L E C T U R E S A T U N I V E R S I T Y O F B U E N O S A I R E S 9 4 3 principle of relativity, which in the strictest sense, means that they appear the same to all those observers that belong to systems that move relatively with uniform, rectilinear mo- tion. If, for example, a person stands on a spring-loaded scale, the reading it gives of his weight is the same as it would be at rest on the pavement or inside an elevator that moves with uniform motion. As is well known, this means that it is not possible to determine the movement of a system when it is rectilinear and uniform by means of internal experiments of a mechanical nature. Further, the mechanical laws pertaining to those systems continue to control the others. Michelson’s experiment also shows that it is impossible to attain knowledge of the uni- form, rectilinear motion of the system through optical experiments, which means that that principle of relativity is valid not only for mechanics, but also for optics. However, the question arises as to whether in this branch of physics it is possible to log- ically accept the validity of the law of special relativity, since there is a fact that seems to contradict it because if one establishes the postulate, which coincides with the experiment, that the speed of light is constant, or rather, independent of the velocity of the source that emits it, the result would be that the distance covered in one second by the waves along a train car at rest would have a different value from that which would be observed if it were in motion, which specifically contradicts that principle, since the law of propagation would be different for two systems that move relatively. The constancy of the velocity of light, therefore, seems irreconcilable with the postulate of relativity. That this is just an illusion will be demonstrated at the next lecture. Professor Einstein Continues His Lecture Series [30 March 1925] Contradiction between the Results of Michelson’s Experiment and the Principle of the Con- stancy of the Speed of Light Dr. Einstein began with a reminder that in the last lecture he had made evident the con- tradiction that apparently exists between the results of Michelson’s experiment and the fact, imposed by nature, that the speed of light is independent of the movement of the source that emits it. He again emphasized that Michelson’s experiment proves that the velocity of the system (Earth) does not influence the optical phenomena that take place in it, or rather, that it is not possible to predict the movement of a body when it is rectilinear and uniform by means of optical experiments carried out on it. According to this, the principle of relativity would also be applicable to optical phenomena. As I mentioned a moment ago—he went on—that principle seems to clash with the experimental fact of the constancy of the speed of light. We are therefore in the presence of irreconcilable experimental facts, he added. But one must ask, what does it mean to talk about contradictory experimental facts? Can nature