Thermal radiation can be treated thermodynamically as a gas of photons with internal energy U=u(T)V and pressure p=u(T)/3, where u(T) is the energy density. Show that:
(a) the entropy density s is given by s=4p/T;
(b) the Gibbs function G=0;
(c) the heat capacity at constant volume Cv=3s per unit volume;
(d) the heat capacity at constant pressure, Cp, is infinite. (What on earth does that mean?)