantization of the radiation field in terms of a gas of photons, the SB law received a microscopic interpretation providing also the value of the SB constant on the basis of a set of universal constants including the quantum action constant $h$. However, the successive consideration by Planck (1912) of the zero-point energy contribution was found to be responsible of another divergence of the radiation energy-density for the single photon mode at high frequencies. This divergence is of pure quantum origin and is responsible for a vacuum-catastrophe, to keep the analogy with the well-known ultraviolet catastrophe of the classical black-body radiation spectrum, given by the Rayleigh-Jeans law in 1900. As a consequence, from a rigorous quantum-mechanical derivation we expect the divergence of the SB law (here called second Boltzmann paradox). In this paper we revisit the SB law by accounting for genuine quantum effects associated with Planck energy quantization and Casimir size quantization thus resolving both Boltzmann paradoxes