 Derivations in this group are distinctive because they demonstrate that mass-energy equivalence is a consequence of the changes to the structure of spacetime brought about by special relativity. The relationship between mass and energy is independent of Maxwell’s theory or any other theory that describes a specific physical interaction. We can get a glimpse of this by noting that to derive \(E_o = mc^2\) by analyzing a collision, one must first define the four-momentum \(\mathbf{p}\), the “space-part” of which is relativistic momentum \(\mathbf{p}_{\rel}\), and relativistic kinetic energy \(T_{\rel}\), since one cannot use the old Newtonian notions of momentum and kinetic energy. 