9 4 2 A P P E N D I X F Lorentz provided the theoretical explanation of that result by using Maxwell’s equations applied to a vacuum. As we know, electrical and magnetic fields and electrical current all play a role in them. He formulated the hypothesis that matter partially drags the ether, and with it the electrical and magnetic fields, because electricity is connected to it in elastic form that is, it can create small oscillations with respect to matter. With this simple hypoth- esis one can calculate the motion of the charge relative to the ether by knowing the motions that we have just mentioned. This theory of Lorentz’s reconciles Fizeau’s result with the immobility of the ether that he assumed. Lorentz’s theory thus postulates the existence of a privileged frame of reference: the electromagnetic ether at rest. The laws of nature would be valid in their simple forms, only with respect to such a frame. If ether at rest really does exist, the earth must inevitably move relative to it, which would be demonstrable through optical or electrodynamic experiments carried out in the laboratory. It was surprising that such a relative displacement did not show up in experi- ments made with that precise objective in mind. Lorentz theoretically deduced that according to his theory, the observable phenomena depended, at first glance, upon the relation between the velocity of the Earth and that of light. Since it was a question of such small differences, success could be anticipated only by resorting to the most delicate methods of optics, namely, interferential methods. Michelson and Morley, then Morley alone, and recently Morley and Miller, have carried out such an experiment by observing the interference of two beams of light that propagate, one in the direction of the Earth’s movement and the other in a perpendicular direction. Each of these beams advances first in one direction and then in the opposite, reflecting in mirrors placed perpendicular to the beams in order to interfere later. These beams should move at different speeds as a result of the unequal propagation of light in those directions, since in relation to one of them, the component of the velocity of the Earth is nil. Despite that fact that these effects could be observed at one percent of what the theory had predict- ed—assuming the existence of a fixed ether—the experiments repeatedly yielded the same negative result. This experiment seemed to indicate that the ether was totally dragged by the Earth. The partial drag observed in Fizeau’s experiment could be explained, assuming that large bodies drag the ether, but not small masses like the ones in the water that moves in that experiment. This assumption is destroyed by the phenomenon of the aberration of light, which is absolutely irreconcilable with the idea that the Earth drags that hypothetical substance. Thus, it is impossible to explain Michelson’s negative results by assuming the drag of the ether, since this supposition openly contradicts the hypothesis to which the re- cently described phenomenon leads. In order to reconcile these results, it is necessary to re- sort to other ideas. Up to this point we have talked about optics, and it is unavoidable, having come this far, to look back at the phenomena of mechanics. In experimental terms there is an essential dif- ference between optical and electrodynamic phenomena and mechanical ones, since while in the former, according to Lorentz’s theory, there exists a privileged system—the ether at rest—in the latter this does not occur. Mechanical phenomena are ruled by the so-called