For several compelling reasons, many physicists have come to accept that mass is invariant,65 even though there is no proof—theoretical or experimental—that it is. If mass is speed-independent, then E0 = mc2 and only the internal en- ergy, the rest-energy, of an object determines its mass. However, if mass is relativistic, then E = mRc2 and the kinetic energy of a system moving as a whole also contributes to its mass.