Consider a science fiction universe in which the number of photons N is constant, at a concentration of 1020 cm–3. The number of thermally excited photons we assume is given by the result of Problem 4.1, which is Ne = 2.404 Vτ3/π2/h3c3. Find the critical temperature in K below which Nε < N. The excess N – Nε will be in the photon mode of lowest frequency; the excess might be described as a photon condensate in which there is a large concentration of photons in the lowest mode. In reality there is no such principle that the total number of photons be constant, hence there is n o photon condensate.