Okay, we’re defining the mass of a system as its total energy in its zero momentum frame? If so, I’m not sure I see yet why this definition is justified, but now I understand most of your comments so far. But not this one:
I don’t see why this is true at all. If the individual particles are moving at a speed v = 0.6 (c = 1), then they each have total energy E = 1.25m[sub]e[/sub], and the total energy of the system is E = 2.5m[sub]e[/sub], not 2m[sub]e[/sub]. What am I doing wrong?