There actually is a law of conservation of mass (it's the same law, because mass...

mr_mitm · 2026-05-16T13:28:39 1778938119

Mass is not the same as energy. Mass can be converted to energy or has energy, but a photon, for example, is massless while carrying energy.

tardedmeme · 2026-05-16T20:33:59 1778963639

That is incorrect. Photons have mass. They have no rest mass. They also cannot rest, so you might wonder how relevant that is.

mr_mitm · 2026-05-16T20:41:34 1778964094

The concepts of rest mass and relativistic mass are considered outdated.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 · 2026-05-17T20:39:25 1779050365

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.