Fig. 15. Ether-drift house at Mount Wilson in 1924-1926.
that of 1921. The house was about twenty-two feet square, and there were canvas windows all around as before; but instead of the corrugated iron for the sides, beaver-board was used, as this material is less absorbent of heat from the sun. Large pieces of canvas were placed over the entire house and at the end, to protect the house from the direct rays of the sun, greatly facilitating the making of observations throughout the period of daylight. The interferometer, Fig. 16, had the improved mirror mountings, protection from heat, improved light-source, large viewing telescope and other refinements which had been developed in the laboratory tests at Cleveland in 1923 and 1924.
This series of observations, of September, 1924, at Mount Wilson, was undertaken in a wholly unprejudiced but very confident state of mind. The extended laboratory tests had involved every suggested source of instrumental and external disturbance and had proved that none of these was operative in the experiment. The method of observing was so developed that there was perfect confidence in the readings. It was
Fig. 16. The ether-drift interferometer as used at Mount Wilson in 1924-1926.
felt that if any of the suspected disturbing causes had been responsible for the previously observed effects, now these were removed, the result would be a true null effect. Such a conclusion would have been accepted with entire satisfaction; and indeed it was almost expected. On the other hand, if the observations continued to give the positive effect, it would certainly have to be considered as real.
Ten sets of readings, consisting of 136 turns of the interferometer, were made on September 4, 5 and 6, 1924. These observations all show a positive periodic displacement of the interference fringes, as of an ether-drift, of the same magnitude, about ten kilometers per second, as had been obtained in previous trials. Part of these observations were made with the glass case over the light-path covered with corrugated paper board, which had been found in the Cleveland experiments to exclude all effects of radiant heat; the results were not altered in any way by this covering. The effects were shown to be real and systematic, beyond any further question.
In spite of the long continued efforts, it had so far been impossible to account for the effects observed in the interferometer as being due to terrestrial causes or to experimental errors. Very extended calculations were made in the effort to reconcile the observed effects with the accepted theories of the ether and of the presumed motions of the earth in space. The observations had been repeated at various epochs to test one after another of the hypotheses which had been suggested. At the end of the year 1924 when a solution seemed impossible, a complete calculation was made, for all hours of the day and for twenty-four epochs during the year, of the then expected effects due to the orbital motion and the apparent motion towards Hercules. This indicated that the effect to be expected had its greatest magnitude in April and that the minimum in April should be two and a half times as great as the effect at the time of the observations which had been made in September and that the maximum effect in April should be four and a half times as great. Furthermore, the effect in September would be directed to the northward at all times of the day while in April the azimuth of the effect would move progressively all around the horizon, the maximum value being attained
at midnight with a direction exactly east and again at noon with a direction exactly west. Observations for verifying these contrasting predictions were made at Mount Wilson between March 27 and April 10, 1925. The effect was equal in magnitude to, but not larger than, the effects previously observed; it was not directed successively to all points of the compass, that is, it did not point in directions 90° apart at intervals of six hours. Instead of this, the direction merely oscillated back and forth through an angle of about 60°, having, in general, a northerly direction, as before. This proved that the presumptions as to the absolute motion of the earth, upon which these calculations were based, were invalid.
General Analysis of the Ether-Drift Problem
The various component motions involved
Previous to 1925, the Michelson-Morley experiment had always been applied to test a specific hypothesis. The only theory of the ether which had been put to the test is that of the absolutely stationary ether through which the earth moves without in any way disturbing it. To this hypothesis the experiment gave a negative answer. The experiment was applied to test the question only in connection with specific assumed motions of the earth, namely, the axial and orbital motions combined with a constant motion of the solar system towards the constellation Hercules with the velocity of about nineteen kilometers per second. The results of the experiments did not agree with these presumed motions. The attention was given almost wholly to this velocity of the ether drift, and no attempt was ever made to determine the apex of any indicated motion. The experiment was applied to test the Lorentz-FitzGerald hypothesis that the dimensions of bodies are changed by their motions through the ether; it was applied to test the effects of magnetostriction, of radiant heat and of gravitational deformation of the frame of the interferometer. Throughout all these observations extending over a period of years, while the answers to the various questions have been “no,” there has persisted a constant and consistent small effect which has not been explained.
The ether-drift interferometer is an instrument which is generally admitted to be suitable for determining the relative motion of the earth and the ether, that is, it is capable of indicating the direction and the magnitude of the absolute motion of the earth and the solar system in space. If observations were made for the determination of such an absolute motion, what would be the result, independent of any “expected” result? For the purpose of answering this general question, it was decided to make more extended observations at several epochs when the earth is in contrasting positions in its orbit and this was done in the months of April, August and September, 1925, and in February, 1926.
It may be asked: why was not such a procedure adopted before? The answer is, in part, the fact already stated that the purpose had been the verification of certain predictions of the so-called classical theories; and, in part, that it is not easy to develop a new hypothesis, however simple, in the absence of direct indication. Probably a considerable reason for the failure is the great difficulty involved in making the observations at all times of day at any one epoch. Very few, if any, scientific experiments require the taking of so many and continuous observations of such extreme difficulty; it requires greater concentration than any other known experiment. Half the time, perhaps, the observations are interrupted before they become numerous enough to be useful, because of excessive displacement of the fringes by temperature changes or by earth or aerial vibrations. The mere adjustment of an interferometer for white-light fringes and the keeping of it in adjustment, when the light path is 210 feet long, made up of sixteen different parts, and when it is in effect in the open air, requires patience as well as a steady “nerve” and a steady hand. Professor Morley once said, “Patience is a possession without which no one is likely to begin observation of this kind.”
The absolute motion of the earth may be presumed to be the resultant of two independent component motions. One of these is the orbital motion around the sun, which is known both as to magnitude and direction. For the purposes of this study, the velocity of the orbital motion is taken as 30 kilometers per second and the
