2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/147334A thermodynamical minimum principle valid for photon radiation is shown to hold for arbitrary geometries. It is successfully extended to neutrinos, in the zero mass and chemical potential case, following a parallel development of photon and neutrino statistics. This minimum principle stems more from that of Planck than that of classical Onsager- Prigogine irreversible thermodynamics. Its extension from bosons to fermions suggests that it may have a still wider validity.18 pages, LaTeX, requires REVTeX, no figures, accepted by J. Stat. Phys.; minor clarifications and additionsAstrophysicsStatistical Mechanicstext