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[Vol. LXI, No. 1590
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 wave-lengths 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, in Cleveland, Professor Michelson, then professor of physics at Case School of Applied Science, in collaboration with Professor Edward W. Morley, of Western Reserve University, made certain important developments of method and apparatus and used the interferometer in an effort to determine whether the motion of the earth through space produces the effect upon the velocity of light predicted by theory. Unfortunately we do not know in what absolute direction the earth is going through space 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 presumed ether 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 cone whose axis 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 horizontal 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. A drift perpendicular to the plane of the interferometer will produce no effect whatever; it is quite possible that this condition may occur at certain times of the year.
It is not possible at this time to explain the details of the principles involved. The observations are made by looking through a telescope at the system
of interference fringes formed by the two beams of light. As the interferometer is rotated on its axis, an ether drift would cause the whole system of fringes to oscillate, moving first to one side and then to the other, this effect being periodic in each half revolution of the interferometer about its vertical axis. For a relative motion of the earth and the ether equal to the orbital velocity of the earth, that is 30 km/sec, the displacement in the original Michelson-Morley experiment would have been 4/10th of a fringe.
In November, 1887, Michelson and Morley announced the conclusions drawn from their observations made in July of that year 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 than one sixth of the earth’s orbital velocity and certainly less than one fourth.” (That is, it is less than 71/2 kilometers per second.) This result was considered by many as a null result, often called a negative result, and by some was thought to throw grave doubts upon the validity of the hypothesis of the luminiferous ether. There is a significant “Supplement” to this report which begins with the following sentence: “But it is not impossible that at even moderate distances above the level of the sea, at the top of an isolated mountain peak, for instance, the relative motion might be perceptible in an apparatus like that used in these experiments.”2
At the International Congress of Physics, held in Paris in 1900, Lord Kelvin gave an address in which he considered theories of the ether. He remarked that “the only cloud in the clear sky of the theory was the null result of the Michelson-Morley experiment.” Professor Morley and the writer were present, and in conversation with Lord Kelvin he expressed the conviction that the experiment should be repeated with a more sensitive apparatus. 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 224 feet, equal to about 150,000,000 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 1.5 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
2 Michelson and Morley: “Relative motion of the earth and the luminiferous ether, ” Am. Jl. of Sci., 34, 333 (1887); Phil. Mag., 24, 449 (1887); Jl. de Phys., 7, 444 (1888).
June 19, 1925]
of wood was used in 1902 and 1903, but the changes in the wooden frame due to the variations in humidity and temperature made accurate observations impossible. 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 constructed of structural steel, and was erected in a basement room in the Physical Laboratory of Case School of Applied Science in Cleveland, and observations were made in 1904 and 1905. The results of these observations were published in The Philosophical Magazine for May, 1905. They were stated as follows: “We may therefore declare that the experiment shows that if the ether near the apparatus did not move with it; the difference in velocity was less than 3.5 kilometers per second unless the effect on the materials annulled the effect sought. 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
It was at this time that Einstein became interested; and later in the year, 1905, he published a paper on “The electrodynamics of moving bodies.”4 This paper was the first of a long series of papers and treatises by Einstein and others which has developed into the present theory of relativity. In this first paper, Einstein states the principle of the constancy of the velocity of light, tending to show that for an observer on the moving earth, the measured velocity of light would be constant, regardless of the direction or amount of the earth’s motion. The whole theory was related to physical phenomena, largely on the assumption that the ether-drift experiments had given a definite and exact null result. This interpretation of the experiment was not acceptable to the writer, and further observations were undertaken to determine this particular question.
In the autumn of 1905, Morley and Miller removed the interferometer from the laboratory basement to a site on Euclid Heights, Cleveland, at an altitude of about 300 feet above Lake Erie, and free from obstruction of buildings. Five sets of observations were made in 1905-1906, which give a definite positive effect of about 1/10 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
3 Morley and Miller: ‘‘ An experiment to detect the FitzGerald-Lorentz effect, ” Phil. Mag., 9, 680 (1905); Proc. Am. Acad. Arts and Sci., 41, 321 (1905) ; “On the theory of experiments to detect aberrations of the second degree,7 ’ Phil. Mag., 9, 669 (1905).
4 Einstein: 11 Zur electrodynamik bewegter Körper, Ann. der Physilc, 17, 891 (1905).
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. It seemed desirable that further observations should be carried out at a much higher altitude, but numerous causes prevented the resumption of observations. The publication of reports on the solar eclipse of 1919, which were interpreted as confirming the theory of relativity, revived the interest in the ether-drift experiments. A generous friend provided ample funds to cover the considerable expense involved. The site of the Mount Wilson Observatory near Pasadena, California, at an elevation of about 6,000 feet, seemed to be a suitable place for further trials. Through the kindness of President Merriam, of the Carnegie Institution at Washington, and of Directors Hale and Adams, the experiments were resumed by the writer in March and April, 1921, at the Mount Wilson Observatory. The apparatus was substantially the same as that used by Morley and Miller in 1904, 1905 and 1906. Observations were also made in the latter part of the year 1921 and again in 1924 and 1925.
At the Mount Wilson station, about 5,000 single measures of the ether-drift have been made at various times of the day and night. These have been reduced in 204 different sets, each set consisting of observations made within one hour’s time. The observations correspond to four different epochs of the year, as follows: I. April 15, 1921, 117 sets of observations; II. December 8, 1921, 42 sets; September 5, 1924, 10 sets; and April 1, 1925, 35 sets.
The very first observations made in March, 1921, gave a positive effect such as would be produced by a real ether drift, corresponding to a relative motion of the earth and ether of about ten kilometers per second. But before announcing such a result it seemed necessary to study every possible cause which might produce a displacement of fringes similar to that caused by the ether drift. The causes suggested were magnetic deformation of the steel frame of the interferometer and the effects of radiant heat. In order to eliminate the effects of radiant heat the metal parts of the interferometer were completely covered by cork about one inch thick. Fifty sets of observations were made under these conditions, showing the periodic displacement of the fringes due to the drift agreeing with the first observations.5
5 Miller: "Ether-drift experiments at Mount Wilson Observatory," Phys. Rev., 19, 407 (1922); Science, 55, 496 (1922).