8 8 4 A P P E N D I X H with the exception of Mercury, for which it reaches an approximate value of per cen- tury. [Already] Leverrier discovered it, but classical mechanics could not explain it. Second. The deflection of light in a gravitational field. For the solar field, the corre- sponding deflection should be , as was confirmed in 1919. Third. We have seen that the relative rate of two clocks depends on their position in the [gravitational] field. The light emitted by a [chemical] element in the Sun should differ from that emitted on Earth by the same element, and this difference must be noticeable by a shift in [the spectrum of] rays. Due to the smallness of the difference, this phenomenon is difficult to observe. Most physicists believe it exists and has the magnitude predicted by the theory. Nevertheless, doubts remain as to its value. The most essential part remains to be said. Geometry, that is, the law for locating bodies, is not a law given a priori, but depends on the gravitational field, and this depends on the bodies. This means that geometry depends on the position of bodies and on all other phys- ical phenomena and, as a result, it cannot be taken for the basis of physics. In my next lecture I will speak of problems of purely speculative interest, for which I must make use of mathematics. As our readers will observe, El Debate has given a most exact account of the first two lectures and will attempt to do the same with the third, within the limits imposed by the na- ture of the topic. THIRD LECTURE OF EINSTEIN Before a public as numerous as that which attended the two previous lectures, Einstein delivered his third lecture, devoted to the latest results of the theory. He began by saying: “We have seen that the special theory was imperfect from the philosophical viewpoint. Now we want to discuss the points of imperfection in the general theory and the attempts made to avoid such weak points.” It is clear that all theories leave something to be desired. They are incomplete by any criterion. As to the general theory, it is necessary, in the first place, to consider Mach’s point of view which has achieved considerable importance in the theory of relativity. Mach was not satisfied with his analysis of classical mechanics. From a descriptive standpoint, motion is only relative, and one cannot give an exact sense to the concept of absolute motion. Like Newton, Mach saw very well that in ordinary mechanics everything is based on accelera- tion relative to space, that is, to something absolute. To avoid this he conceived that inertia, rather than working against absolute acceleration, worked against a relative acceleration. If we have two masses and we accelerate one of them, its inertia or resistance reacts against acceleration with respect to the second mass, and, in general, he thinks that inertia is a re- sistance to the acceleration of the mass with respect to all the rest of the masses in the uni- verse. He sought an analytic expression for this but could not find it because actio in distans 40″ 1″, 7
Previous Page Next Page