 In general, derivations in this group use the same style of reasoning. One typically begins by considering an object that either absorbs or emits electromagnetic radiation (typically light) of total energy \(E_o\) in equal and opposite directions. Because light carries both energy and momentum, one then uses the conservation principles for those quantities and the standard heuristic in relativity of considering the same physical process from two different inertial frames that are in a state of relative motion to show that in order for the conservation principles to be satisfied, the mass (i.e., rest-mass) of the emitting or absorbing object must increase or decrease by an amount \(E_o /c^2\). For a more detailed description of a simplified derivation in this group, see  Section 1.5 