 Einstein’s original derivation of mass-energy equivalence is the best known in this group. Einstein begins with the following thought-experiment: a body at rest (in some inertial frame) emits two pulses of light of equal energy in opposite directions. Einstein then analyzes this “act of emission” from another inertial frame, which is in a state of uniform motion relative to the first. In this analysis, Einstein uses Maxwell’s theory of electromagnetism to calculate the physical properties of the light pulses (such as their intensity) in the second inertial frame. By comparing the two descriptions of the “act of emission”, Einstein arrives at his celebrated result: “the mass of a body is a measure of its energy-content; if the energy changes by \(L\), the mass changes in the same sense by \(L/9 \times 10^{20}\), the energy being measured in ergs, and the mass in grammes” (1905b, p. 71). A similar derivation using the same thought experiment but appealing to the Doppler effect was given by Langevin (1913) (see the discussion of the inertia of energy in Fox (1965, p. 8)). 