>>12268224Basically the previous understanding was that if "photon 1" is at position 1 and "photon 2" is at position 2, swapping the two so that photon 1 is at position 2 and photon 2 is at position 1 is a new "configuration" or state of the system. Basically he showed they this new configuration is actually the same as the previous, because photons can't really be "numbered", in both cases its just 2 photons at 2 positions, which fixed the derivation of Planks law.
Easiest to think of it with coins, if you flip two coins the usual probability of 2 heads is 1/4, because you can get 2H, 2T, 1H1T, 1T1H. In this weird way for photons 1H1T and 1T1H are the same so its 1/3.
