I think the original explanation was terrible I also think that the explanation based on g E c 2 makes no sense whatsoever In effect it encourages the people that claim that the photons have an equivalent mass m E c 2 which is totally wrong Come to think of it the whole page misses the fact that current experiments put a limit of 6 10 17 eV on the photon mass with newer experiments set to lower this limit This limit is in total contradiction with any type of E c 2 derivation If you don t like my explanation of photon in the box could you please remove the entry altogether It is very misleading and badly written as originally set Thank you