[mr_mitm]

Yes, there is no law of conservation for mass like there is for energy. Fusion is a good example for why it's not conserved. The teacher was right.

[dguest]

He was right that it violates conservation of mass. He was completely wrong that it violated it by adding 2 atomic mass units when hydrogen fuses.

In reality heavier isotopes of hydrogen fuse, conserving the total number of nucleons, but the resulting hydrogen has a lower rest mass than the parent particles. The extra mass is released as energy and the total energy is conserved.

By his logic the system either violated energy conservation (by creating nucleons while releasing energy) or was endothermic (creating nucleons from the surrounding energy).

[tardedmeme]

There actually is a law of conservation of mass (it's the same law, because mass is energy) and it only appears violated if you forget about the particles that are zooming away at the speed of light. Of course the mass of a system changes if mass can flow in and out.

[mr_mitm]

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]

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]

The concepts of rest mass and relativistic mass are considered outdated.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.