RELATIVITY AND REALITY

Press, Volume LXXIV, Issue 22343, 5 March 1938, Page 24

RELATIVITY AND REALITY

IO THE EDITOR Or WE PRESS. Sir, —Many articles and letters in newspapers and magazines seem to suggest that the relativity creed contains such fantastic propositions that however much they appeal to philosophers, cannot be taken seriously by the matter-of-fact individual who prides himself on believing only what he sees, and distrusts suggestions which seem to contradict his own personal experience. But this is not so. Relativity is a theory of principle. To understand it the principles on which it rests must be grasped. Before stating these, however, it is necessary to indicate that the theory consists of two .. distinct sections—the special and the general relativity theories. Since the earliest times it has been known that in describing the motion of a body, we must refer to another body. In physics the bodies to which motions arc spatially referred are termed systems of co-ordinates, but the slate of j motion of a system of co-ordinates cannot be chosen arbitrarily if the laws of mechanics are to hold good—it must be free from twisting and j acceleration. The special relativity theory is then the application of the following proposition to any natural l process:—"Every law of nature which holds good with respect to a co-ordin-ate system K must also hold good for uny other system KJ, provided K and Kl are in'uniform movement of translation." The second principle on which rests the special relativity theory is that of the constancy of the velocity of light in a vacuum. The two principles do not seem at first to be logically compatible but reconciliation was achieved by making a change in kinematics. It became evident that a statement of the coincidence of two events could have meaning only in connexion with a system of co-ordinates. Physics had to be modified. The most notable change was a new law of motion for (very rapidly) moving mass-points, and the result was that the inertia of a material system must depend on its energy content. Hence inert mass was apparently nothing else than latent energy. The doctrine of the conservation of mass lost its independence and became merged in the doctrine of conservation of energy. Although it may be necessary for our descriptions of nature to employ systems of co-ordinates that we have selected arbitrarily, the choice should not be limited in any way so far as their state of motion is concerned. The theory of relativity was found to be in conflict with the identity of inert and heavy masses experiment. The theory, therefore, had to be generalised to include the laws of gravitation, but this contracted Euclidean geometry; hence the phrase, "a ways in space." The fundamental concepts, "straight plane," etc., accordingly lose their exact meaning in physics. In the generalised theory of relativity, kinematics is no longer one of the absolute foundations of general physics. The geometrical states of bodies and the ratei of clocks depend in the first place on their gravitatioi-al fields, which again are produced by the material systems concerned. Thus the new theory of gravitation diverges widely from that of Newton with respect to its basal principle. Again, it is evident that it is not possible to attribute an absolute to the notion of acceleration, no more than to the notion of velocity. Similarly it is not reasonable to attribute to a body a "resistance to acceleration" in the absolute sense. Further, this resistance ought to be so much greater where there is in the neighbourhood of the body more inert masses not in accelerated movement. Conversely, this resistance ought to disappear when the masses participate in the acceleration of the body; and the equations of the gravitational field contain these different aspects of this resistance, which Einstein calls the relativity of inertia. Gradually these laws of space and time are being disclosed so that in the light of greater knowledge so gained we may perhaps be permitted to quote Dr. C. C. Farr, who altered Milton's words slightly and said, instead, "These are Thy wondrous works, Parent of good; Thyself how glorious then!"— Yours, etc., W. P. B. CLEARY. March 3, 1938.