The triumph of Bohr consisted in his demonstrating, once again, that Einstein's subtle argument was not conclusive, but even more so in the way that he arrived at this conclusion by appealing precisely to one of the great ideas of Einstein: the principle of equivalence between gravitational mass and inertial mass, together with the time dilation of special relativity, and a consequence of these—the gravitational redshift. Bohr showed that, in order for Einstein's experiment to function, the box would have to be suspended on a spring in the middle of a gravitational field. In order to obtain a measurement of the weight of the box, a pointer would have to be attached to the box which corresponded with the index on a scale. After the release of a photon, a mass m could be added to the box to restore it to its original position and this would allow us to determine the energy E = m c 2 that was lost when the photon left. The box is immersed in a gravitational field of strength g , and the gravitational redshift affects the speed of the clock, yielding uncertainty Δ Δ t in the time t required for the pointer to return to its original position. Bohr gave the following calculation establishing the uncertainty relation Δ Δ E Δ Δ t ≥ ≥ h .