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--- On Sun, 6/28/09, John Denker <jsd@av8n.com> wrote:
"It doesn't "come from" anywhere. You shouldn't assume it needs to
"come from" anywhere. Asking where it comes from has no physical
significance, because mass is not conserved. There is no reason
why it should be."
It comes from the past. If an isolated system (e.g. positronium) had a certain rest mass before reaction (e.g., annihilation), it has the same rest mass after reaction. In case of annihilation, the system of emerging two photons has the rest mass exactly equal to the initial mas of positronium. The system has changed beyond recognition, but its rest mass remains as before. The rest mass of the new state comes from the rest mass of the initial state.This is what conservation means.
.
" Energy is conserved. Rest energy (by
itself) is not conserved. There is no reason why it should be".
The reason is that the rest energy is also energy. Would you deny this?