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).
the knoll and four concrete piers were formed to support the iron mercury tank at a suitable height. This was enclosed in a light housing, Fig. 12, twenty feet square and about twelve feet high at the ridge of the roof. The sides of the house were enclosed with sheets of corrugated
Fig. 12. Interferometer house on “Ether Rocks," Mount Wilson.
iron, except that at a height of from four to seven feet above the floor, on all sides there were continuous “windows” of white canvas cloth. The canvas was attached to a series of frames so that the windows could be opened on all sides at the level of the interferometer, for a width of three feet. In the south end was a small door with iron and canvas panels to match the sides of the house. A rough flooring was placed a little above the rock; on this floor a smooth circular track was constructed on which the observer could walk comfortably while following the interferometer as it turns slowly on its axis. This house was purposely constructed with wide cracks at the various joints in the sides and floor and under the eaves, so that there should be a very free circulation of air to secure equalization of temperature with the outside air. The opening of the windows on all sides greatly facilitated this condition. In order to secure sufficient darkness for the observation of the fringes in the daytime, sheets of thin black paper were placed over the canvas windows and over such holes and cracks as admitted too much light. Electric current was supplied to the house and several fixed and portable lamps were available. Common and
precision thermometers were hung on each side of the house and were read at the beginning and end of each set of observations. A barograph and a thermograph were carried at all times on the interferometer itself. An anemometer was attached to the roof of the house. A copy of the Mount Wilson Observatory meteorological records was also secured for the duration of the observations. These general arrangements apply to all subsequent experiments.
Observations were begun on April 8 and continued till April 21, 1921, by means of the apparatus and methods employed by Morley and Miller in 1904 and 1905, with certain modifications and developments in details. The first observations of sixty-seven sets consisting of 350 turns 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. 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 suggested were radiant heat, centrifugal and gyrostatic action, irregular gravitation effects, yielding of the foundation, magnetic polarization and magnetostriction. In order to test the first, the metal parts of the interferometer were completely covered with cork about one inch thick, and fifty sets of observations consisting of 273 turns were made; there was a periodic displacement of the fringes, as in the first observations, showing that radiant heat is not the cause of the observed effect.
Observations of December, 1921. Concrete interferometer
In the summer of 1921, the steel frame of the interferometer was dismounted and a base of one piece of concrete, Fig. 13, reinforced with brass, was cast in place on the mercury float. All the metal parts which were supported on the concrete base were made of aluminum of brass. 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, consisting of 422 turns, were made with the nonmagnetic interferometer. These show a positive effect as of an ether drift, which is
