The Ether-Drift Experiment

by Prof. Dayton C. Miller

(Cleveland Plain Dealer, 10 March 1940, "All Feature Section" p.1 & 6)


The wave theory of light received general acceptance in the 19th century, and this required the hypothesis of a luminiferous ether, filling all space, even that occupied by material bodies, through which bodies such as the sun and planets move freely without disturbing the other.  Scientists sought to prove the existence of the ether by some direct experiment, and in 1878 Maxwell of Cambridge University, England, first proposed an optical experiment for this purpose.


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. The experiment is based upon the argument that if the speed of light were determined by an observer on the moving earth, the apparent speed would be different according to whether the observer is moving in the line which the light is travelling or at right angles to this line.  The detection of such an effect, would be accepted as proof of the existence of the ether.


An incomplete analogy is given by a power boat on a river, the speed of which would be different when sailing down stream from the speed with the same power when sailing across the stream.  Not only would the speed across stream be slower, but in order to reach a point directly opposite the starting point, the boat would have to be headed somewhat upstream.


The late Prof. A.A. Michelson accepted the challenge of Maxwell's suggestion, and in 1881 devised the remarkable instrument called the interferometer, by means of which it is possible to compare the speed of a beam of light which travels in the line of motion of the earth in space, directly with the speed of a beam at right angles to this motion, by detecting the difference in the two speeds.  In the interferometer a beam of light is literally split in two by a "half-silvered" mirror, and the two beams 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 beams travel equal paths with equal speeds, the reunited beams of light will blend with the waves of concordance. If, however, the speeds are different in the two paths, the two beams come together with differences in the phases of the waves, producing effects which are observed as the "interference fringes".


The interferometer enables one to detect exceedingly small differences in the relative speeds of the light in the two paths, the measurements being made in terms of the wave lengths of light.  Michelson's first apparatus proved inadequate as to sensitivity and stability.


He came to Case School of Applied Science in 1882 as the first professor of physics, and became associated with Prof. Edward W. Morley of Western Reserve University.  Prof. Morley proposed several important developments in the interferometer.  The light path was increased in lengthy by multiple reflections, and the optical parts were mounted on a stone block five feet square and one foot thick.  The stone was floated on a large basin of mercury so that it could be easily  rotated to bring one of the light paths of the interferometer into line with the earth's movement in space.


With this instrument, the famous "Michelson-Morley Ether-Drift Experiment" was performed in Cleveland, in July 1887.  Again the results were inconclusive and the instrument was of insufficient sensitivity for the delicate measurements.


Prof. Michelson left Case School in 1889.  In 1890 Dayton C. Miller entered the faculty and later became associated with Prof. Morley in a repetition of the ether-drift experiment on a still larger scale.  They constructed an interferometer about four times as sensitive as the one used in the earlier experiment, having a light path of 214 feet, equal to about 112,000,000 wave lengths.


Observations were made with this apparatus in 1904.  The results were expressed as follows:


"If the ether near the apparatus did not move with it, the difference in velocity was less than 3.5 kilometers a second, unless the effect on the materials annulled the effect sought. We desire to place the apparatus on a hill, covered only with transparent covering, to see if an effect can be there detected."


It was at this time that Einstein became interested; and in November 1905 he published a paper on "The Electrodynamics of Moving Bodies".  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 the first paper, Einstein makes the postulate that for an observer on the moving earth the measured velocity of light must be constant regardless of the direction of amount of 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, Cal., at an elevation of about 6,000 feet, appeared to be a suitable place for further trials.  An elaborate program of experimentation was prepared.


When observations are being made, the apparatus is kept in rotation on the mercury float, and the observer must walk around in a circle about twenty feet in diameter, keeping his eye at the moving eyepiece of the telescope attached to the interferometer.  The observer must not touch the interferometer in any way and yet he must never lose sight of the interference fringes which are seen only through the small aperture of the eyepiece, about a quarter of an inch in diameter.


A set of observations involve twenty turns of the apparatus and several hundred single "readings" and occupies about eighteen minutes of time.  The Mount Wilson series of observations involves more than 200,000 individual interferometer readings, being more than twice as many readings as had been made in all the ether-drift observations previous to this time.  This required that the observer should walk in the dark, in a small circle, for a total distance of 190 miles.


In 1933 the writer published the complete study of all the ether-drift observations with conclusions which may be summarized as follows:


"A relative motion of the earth and the ether should produce an effect which, as observed in the interferometer, would vary both in magnitude and direction as the earth rotates on its axis and as it revolves in its orbit; the effect further depends upon the latitude of the station of observation.  The actual observations indicate an ether drift of ten kilometers per second which varies in a manner wholly consistent with the theoretical requirements; however, the observed velocity of the drift is smaller than had been expected, as though the ether through which the interferometer is being carried by the earth's motion was not  absolutely at rest."


"A comparison of the effect of the orbital motion of the earth as observed in the interferometer with its known value leads to the conclusion that the absolute motion of the solar system in space has a velocity of 208 kilometers per second.  This cosmical motion is towards an apex located in the southern constellation Dorado, the Sword Fish, in the midst of the Great Magellanic Cloud of stars."


"In order to account for the results here presented, it seems necessary to accept the Lorentz-Fitzgerald theory of the contraction of matter moving through the ether, or to postulate a viscous or dragged ether as proposed by Stokes."


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