Dayton C. Miller. Significance of the ether-drift experiments of 1925 at Mount Wilson //Science, Vol. 63 (1635), April 30, 1926

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Vol. LXIII April 30, 1926 No. 1635


Significance of the Ether-Drift Experiments of 1925 at Mount Wilson: Professor Dayton C. Miller...... 433

Existing Practices of Polluting Public Water

Courses: Dr. George Wilton Field----------------------------------443

Awards of the John Simon Guggenheim Memorial

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Scientific Events:

The Oceanographic Station at Salammbo, North Africa; The Bawson MacMillan Sub-Arctic Expedition of the Field Museum; The First Meeting of the Eastern Section of the Seismological Society; Tribute to Professor Charles Schuchert 448

Scientific Notes and News.__________________________________________________________450

University and Educational Notes__________________________________________454

Discussion and Correspondence:

The Habits of the Grumon: Dr. David Starr Jordan. Adsorption Mechanisms: Maurice L. Huggins and John Field, 2nd. Who discovered Vit amines? Dr. Casimir Funk. Citations of Scientific Literature: Professor Edwin G. Boring,

E. W. Gifford. Bailroad Passes for Scientific

Work: Dr. Herbert W. Rogers______________________________________________454

Scientific Books:

Bon/ne and Bonne-Wepster’s Mosquitoes of Surinam: Dr. L. O. Howard and Dr. Harrison G.

Scientific Apparatus and Laboratory Methods:

A Simple Method for Observation of the Living

Chick Embryo: Т. C. Byerly_____________________________________________458

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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. The ether was at first 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 the earths motion has been considered from the early days of the wave theory. 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 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. A second explanation was proposed, based on the wave theory, which seemed almost as simple as the former, but it failed to account for the fact, later proved by experiment, that the aberration is unchanged when observations are made with a telescope filled with water. Fresnel developed the theory which has been generally accepted, first, that the ether is at rest in free space and in opaque bodies, while, second, in the interior of moving transparent bodies it is supposed to move with a velocity less than the velocity of the w2-1

body in the ratio ——, where n is the index of re-n

fraction. 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 and in opaque bodies, has always been in doubt.

Several physicists have sought to prove the existence of the stationary ether by direct experiment. The most fundamental of such proposals was that of Professor A. A. Michelson, made in 1881, based upon the idea that the ether as a whole is at rest and that light waves are propagated in the free ether in any direction and always with the same velocity with re-

1 Address of the president of the American Physical Society, read at Kansas City, December 29, 1925.



[Vol. LXIII, No. 1635

spect to the ether. It was also assumed that the earth in its orbital motion around the sun passes freely through this ether as though the latter were absolutely stationary in space. The experiment proposed to detect a relative motion between the earth and the ether, and it is this relative motion which is often referred to as “ether-drift.” The experiment is based upon the argument that the apparent velocity of light would vary according to whether the observer is carried by the earth in the line in which the light is traveling or at right angles to this line. The .velocity of light is three hundred thousand kilometers per second, while the velocity of the earth in its orbit is one ten thousandth part of this, thirty kilometers per second. The actual motion of the earth is at all times the resultant of the motion of the earth in its orbit,, varying in direction and having a velocity of thirty kilometers per second, and of the constant motion of the sun (including the whole solar system), in an unknown direction and with an unknown velocity. Therefore, the actual relative motion of the earth and ether is unknown, and it may be less than thirty kilometers per second or very much greater. If it is assumed that the relative motion is equal to that of the earth in its orbit, and if it were possible to measure the direct effect of this motion on the apparent velocity of light, then the velocity measured in the line of motion should differ from the apparent velocity at right angles to this line, by thirty kilometers per second, or by one part in ten thousand. This is what is called a “first order effect”; but, unfortunately, there is no known method of measuring the velocities under such simple conditions. All methods require the ray of light to travel to a distant station and back again to the starting point, and a positive effect of the earth’s motion on the ray going outward would be neutralized by a negative effect on the returning ray. But, for a moving observer, it was shown that the neutralization would not be quite complete; the apparent velocity of the ray going and coming in the line of the earth’s motion would differ from the apparent velocity of the ray going and coming at right angles, in the ratio of the square of the velocity of the earth to the velocity of light, that is, by an amount equal to one part in (10,000)2 or to one part in 100,000,000. The only effect which can be experimentally determined, therefore, is exceedingly minute; it is a “second order effect.”

A remarkable instrument known as the “interferometer,” which had been invented by Professor Michelson, is capable of detecting a change in the velocity of light of the small amount involved in ether drift. In this experiment a beam of light is literally split in two by a thin film of silver, on what is called the “half-silvered mirror”; the coating of silver is thin enough to allow about half of the light to pass

straight through, while the other half is reflected in the usual manner. These two beams of light may thus 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 paths are optically equal, that is, if there are exactly the same number of wavelengths of light in each, the reunited portions will blend with the waves in concordance. If, however, one path is a half-wave longer than the other, the waves will come together in “opposite phase,” the crest of one coinciding with the trough of the other. These and other phase relations between the two rays produce effects called “interference fringes,” observation of which enables one to detect slight changes in the velocity of light in the two paths.

In the year 1887, at Case School of Applied Science, in Cleveland, Professor Michelson, in collaboration with the late Professor Edward W. Morley, of Western Reserve University, made certain important developments of method and apparatus and used the interferometer in the now famous “Michelson-Morley experiment,” in an effort to determine whether the motion of the earth through space produces the effect upon the velocity of light as predicted by theory. Unfortunately we do not know in what absolute direction the earth is going and so it is not possible to place the interferometer certainly in this direction. Therefore, the whole apparatus is mounted on a base which floats on mercury so that it can be turned to all azimuths of the horizontal plane of observation in the effort to find the direction of the drift. The rotation of the earth on its axis causes the plane of the interferometer to move as though it were on the surface of a rotating cone, the axis of which coincides with that of the earth and thus to take many different space orientations. It is only that component of the actual drift which lies in the plane of the interferometer at the moment of observation which can be observed. Therefore, the apparent azimuth and magnitude of the drift should change with the time of observation.

The full significance of the ether-drift experiments of# 1925 can be presented only by considering the interpretations given to the experiments made previously. For this reason a historical summary of all the experiments will be given.

In July, 1887, Michelson and Morley made six sets of observations for the ether-drift effect, one at noon and one at six o’clock in the evening, on each of three days, July 8, 9 and 11. This constitutes the whole of the observations made by Michelson and Morley. In November, 1887, they announced their conclusions as follows: “Considering the motion of the earth in its orbit only . . . the observations show that the relative motion of the earth and the ether is probably less