[mr_mitm]

The concepts of rest mass and relativistic mass are considered outdated. In modern physics, "mass" means what they meant by "rest mass".

Here some indication I'm not making this up: https://hsm.stackexchange.com/questions/2465/when-and-why-di...

In any case, I never use those concepts, and I know no professional particle physicist that does. By "mass", I mean rest mass.

[tardedmeme]

When you put a photon in a stationary box, the "relativistic mass" of the photon becomes part of the "rest mass" of the photon-box system. You can't ignore it.