Schrollini wrote:It's rest mass, of course. After all, the rest mass is defined as the Lorentz invariant of the energy-momentum four-vector.

What if I've got a mirror box containing a huge amount of light? If the box itself, without the light, has a mass of 1 kg, and the total energy of the light is c

^{2} kg, the total mass is 2 kg. I would have thought that when treating the box with light as a single thing, the internals of which we aren't interested in, we'd treat it all as 2 kg of mass, as rest mass, but that when taking an interest in the light, we'd probably start treating the 1 kg of light as c

^{2} kg of energy, or whatever, as that would then be more convenient.

My complaint is with the habitual dropping of the momentum term from the relativistic energy equation. This creates confusion about the energy of photons and leads to abominations like "relativistic mass". Sure, E = mc^{2} when p = 0, but that's like saying the equation of a circle is r^{2} = x^{2} when y = 0. It's leaving out some rather important information.

I thought it was more of a old-school thing, rather than actually missing stuff out.

Suppose I've got a spring with two, equal masses on the ends. Initially, the spring is held in a compressed state, and stores a huge amount of energy, c

^{2} kg. Each attached mass is 1 kg. The spring itself is so light it's mass is negligible. So, in total, there's 3 kg of rest mass. The spring is released, and the masses start accelerating away. At some point, when the masses are moving at their fastest before stretching the spring and slowing down, each mass has 1 kg of rest mass and c

^{2}/2 kg of kinetic energy. What is the total mass of the system as a whole? I would have said 3 kg. Is the extra kilogram rest mass?

I think I do understand that it's useful to distinguish between rest mass and other mass-energy, and that being disciplined about it is a good idea, but I'm not sure that being dogmatic about it is always quite so helpful.

Edited to add the following:-

lgw wrote:For energy, I find it baffling, perhaps because it's such a nebulous catch-all term. How the heck could potential energy couple to anything? How does a spring "know" to interact more strongly with the Higgs field when compressed - or worse, a stretched rubber band, where the potential energy depends on the nearby temperature (energy of other molecules at some distance).

Are you thinking that energy gets corresponding mass because of the Higgs field, that mass-energy equivalence is due to the Higgs field? That's what it sounds like, but that's just wrong. If you weighed a 1 kg mirror box containing c

^{2} kg of energy in the form of light, you'd find it would have a total mass of 2 kg, without any of the photons interacting with the Higgs field at all.

I am male, I am 'him'.