Miller D.C. The Ether-Drift Experiment and the Determination of the Absolute Motion of the Earth // Reviews of modern physics, Vol.5, July 1933

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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

direction changes continuously through the year, at all times being tangential to the orbit. The second component is the cosmical motion of the sun and the solar system. Presumably this is constant in both direction and magnitude but neither the direction nor magnitude is known; the determination of these quantities is the particular object of this experiment. The well-known motion of the solar system towards the constellation Hercules, with a velocity of 19 kilometers per second, is only a relative motion of the sun with regard to the group of nearby stars and it may give no information as to the motion of the group as a whole. In fact, the previous ether-drift experiments have clearly shown that the motion towards Hercules is not a component of the absolute motion of the earth. The rotation of the earth on its axis produces a velocity of less than four-tenths of a kilometer per second in the latitude of observation and is negligible as far as the velocity of absolute motion is concerned; but this rotation has an important effect upon the apparent direction of the motion and is an essential factor in the solution of the problem. However, since the orbital component is continually changing in direction, the general solution is difficult; but by observing the resultant motion when the earth is in different parts of its orbit, a solution by trial is practicable. For this purpose it is necessary to determine the variations in the magnitude and in the direction of the ether-drift effect throughout a period of twenty-four hours and at three or more epochs of the year.

The interferometer continually rotates in a horizontal plane about a vertical axis at the latitude of the observatory. As the earth rotates on its axis, the axis of the interferometer extended may be considered as the generating element of a cone, the apex of which is at the center of the earth. The earth in its orbital motion carries this cone around the orbit, the axis of the cone, the earths axis, always pointing in the same direction in cosmic space. At the same time this system with rotations about three different axes is being translated through space in an unknown manner. It is presumed, further, that the ether-drift interferometer will detect only that single component of the complicated combination of translations and rotations which

at the instant lies in the optical plane of the interferometer; it gives the magnitude and direction of this one component. Fig. 17 shows a globe with a model representing an interferometer attached at a point corresponding to Mount Wilson. The wire extending from the pole of the globe indicates the direction of an assumed

Fig. 17. Models illustrating the diurnal variation in the magnitude and direction of the ether-drift effect.

resultant absolute motion. With the earth in the position shown in the left view, the projection of the motion indicated by the wire, on the plane of the interferometer, passes through the north and south points and has a magnitude less than the full value of the motion. When the earth has turned on its axis to the position shown in the middle view, the projection of the absolute motion in the plane of the interferometer lies to the west of north; as the interferometer rotates on its axis the telescope will detect the maximum component when it points west of north. When the earth has turned to the position shown at the right, the projected component of motion will again be north and south and will have a maximum value, slightly less than the full value. Thus there is a diurnal variation in the observed azimuth of ether-drift. It is evident, further, that the angle which the absolute motion makes with the plane of the interferometer varies throughout the day as the interferometer is carried around on the cone described by its axis. In the illustration, the absolute motion most nearly coincides with the plane of the interferometer in the right view which corresponds to a maximum observed effect; in the left view, the motion is more nearly perpendicular to the plane of the interferometer and the effect is a minimum. It follows that there is a diurnal variation in the magnitude of the effect and this is quite independent of the



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