Fig. 10. The Morley-Miller ether-drift interferometer arranged for tests of the Lorentz-FitzGerald
hypothesis, 1904.
cast iron, fastened by bolts; each frame carries four mirrors. Against the corners of each of these frames rest four pine rods, about 2 centimeters in diameter and 425 centimeters long. Each rod is supported throughout its length by a brass tube,
2.5 centimeters in diameter, and each pair of tubes is joined together in a vertical truss, as shown in Fig. 10. Against the farther ends of the wood rods, rest the frames which hold the other sets of mirrors. Each of the latter frames is freely suspended by two thin steel ribbons and is held firmly against the pine rods and through these against one of the two fixed mirror-holders; contact is maintained by means of adjustable spiral springs. Thus the distances between the opposite systems of mirrors depend upon the pine rods only, while the whole optical system is adequately supported by the steel cross.
The first observations with this apparatus were made in July, 1904, and consisted of 260 turns of the interferometer arranged in two series. 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. On the dates chosen for the observations there were two times of the day when the resultant of these motions, about 33.5 kilometers per second, would lie in the plane of the interferometer, 11:30 o’clock, a.m., and 9:00 o’clock, p.m. The calculated azimuths of the motion would be different for
these two times but the velocities would be the same and 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 effects having positive magnitudes but having nearly opposite phases; when these were combined, the half sum was nearly zero. This small result was opposed to the theory then under consideration and it seemed impossible to reconcile the observations with the known orbital motion of the earth. The report of these experiments, published in the Philosophical Magazine,9 in May, 1905, concludes with this statement: “If pine is affected at all as has been suggested, it is affected to the same amount as is sandstone. 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.” The two curves for the ether-drift obtained from the morning and the evening observations of July, 1904, are shown in Fig. 11, being superimposed, as explained above; the lower curve represents the mean displacement thus obtained, which is the result given in the published account of these experiments.
In accordance with the results set forth later in this report, this procedure of 1904 was incorrect,
9 E. W. Morley and D. C. Miller, Phil. Mag. [6] 9, 680 (1905); Proc. Am. Acad. Sci. 41, 321 (1905).
Fig. 11. Method of combining the ether-drift observations of July, 1904, now considered erroneous.
being based upon an erroneous hypothesis as to the resultant absolute motion of the earth. The morning and evening observations each indicate a velocity of ether drift of about 7.5 kilometers per second; these values are charted in Fig. 4 in relation to the magnitudes predicted by the new hypothesis of a much larger predominating cosmic motion of the solar system and show reasonable consistency.
Observations by Morley and Miller in 1905
In 1905, the interferometer was mounted in a temporary hut on a site in Cleveland Heights, free from obstruction by buildings and at an altitude of about 285 meters. The house was provided with glass windows at the level of the interferometer so that there should be no opaque screens in the plane of drift. The test of the contraction hypothesis was continued; the wooden rods which determined the length of the optical path in the experiments of 1904 were omitted and all the mirror frames were fastened to the steel base, so that, for contrast, the optical distances were now determined by the steel. The program also included an investigation of an ether drift with the apparatus at the higher elevation and free from obstruction by buildings. The observations made here in July, October and November, 1905, consisting of 230 turns in three sets, showed a very definite positive effect slightly larger than that previously obtained, but still too small to be reconciled with the expectation. The velocity of relative motion of the
earth and ether obtained from the observations made in October is 8.7 kilometers per second; this is shown on the chart, Fig. 4; as compared with the result to be expected from the new theory here presented, the agreement is almost perfect. Plans were made for testing various modifications of theory but before these were carried out circumstances beyond control required that the interferometer be dismounted. Professor Morley retired from active service and it devolved upon the writer to continue the experiments. It seemed desirable that further observations should be carried out at a much higher altitude but several causes prevented the immediate resumption of the work. Other interests developed and though the expectation of continuing the experiments persisted, a long delay ensued.
The Inception of the Theory of Relativity, 1905
The Theory of Relativity had its inception at this time when Einstein published his paper entitled Zur electrodynamik bewegter Körper, in November, 1905,10 and was elaborately developed in succeeding years. The tests of the theory of relativity, made at the solar eclipse of 1919, were widely accepted as confirming the theory. Since the Theory of Relativity postulates an exact null effect from the ether-drift experiment which had never been obtained in fact, the writer felt impelled to repeat the experiment in order to secure a definitive result. An elaborate program was prepared and ample funds to cover the very considerable expense involved were very generously provided by Mr. Eckstein Case of Cleveland.
The Mount Wilson Experiments, 1921 Observations of April, 1921. Steel interferometer
Through the courtesy of the Carnegie Institution of Washington, the ether-drift interferometer was set up on Mount Wilson in March, 1921, on the grounds of the Mount Wilson Observatory, on Rock Crusher Knoll or “Ether Rock” as it came to be called, near the site of the 100-inch telescope, at an altitude of about 1750 meters. A concrete foundation was laid on the bare rock of
10 A. Einstein, Ann. d. Physik 17, 891 (1905).
