Dayton C. Miller. Significance of the ether-drift experiments of 1925 at Mount Wilson //Science, Vol. 63 (1635), April 30, 1926

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[Vol. LXIII, No. 1635

spect to the ether. It was also assumed that the earth in its orbital motion around the sun passes freely through this ether as though the latter were absolutely stationary in space. The experiment proposed to detect a relative motion between the earth and the ether, and it is this relative motion which is often referred to as “ether-drift.” The experiment is based upon the argument that the apparent velocity of light would vary according to whether the observer is carried by the earth in the line in which the light is traveling or at right angles to this line. The .velocity of light is three hundred thousand kilometers per second, while the velocity of the earth in its orbit is one ten thousandth part of this, thirty kilometers per second. The actual motion of the earth is at all times the resultant of the motion of the earth in its orbit,, varying in direction and having a velocity of thirty kilometers per second, and of the constant motion of the sun (including the whole solar system), in an unknown direction and with an unknown velocity. Therefore, the actual relative motion of the earth and ether is unknown, and it may be less than thirty kilometers per second or very much greater. If it is assumed that the relative motion is equal to that of the earth in its orbit, and if it were possible to measure the direct effect of this motion on the apparent velocity of light, then the velocity measured in the line of motion should differ from the apparent velocity at right angles to this line, by thirty kilometers per second, or by one part in ten thousand. This is what is called a “first order effect”; but, unfortunately, there is no known method of measuring the velocities under such simple conditions. All methods require the ray of light to travel to a distant station and back again to the starting point, and a positive effect of the earth’s motion on the ray going outward would be neutralized by a negative effect on the returning ray. But, for a moving observer, it was shown that the neutralization would not be quite complete; the apparent velocity of the ray going and coming in the line of the earth’s motion would differ from the apparent velocity of the ray going and coming at right angles, in the ratio of the square of the velocity of the earth to the velocity of light, that is, by an amount equal to one part in (10,000)2 or to one part in 100,000,000. The only effect which can be experimentally determined, therefore, is exceedingly minute; it is a “second order effect.”

A remarkable instrument known as the “interferometer,” which had been invented by Professor Michelson, is capable of detecting a change in the velocity of light of the small amount involved in ether drift. In this experiment a beam of light is literally split in two by a thin film of silver, on what is called the “half-silvered mirror”; the coating of silver is thin enough to allow about half of the light to pass

straight through, while the other half is reflected in the usual manner. These two beams of light may thus be made to travel paths at right angles to each other. At the end of the desired path each beam is reflected back upon itself and the two come together where they first separated. If the two paths are optically equal, that is, if there are exactly the same number of wavelengths of light in each, the reunited portions will blend with the waves in concordance. If, however, one path is a half-wave longer than the other, the waves will come together in “opposite phase,” the crest of one coinciding with the trough of the other. These and other phase relations between the two rays produce effects called “interference fringes,” observation of which enables one to detect slight changes in the velocity of light in the two paths.

In the year 1887, at Case School of Applied Science, in Cleveland, Professor Michelson, in collaboration with the late Professor Edward W. Morley, of Western Reserve University, made certain important developments of method and apparatus and used the interferometer in the now famous “Michelson-Morley experiment,” in an effort to determine whether the motion of the earth through space produces the effect upon the velocity of light as predicted by theory. Unfortunately we do not know in what absolute direction the earth is going and so it is not possible to place the interferometer certainly in this direction. Therefore, the whole apparatus is mounted on a base which floats on mercury so that it can be turned to all azimuths of the horizontal plane of observation in the effort to find the direction of the drift. The rotation of the earth on its axis causes the plane of the interferometer to move as though it were on the surface of a rotating cone, the axis of which coincides with that of the earth and thus to take many different space orientations. It is only that component of the actual drift which lies in the plane of the interferometer at the moment of observation which can be observed. Therefore, the apparent azimuth and magnitude of the drift should change with the time of observation.

The full significance of the ether-drift experiments of# 1925 can be presented only by considering the interpretations given to the experiments made previously. For this reason a historical summary of all the experiments will be given.

In July, 1887, Michelson and Morley made six sets of observations for the ether-drift effect, one at noon and one at six o’clock in the evening, on each of three days, July 8, 9 and 11. This constitutes the whole of the observations made by Michelson and Morley. In November, 1887, they announced their conclusions as follows: “Considering the motion of the earth in its orbit only . . . the observations show that the relative motion of the earth and the ether is probably less

April 30, 1926]



than one sixth of the earth’s orbital velocity and certainly less than one fourth.”2 (That is, it is less than seven and one half kilometers per second.) It is to be noted that this experiment was designed and carried out solely to detect the influence of the earth’s orbital motion, which should have different values at the two times of day chosen for observation, and that the smallest quantity which could be measured with certainty was one fourth of the expected effect.

In 1895, Lorentz and FitzGerald suggested that the motion of translation of a solid through the ether might produce a contraction in the direction of the motion, with extension transversely, the amount of which is proportional to the square of the ratio of the velocities of translation and of light, and which might have a magnitude such as to annul the effect of the ether-drift in the Michelson-Morley interferometer. The optical dimensions of this instrument were determined by the base of sandstone on which the mirrors were supported. If the contraction depends upon the physical properties of the solid, it was suggested that pine timber would suffer greater compression than sandstone, while steel might be compressed in a lesser degree. If the compression annuls the expected effect in one apparatus,, it might in another apparatus give place to an effect other than zero, perhaps with the contrary sign.

The writer, in collaboration with Professor Morley, constructed an interferometer about four times as sensitive as the one used in the first experiment, having a light path of 214 feet, equal to about 130,000,000 wave-lengths. In this instrument a relative velocity of the earth and ether equal to the earth’s orbital velocity would be indicated by a displacement of the interference fringes equal to 1.1 fringes. This is the size of the instrument which has been used ever since. The optical parts were all new and nothing was used from the original apparatus excepting the mercury tank and its wooden float.

Such an instrument with a base made of planks of pine wood was used at Cleveland, in 1902, 1903 and 1904, for the purpose of directly testing the Lorentz-FitzGerald effect, but the changes in the wooden frame due to the variations in humidity and temperature made it difficult to obtain accurate observations. A new supporting frame was designed by Professor F. H. Neff, of the Department of Civil Engineering of Case School of Applied Science, the purpose being to secure both symmetry and rigidity. This frame, or base, was made of structural steel and was so arranged that the optical dimensions could be made to depend upon distance-pieces of wood, or upon the steel

2 Michelson and Morley,1 ‘ Relative Motion of the Earth and the Luminiferous Ether/y Am. Jl. of Sci.f 34, 333 (1887); Phil. Mag,, 24, 449 (1887); Jl. de Phys., 7, 444 (1888).

frame itself. Observations were made with this apparatus in 1904. The procedure was based upon the effect to be expected from the combination of the diurnal and annual motions of the earth together with the presumed motion of the solar system towards the constellation Hercules with a velocity of 17.7 kilometers per second. On the dates chosen for the observations there were two times of the day when the resultant of these motions would lie in the plane of the interferometer, about 11:30 o’clock, A. М., and 9:00 o’clock, P. M. The calculated azimuths of the motion would be different for these two times. The observations at these two times were, therefore, combined in such a way that the presumed azimuth for the morning observations coincided with that for the evening. The observations for the two times of day gave results having positive magnitudes but having nearly opposite phases; when these were combined, the result was nearly zero. The result, therefore, was opposed to the theory then under consideration; but according to the ideas which will be set forth later in this address it now seems that the superposition of the two sets of observations of different phases was based upon an erroneous hypothesis and that the positive results then obtained are in accordance with a new hypothesis as to the solar motion. Our report of these experiments published in the Philosophical Magazine for May, 1905, concludes with the following statement: “Some have thought that this experiment only proves that the ether in a certain basement room is carried along with it. We desire therefore to place the apparatus on a hill to see if an effect can be there detected.”3

In the autumn of 1905, Morley and Miller removed the interferometer from the laboratory basement to a site on Euclid Heights, Cleveland, free from obstruction by buildings, and having an altitude of about three hundred feet above Lake Erie and about eight hundred and seventy feet above sea-level. Five sets of observations were made in 1905-1906, which give a definite positive effect of about one tenth of the then “expected” drift. There was a suspicion that this might be due to a temperature effect, though there was no direct evidence of this. A plan was made for putting this surmise to the test after a summer’s vacation. We had erected the interferometer on land owned by a friend; during our vacation absence, the land was sold and the new owner ordered the immediate removal of the interferometer. Professor Morley retired from active work in 1906 and it devolved upon the present writer to continue the experiments.

з Morley and Miller, “An Experiment to detect the Fitz-Gerald-Lorentz Effect, ” Phil. Mag., 9, 680 (1905); Proc. Am. Acad. Arts and ScL, 41, 321 (1905); “On the Theory of Experiments to detect Aberrations of the Second Degree,” Phil. Mag., 9, 669 (1905).