 There is nothing unique about energy absorption in these examples. As the battery loses energy, say by powering a device, or the brick emits thermal energy as it cools, its mass decreases. Consequently, imagine a suitably idealized closed system in which two objects \(B_1\) and \(B_2\) are in a state of relative rest. If \(B_1\) radiates an amount of energy \(E_o\) and \(B_2\) fully absorbs that same amount of energy, the mass of \(B_1\) decreases by \(E_o /c^2\) while the mass of \(B_2\) increases by the same amount. This physical change in the masses of \(B_1\) and \(B_2\) is a novel prediction in special relativity. For in Newtonian physics, there is no relationship at all between the inertial mass of a body and the amount of energy it radiates or absorbs. This is why Einstein was led to conclude that “If the theory agrees with the facts, then radiation transmits inertia between emitting and absorbing bodies” (Einstein 1905b). If special relativity is supported by empirical evidence, the inertial mass of an object can change, not because we have chopped off a piece of the object or attached more stuff to it, but merely because the object has radiated or absorbed energy. To physicists and philosophers trained exclusively in the Newtonian tradition, this result would have seemed perhaps extraordinary but certainly revolutionary. 