The Ether-Drift Experiment, Historical 1878-1881
THE general acceptance of the theory that light consists of wave motion in a luminiferous ether made it necessary to determine the essential properties of the ether which will enable it to transmit the waves of light and to account for optical phenomena in general. Theories of the ether are intimately associated with theories of the structure of matter and these are among the most fundamental in the whole domain of physical science. The ether was presumed to fill all space, even that occupied by material bodies, and yet to allow all bodies to move through it with apparent perfect freedom. The question of whether the ether is carried along by moving bodies such as the earth has been considered since the early days of the wave theory. The discovery of the aberration of light, in 1728, was soon followed by an explanation according to the then accepted corpuscular theory of light. The effect was attributed to a simple composition of the velocity of light with the velocity of the earth in its orbit. Fresnel proposed an explanation based on the wave theory, which has been generally accepted, which presumes first that the ether is at rest in free space; and, second, that the ether density is different in different substances and that the velocity of propagation of light in any substance varies inversely as the square root of the ether density. These two hypotheses give a complete and satisfactory explanation of aberration; the second is considered to have been proved by the experiments of Fizeau and of Michelson and Morley on the velocity of light in moving media; the first hypothesis, that of an ether at rest in space, has always been in doubt.
The first suggestion of a method for measuring the relative motion between the earth and the ether by means of an optical experiment was made by James Clerk Maxwell in the article on Ether, which he contributed to Vol. VIII of the 9th Edition of the Encyclopaedia Britannica, published in 1878. It is assumed that the ether as a whole is at rest, that light waves are propagated in the free ether in any direction and always with the same velocity with respect to the ether and that the earth in its motion in space passes freely through the ether without disturbing it. The
experiment is based upon the argument that the apparent velocity of light would be different according to whether the observer is carried by the earth in the line in which the light is travelling or at right angles to this line. It would thus be possible to detect a relative motion between the moving earth and the stationary ether, that is to observe an ether drift. The orbital motion of the earth has a velocity of thirty kilometers per second, while the velocity of light is ten thousand times as great, three hundred thousand kilometers per second. If it were possible to measure the direct effect of the earths orbital motion on the apparent velocity of light, then the velocity measured in the line of motion should differ from the velocity at right angles to this line by thirty kilometers per second, that is by one part in ten thousand. This would be a first-order effect. Maxwell explains that, since all practicable methods require that the light shall travel from one station to another and back again to the first station, a positive effect of the earths motion on the ray going outward would be neutralized by a negative effect on the returning ray, except that on account of the motion of the observer during the time the light is travelling the neutralization would not be quite complete, and a second-order effect, proportional to the square of the ratio of the velocity of the earth to the velocity of light, would be observable. Maxwell concludes with the statement, The change in the time of transmission of the light on account of a relative velocity of the aether equal to that of the earth in its orbit would be only one hundred-millionth part of the whole time of transmission, and would therefore be quite insensible.
The late Professor Albert A. Michelson accepted the challenge of Maxwells suggestion and while attending the University of Berlin in 1880-1881, he devised the remarkable instrument universally known as the Michelson interferometer which was especially adapted to the ether-drift experiment.1, 2 In the interferometer a
1 A. A. Michelson, Phil. Mag. [5] 13, 236 (1882); Am. J. Sci. 23, 395 (1882); H. A. Lorentz, Astrophys. J. 68, 345 (1928); Thos. Preston, Theory of Light, 5th ed., 229, 566 (1928); R. W. Wood, Physical Optics, 2nd ed. 265, 672 (1911).
2 W. M. Hicks, Phil. Mag. [6] 3, 9, 256, 555 (1902);
beam of light is literally split in two by a halfsilvered mirror, and the two beams of light may 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 right-angled paths are optically equal, the reunited beams of light will blend with the waves in concordance. If, however, the paths of the light in the interferometer differ either in actual length or in the optical properties of the medium through which the light passes, differences of phase will result which may be observed as interference fringes. Observation of these fringes enables one to detect exceedingly small changes in the relative velocities of the light in the two paths of the interferometer, the measurements being made in terms of the wave-length of the light.
Michelson at once applied his interferometer to detect the relative motion of the earth and the ether as proposed by Maxwell. Alexander Graham Bell provided for the construction of the new instrument, Fig. 1, which was made by
Fig. 1. Michelsons ether-drift interferometer of 1881.
Schmidt & Haensch of Berlin. The half-silvered mirror was placed over the central axis, and two arms at right angles, each 120 centimeters long, carried the end mirrors. The apparatus could be set with the telescope arm pointing in different azimuths and it should be possible to detect the effect of the orbital motion of the earth when the light travels in the direction of this motion and at right angles to it.
The first trials of the ether-drift experiment were made at the Physikalisches Institut of the
Nature 65, 343 (1902); E. W. Morley and D. C. Miller, Phil. Mag. [6] 9, 669 (1905); A. Righi, Comptes Rendus 168, 837 (1919); 170, 497, 1550 (1920); 171, 22 (1920). E. R. Hedrick, Astrophys. J. 68, 374 (1928).
University in Berlin; but the disturbances produced by street traffic made it impossible to see the fringes except in the middle of the night. The experiment was transferred to the Observatory in Potsdam, the interferometer being mounted in a hollow place in the lower part of the brick pier which supported the big telescope. The report of the experiment, published in 18813 (with a correction explained in the paper of 1887),4 states that, considering only the motion of the earth in its orbit, the displacement of the interference fringes to be expected would be 0.04 of the fringe width; the displacements actually observed varied from 0.004 to 0.015 of a fringe width and were considered to be merely errors of experiment. The conclusion was that the hypothesis of a stationary ether was not confirmed.
The Michelson-Morley Experiments, Cleveland, 1887
While he was still in Europe, in 1881, Michelson was appointed to the Professorship of Physics in the newly organized Case School of Applied Science in Cleveland and thus became acquainted with the late Professor Edward W. Morley, Professor of Chemistry in Western Reserve University, these two institutions being located side by side. Professor Morley proposed several important developments in the interferometer and in the method of using it, so that it became adequate to measure the then expected effect in the ether-drift experiment. Having secured an appropriation from the Bache Fund of the National Academy of Sciences, a new interferometer was constructed, embodying these improvements; the optical parts were made by the late John A. Brashear of Pittsburgh. In order to avoid disturbances of vibration and distortion, the optical parts were mounted on a solid block of sandstone, Fig. 2, which was floated on mercury contained in a circular tank of cast iron. This support by floatation made it possible to turn the interferometer to different azimuths while observations were in progress. The practicable limit for the size of the stone base was 150
3 A. A. Michelson, Am. J. Sci. [3] 22, 120 (1881).
4 A. A. Michelson and E. W. Morley, Am. J. Sci. [3] 34, 333 (1887); Phil. Mag. [5] 24, 449 (1887); J. de Physique [2] 7, 444 (1888).
