9 4 4 A P P E N D I X F contradict itself? Our representations of it can be irreconcilable, but not the facts of the experiment. Forced to choose the former or the latter, the physicist has only one option: to accept the dictates of nature. The Two Fundamental Postulates The experiments therefore oblige us, he continued, to establish the following two funda- mental postulates: 1. That the principle of relativity is applicable to all natural phenomena, which simply means establishing the identity of natural laws for all systems that move relatively with rec- tilinear, uniform movement. (The result of Michelson’s experiment). 2. That the speed of light is constant, independent of the movement of the source that emits it. These postulates, Dr. Einstein reiterated, are contradictory in the light of classical sci- ence. Relativity of the Concepts of Time and Space The difficulty in resolving this incongruence arises, he said, from inaccurate hypotheses concerning the concepts of geometry and time. I will restrict my remarks to showing, in the simplest way possible, he added, what these concepts are and the nature of the changes it is necessary to introduce in them. He demonstrated that in the light of classical science, if there are two systems, that is to say, two bodies that move relatively with uniform, rectilinear movement, and if correspond- ing to these there are two locations where different events and the times in which they take place can be verified, the distances between the locations of any two events and the time elapsed between them turn out to be identical for observers of those two systems. Two events that are simultaneous for a given observer, therefore, are simultaneous for another observer that moves with respect to the first one in the described manner, and an analogous process takes place with distances. They are, he said, immediate consequences of adopting our intuitive notions of space and time for nature. We postulate a priori that space and time are absolutes. If one attempts to work with such concepts, he continued, it is impossible to reconcile those postulates. By passing from one system of reference to another, that is, by introducing coordinates that correspond to the second system into the laws of the phenomena observed within the first, instead of the coordinates and the time that correspond to that first system, taking advantage of the simple relationships provided by classical mechanics, one obtains expressions that are not identical to the first, contrary to what that principle demands. If we give some thought to our concept of geometry, he went on, we will see that it is purely intuitive. Euclid’s geometry is of such a nature. In it, distances, which are purely sub- jective, have an absolute character. It would therefore make no sense to ask if it is true or false. But if we concretize the notion of a straight line segment, considering it as the dis- tance between two lines of a nearly solid bar, and if we introduce the assumption that these two lines are always at the same distance, regardless of the motion of the bar, we turn ge-