  Recent experiments with photons equilibrating inside a dye medium in a cavity
have raised the question of whether Bose condensation can occur in a system
with only incoherent interaction with phonons in a bath but without
particle-particle interaction. Analytical calculations analogous to those done
for a system with particle-particle interactions indicate that a system of
bosons interacting only with incoherent phonons can indeed undergo Bose
condensation and furthermore can exhibit spontaneous amplification of quantum
coherence. We review the basic theory for these calculations.
