Comment by rnhmjoj

1 day ago

> - what is a "photon gas"? Is this a state of matter? What is the matter if photons aren't matter?

It's yet another misleading use of the word "photon". A photon gas basically refers to the statistical behaviour of quantised oscillators (typically some atoms/molecules that can vibrate) in the walls of a closed box, called a "cavity". Since the oscillators de-excite by emitting radiation which stays inside the box until it's re-absorbed by the oscillators, you can sort of get away with thinking of it like the distribution of the modes of oscillations of the electromagnetic fields inside the cavity, which is what photons actually are.