 At a time when physicists were moving toward regarding energy-carrying fields, such as the electromagnetic field, as entities in their own right, combining the principle of equivalence with the “inertia of energy” led Einstein to the insight that fields themselves could gravitate. So, for example, Einstein claims in 1907 to show that “radiation enclosed in a cavity possesses not only inertia but also weight” (Einstein 1907b, p. 288). A more contemporary and fairly common way of making the same point is to say that a mirror box with perfectly reflecting walls filled with light (conceived here for convenience as an energy-carrying disturbance in the electromagnetic field) is attracted by gravity by an amount that is greater than the rest mass \(M\) of the box itself. Specifically, if the energy of the light in the box is \(E_L\), gravity acts on the box not as if the box has a mass \(M\) but as if the box has a mass \(M + E_L /c^2\). Although the difference is numerically tiny, strictly speaking a balance with an empty mirror box on one side and an identical mirror box filled with light on the other would not be level. 