 One of the few exceptions to this approach among derivations that use Maxwell’s theory is Einstein’s 1906 derivation (Einstein 1906). In this derivation, Einstein considers a freely-floating box. A burst of electromagnetic radiation of energy \(E_o\) is emitted inside the box from one wall toward a parallel wall. Einstein shows that the principle of mechanics that says that the motion of the center of mass of a body cannot change merely because of changes inside the body would be violated if one did not attribute an inertial mass \(E_o /c^2\) to the burst of electromagnetic radiation (see Taylor and Wheeler 1992, p. 254 for a detailed discussion of this example). 