THE THEORY AND THE TRIAL

Evening Post, Volume CV, Issue 88, 13 April 1923, Page 7

THE THEORY AND THE TRIAL

The eclipse expedition to Walla], 'headed by Dr. W. W. Campbell, director of the Lick Observatory, had as its main object the taking of photographs of thestars in the neighbourhood of the sun for the purpose, as the cable message indicates, of testing Einstein's best-known prediction. It was more than: a prediction; it was an assertion, a necessary consequenoe of the famous theory. It is impossible to do more in small space than to indicate very roughly what the Einstein Theory does. It is common knowledge that tlie earth is in motion round the sun; it is also known that the sun and 1 all the other stars are in motion, and it is presumed that the sun and the solar system are travelling rapidly through space. But attempts to find by experiment any direct evidence of the earth's motion through space have failed. Such experiments have to be performed by using light, which travels at a fixed velocity through space, as the "point of reference"; and the experiments have failed because, no matter whether the observer is travelling in the same direction as the light or in any other direction, light appears always to have tlje aame velocity relative to him. It was suggested before Einstein worked out Tiis theory that this paradox was caused by the actual shortening of measuring apparatus, and of all material bodies, in the direction of motion, and the amount of that shortening, which varies with, the velocity of the body,, was expressed in a simple mathematical formula. This idea, was supported by a number of observations in electrical and other research. . . ■ .

Einstein adopted as one of the facts of nature the impossibility of detecting absolute motion through space, as a consequence of the invariable measured velocity of light. With this starting point he constructed an elaborate mathematical argument which led first to a confirmation of the idea of the contraction of matter by motion, and thence to the conclusion that velocity also, modifies mass and time. " Thus neither size, nor weight, nor. time remains absolute; if a body has high velocity its length is less, its mass' greater, and its time longer than they would be if its motion were less rapid. Einstein's consideration of accelerating velocities, that is, velocities which- are increasing or decreasing, led to the development of a theory of gravitation which, because it gives gravitational force a slightly different value from that calculated by Newton, is often said to have upset the Newtonian calculation. Einstein, however, approached the matter from an entirely different point of view. Newton took gravitation as a fact, and deduced its laws. Einstein, starting with a consideration of velocities, found gravitation to be a necessary consequence; and that his value so nearly coincides with Newton's is in a sense a remarkable tribute to Newton's work.

Among the numerous conclusions of the Einstein Theory there are very few that can be te3ted by observation. Two of them have- been tested. One relates to a peculiarity in the motion of the planet Mercury, and that peculiarity, apparently unexplainable in accordance with laws, disappears under Einstein's. The other ia the prediction that lijfht rays are bent fay gravitation— a phenomenon that has bseti misleadingly described by saying that light has weight. This is the prediction tested by the y/allal 'photographs and, judging by the items of the cable message, triumphantly •confirmed. The making of the teat was a wprk of extraordinary delicacy, because the shifting of the star images is extremely small, and can_only be detected by microscopic measurement. None of the shifts that were measurable was anything like 1.75 seconds of arc — the maxiimim required by the Einstein Theory, and itself only 9. minute fraction of an inch. That displacement should have been shown by the image of a star photographed just at the edge of the sun's disc, where, in fact, no-star could be seen owing to tlie glare of the corona. Further from the sun the displacements became less and less. The astronomers, having measured the actual displacements, have calculated what they would have been if each star had been at the position of maximum shift, and have in that way arrived a-t their average of 1.74 seconds of arc.