Talk:Energy-matter conservation

Well, I'd be for getting rid of the above stub entirely, or else expanding it a LOT.

The problem is the word "matter." It's got problems, as noted above, because it's not total MASS. In relativity, single observers (in single inertial frames) measure momentum, total energy, and a combination of these called invariant mass (mass for short), all to be separately conserved, in reactions in closed systems. Whenever you see somebody talking about conservation of "energy-matter" you know they're really trying to talk about muddy circumstances in which "matter" has somehow been "turned into" energy, but the additive combination is conserved. But in that case, by "matter" they mean "a sum of rest masses of matter particles" which is complicated and somewhat articifical, because it's never what we measure in a system (where those particles are not at rest, and are often subject to terrific potential binding energies). The sum of rest masses is always something we calculate by taking rest masses out of a book and adding them up. You can actually do that to get the active energy released in nuclear reactions, and that's where this whole idea of "sum of mass-energy conservation" comes from. However, it's (as I said) artificial is some ways. By contrast, total momentum, total energy and invariant mass of many systems is measureable directly. If you have a system on scales, its total momentum is zero, the mass is what it weighs, and its total energy is mass times c^2. During a reaction, none of those things change, if you keep the system closed. That's the most simple kind of conservation. Nothing is converted to anything. Mass is conserved, momentum is conserved, and total energy is conserved.

So anyway, all this has to be explained. Some things in physics you should say a lot about, or else nothing at all. In between always gets makes you say something that is wrong. Steve 20:56, 19 June 2006 (UTC)