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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).
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It seemed desirable that further observations should be carried out at a much higher altitude, but numerous causes prevented the resumption of observations.
It was at this time that Einstein became interested; and in November, 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, postulating that for an observer on the moving earth, the measured velocity of light must 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 of Michelson, Morley and Miller had given a definite and exact null result.
The deflection of light from the stars by the sun, as predicted by the theory of relativity, was put to the test at the time of the solar eclipse of 1919. The results were widely accepted as confirming the theory. This revived the writer's interest in the ether-drift experiments, the interpretation of which had never been acceptable to him.
The site of the Mount Wilson Observatory, near Pasadena, California, at an elevation of about six thousand feet, appeared to be a suitable place for further trials. An elaborate program of experimentation was prepared, and ample funds to cover the very considerable expense involved were very generously provided by Mr. Eckstein Case, of Cleveland. The president and trustees of Case School of Applied Science gave every possible assistance by allowing leave of absence to the writer at such times as were desirable for making the experiments and by providing an assistant for carrying on the very laborious work of calculating and analyzing the observations. Through the kindness of President Merriam, of the Carnegie Institution of Washington, and of Directors Hale and Adams, the ether-drift experiments have been carried on at the Mount Wilson Observatory during the past five years.
Observations were begun in March, 1921, using the apparatus and methods employed by Morley and Miller in 1904, 1905 and 1906, with certain modifications and developments in details. The very first observation 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 ether-drift; among the causes sug-
* Einstein, “Zur electrodynamik bewegter Korper, Ann. der Physih, 17, 891 (1905).
gested were magneto-striction and radiant heat. In order to test the latter the metal parts of the interferometer were completely covered with cork about one inch thick, and fifty sets of observations were made showing a periodic displacement of the fringes, as in the first observations, thus showing that radiant heat is not the cause of the observed effect.
In the summer of 1921 the steel frame of the interferometer was dismounted and a base of one piece of concrete, reinforced with brass, was cast in place on the mercury float. All the metal parts were made of aluminum or brass, thus the entire apparatus was free from magnetic effects and the possible effects due to heat were much reduced. In December, 1921, forty-two sets of observations were made with the non-magnetic interferometer. These show a positive effect as of an ether drift, which is entirely consistent with the observations of April, 1921. Many variations of incidental conditions' were tried at this epoch. Observations were made with rotations of the interferometer clockwise and counter-clockwise, with a rapid rotation and a very slow rotation, with the interferometer extremely out of level, due to the loading of the float on one side. Many variations of procedure in observing and recording were tried. The results of the observations were not affected by any of these changes.5
The entire apparatus was returned to the laboratory in Cleveland. During the years 1922 and 1923 many trials were made under various conditions which could be controlled and with many modifications of the arrangements of parts in the apparatus. An arrangement of prisms and mirrors was made so that the source of light could be placed outside of the observing room, and a further complication of mirrors was tried for observing the fringes from a stationary telescope. Methods of photographic registration by means of a motion picture camera were tried. Various sources of light were employed, inr cluding sunlight and the electric arc. Finally an arrangement was perfected for making observations with an astronomical telescope having an objective of five inches aperture and a magnification of fifty diameters. The source of light adopted was a large acetylene lamp of the kind commonly used for automobile headlights. An extended series of experiments was made to determine the influence of inequality of temperature and of radiant heat, and various insulating covers were provided for the base of the interferometer and for the light path. These experiments proved that under the conditions of actual observation the periodic displacement could not possibly be produced by temperature effects. An ex-
5 Miller, “Ether-drift Experiments at Mount Wilson Observatory, ’ ’ Phys. Bev., 19, 407 (1922); Science, 55, 496 (1922).