8 7 6 A P P E N D I X H “There is,” he says, “a lack of understanding. They say that everything is relative, with- out noting that physics deals with things that are absolute in a certain sense, because it deals with real things. It is totally false to suppose that this theory has anything to do with philo- sophical relativism.” He recalls that the relativity of motion was already known in antiquity, since it was im- possible in ordinary life to conceive of motions that were not relative, at least with respect to the earth. In physics one cannot speak of anything except relative motions. This is not the object of the theory of relativity. The basis of classical mechanics is Galileo’s principle of inertia, according to which a body that is not subject to external forces must move rectilin- early and uniformly. If one states [the principle] in this way, one ends up considering mo- tion as absolute, and any motion would be rectilinear because there is no point of reference. The principle must be completed as follows: there exists a reference body relative to which the body is in uniform motion: we will call the system that the reference body defines an inertial system without it statements lack meaning. Taking into consideration the con- tent of this general principle and that of classical mechanics, we arrive at a general law that we can call the law of restricted relativity. Let us suppose that the law is correct for an in- ertial system k if we take another system , in uniform motion and without rotation, it is easy to see that Galileo’s principle is also valid with respect to . This means that there is an infinity of inertial systems in uniform translatory motion with respect to one another. This is the principle of restricted relativity valid in mechanics it asserts that there is no privileged state of motion because the [inertial] systems are equivalent, and preserve all the laws of motion. In essence, “it is true that from the concept of motion one cannot deduce a priori that there is a privileged system however, from the point of view of physics, it seems a priori possible that its laws will be simpler with respect to a privileged system.” We have seen that this does not occur for mechanics. Thus, the principle of restricted relativity is valid in classical mechanics we say special because this equivalence refers only to systems in relative uniform translatory motion. Now, by having the principle of relativity and preserving the principle of the constancy of the velocity of light in empty space, contradictions arise in optics that make the two prin- ciples appear irreconcilable: applying the theorems of classical kinematics, it seems that the velocity of propagation of light must depend on its direction. Bearing in mind that a terrestrial laboratory fulfills the conditions for an inertial system (due to the movement of the Earth), many experiments have been performed to search for this dependence. The most famous was that of Michelson, which, like all the others, yielded a negative result. Nature appears to show that the principle of special relativity is valid and we thus find ourselves in a difficult situation, because experience contradicts logic and it is therefore necessary to see whether the chain of reasoning that leads to the dependence of the velocity of propagation [of light] on its direction has any weak point. It is necessary to analyze the concepts of space and time. k′ k′