9 9 8 A P P E N D I X L Classical mechanics is based on the principle of inertia, or the principle of Galileo, according to which a body not subjected to any force moves with a rectilinear, uniform motion. According to classical mechanics there is no preferred system of reference, that is, any system of reference which is in motion in relation to another which is under uniform movement can serve as a reference system. The laws of motion for any system of reference are the same as for the original system of reference. Therefore, for infinite systems of ref- erence, all under uniform, rectilinear movement with respect to each other, any of these can be taken as a reference, and none of them being privileged—this is the principle of relative motion in physics. However, in physics, in the study of optics, in the fundamental law of the propagation of light, one cannot find an analogous principle, given that, according to pre-relativity theories, the measurement of the speed of light depends on the chosen system of reference. The theory of relativity, however, removes this discordance, allowing the law of the propagation of light to be expressed in the same form, independent of the reference system, thus extending to other branches of physics the concept of velocity of propagation from classical mechanics. While the concept of relativity for space was already clear, time was always implicitly assumed to be absolute in nature. If one is looking for a rigorous and satisfactory definition of simultaneity, from the point of view of a physicist, one is inclined to draw the conclusion that this concept of simulta- neity is not absolute, but conditioned by the system of reference. Thus, if two events are considered to be simultaneous in one system of reference they would not be so when ob- served from another. Yet if you take the Lorentz transformations in two different space-time coordinate sys- tems, the same law that describes the physical phenomenon of the propagation of light is still valid in both systems. It follows, thus, that the theory of relativity gives an explanation for the negative results of the experiments by Michelson and Morley, proving that the physical phenomenon of the propagation of light will obey the law of relative motion from mechanics. The theory of relativity extends the principle of relativity to other physical phenomena not just to the field of mechanics, allowing for the description of the law of a phenomenon subject to the same form whatever the system of reference. This constitutes, in essence, the principle of restricted relativity. This property is justi- fied, as in mechanics, where the systems of reference alluded to above only differ by a uni- form translation. The generalization to the case of any motion of reference systems is a far more arduous problem and is the subject of the general theory. This will be the topic of the second con- ference to take place at the Polytechnic School. [...]