The behavior of thermal phonons is similar to the photon gas produced by an electromagnetic cavity, wherein photons may be emitted or absorbed by the cavity walls.
12.
This electron may drop back to its lower level in a series of steps, each one of which releases an individual photon back into the photon gas.
13.
It would be similar to an ideal gas or a photon gas ( photons also barely interact with each other ), and it would likewise have a temperature.
14.
In a blackbody enclosure that contains electromagnetic radiation with a certain amount of energy at thermodynamic equilibrium, this " photon gas " will have a Planck distribution of energies.
15.
A very important difference between a gas of massive particles and a photon gas with a black body distribution is that the number of photons in the system is not conserved.
16.
As photon gas expanded and cooled, some fermions would be left over ( in extremely small amounts ~ 10 " 10 ) because low energy photons could no longer break them apart.
17.
By contrast to a material gas where the masses and number of particles play a role, the spectral radiance, pressure and energy density of a photon gas at thermal equilibrium are entirely determined by the temperature.
18.
In a similar manner, even photons ( light quanta ), if trapped in a container space ( as a photon gas or thermal radiation ), would contribute a mass associated with their energy to the container.
19.
If the photon gas is not Planckian, the second law of thermodynamics guarantees that interactions ( between photons and other particles or even, at sufficiently high temperatures, between the photons themselves ) will cause the photon energy distribution to change and approach the Planck distribution.
20.
As shown above, to produce ordinary baryonic matter out of a photon gas, this gas must not only have a very high photon density, but also be very hot the energy ( temperature ) of photons must obviously exceed the rest mass energy of the given matter particle pair.
