Well there are a couple of things to address here. The first is that the invariant mass is invariant - it's the same in any reference frame. So the invariant ma

Well there are a couple of things to address here. The first is that the invariant mass is invariant - it's the same in any reference frame. So the invariant mass of a system will either equal the sum of the masses of the particles or it won't, irrespective of frame. And equality will only hold if there is no internal movement. > Can you make a convincing case that it's reasonable to apply that equation in that way? Even hpcjoe did not object to that, only to the name "rest mass" for the result. We saw that the number you get out was useful upthread, because it allowed predicting if pair production can occur or not.