\b[Ordinary equilibrium blackbody <photons> do not undergo this fixed-number condensation.] Their number adjusts through absorption and emission, while their <chemical potential> stays zero; cooling reduces the equilibrium <photon> number rather than forcing an excess into the <ground state>. This answer concerns the thermal blackbody model, not specially constrained systems where an effective <photon> number is maintained.

Solution

Past exam of the mathematics course of the University of Cambridge / 2015 / ii / Paper 2 / 33C / d / Solution

 Solution (source code)