 Einstein correctly described the equivalence of mass and energy as “the most important upshot of the special theory of relativity” (Einstein 1919), for this result lies at the core of modern physics. Many commentators have observed that in Einstein’s first derivation of this famous result, he did not express it with the equation \(E = mc^2\). Instead, Einstein concluded that if an object, which is at rest relative to an inertial frame, either absorbs or emits an amount of energy \(L\), its inertial mass will correspondingly either increase or decrease by an amount \(L/c^2\). In Newtonian physics, inertial mass is construed as an intrinsic property of an object that measures the extent to which an object resists changes to its state of motion. So, Einstein’s conclusion that the inertial mass of an object changes if the object absorbs or emits energy was revolutionary and transformative. For as Einstein concluded “If the theory agrees with the facts, then radiation transmits inertia between emitting and absorbing bodies” (Einstein, 1905b). Yet, in Newtonian physics, inertia is not the kind of thing that can be transmitted between bodies. 