SCIENCE
Vol. LXI June 19, 1925 No. 1590
CONTENTS
Ether-drift Experiments at Mount Wilson: Professor Dayton C. Miller ............................................................ 617
Risks incurred in the Introduction of Alien Game
Birds: J. Grinnell .............................................................................. 621
Scientific Events:
A National Park in the Belgian Congo; The Society for Cultural Relations with Foreign Countries of the Soviet Union; The Daniel Guggenheim School of Aeronautics; National Research
Fellowships in the Biological Sciences .............................. 623
Scientific Notes and News .................................................................. 626
University and Educational Notes................................................ 629
Discussion and Correspondence:
Bacterial Catalase: Dr. James Walter McLeod.
The Reform of the Calendar: Dr. Charles Clayton Wylie. The Jonas Viles, Jr., Memorial
Scholarship: Professor W. C. Curtis .............................. 630
Scientific Books:
Sinclair Lewis’s Arrowsmith: Dr. Thomas J. Le-
Blanc .................................................................................................................. 632
Scientific Apparatus and Laboratory Methods:
Heavy Mineral Oil as a Permanent Non-volatile Preservative for Valuable Biological Material: Dr.
Joseph C. Chamberlin........................................................................ 634
Special Articles:
Arc Spectrum Regularities for Ruthenium: Dr.
W. F. Meggers and Dr. O. Laporte. Manganese as a Cure for a Chlorosis of Spinach: Dr. Forman
T. McLean and Dr. Basil E. Gilbert.............................. 635
The American Chemical Society:
Physical and Inorganic Chemistry: Arthur E.
Hill........................................................................................................................ 637
Science News...................................................................................................... x
SCIENCE: A Weekly Journal devoted to the Advancement of Science, edited by J. McKeen Cattell and Dublished every Friday by
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Entered as second-class matter July 18, 1923, at the Post Office at Lancaster, Pa., under the Act of March 8, 1879.
ETHER-DRIFT EXPERIMENTS AT MOUNT WILSON1
After the wave theory of light was established, it became necessary to assume the existence of an all-pervading medium in which the waves could be developed and transmitted; this hypothetical medium was called the “ether.” It was endowed with such properties as were necessary for the explanation of observed phenomena. Several physicists sought to prove the existence of the 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 respect 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 300,000 kilometers per second, while the velocity of the earth in its orbit is 1/10,000th part of this, 30 kilometers per second. If the earth’s orbital velocity were directly effective, the two apparent velocities should differ by 30 kilometers per second or by one part in 10,000. However, 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.
1 Read before the National Academy of Sciences, Washington, April 28, 1925.
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SCIENCE
[Vol. LXI, No. 1590
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 wave-lengths 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, in Cleveland, Professor Michelson, then professor of physics at Case School of Applied Science, in collaboration with Professor Edward W. Morley, of Western Reserve University, made certain important developments of method and apparatus and used the interferometer in an effort to determine whether the motion of the earth through space produces the effect upon the velocity of light predicted by theory. Unfortunately we do not know in what absolute direction the earth is going through space 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 presumed ether 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 cone whose axis 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 horizontal 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. A drift perpendicular to the plane of the interferometer will produce no effect whatever; it is quite possible that this condition may occur at certain times of the year.
It is not possible at this time to explain the details of the principles involved. The observations are made by looking through a telescope at the system
of interference fringes formed by the two beams of light. As the interferometer is rotated on its axis, an ether drift would cause the whole system of fringes to oscillate, moving first to one side and then to the other, this effect being periodic in each half revolution of the interferometer about its vertical axis. For a relative motion of the earth and the ether equal to the orbital velocity of the earth, that is 30 km/sec, the displacement in the original Michelson-Morley experiment would have been 4/10th of a fringe.
In November, 1887, Michelson and Morley announced the conclusions drawn from their observations made in July of that year 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 than one sixth of the earth’s orbital velocity and certainly less than one fourth.” (That is, it is less than 71/2 kilometers per second.) This result was considered by many as a null result, often called a negative result, and by some was thought to throw grave doubts upon the validity of the hypothesis of the luminiferous ether. There is a significant “Supplement” to this report which begins with the following sentence: “But it is not impossible that at even moderate distances above the level of the sea, at the top of an isolated mountain peak, for instance, the relative motion might be perceptible in an apparatus like that used in these experiments.”2
At the International Congress of Physics, held in Paris in 1900, Lord Kelvin gave an address in which he considered theories of the ether. He remarked that “the only cloud in the clear sky of the theory was the null result of the Michelson-Morley experiment.” Professor Morley and the writer were present, and in conversation with Lord Kelvin he expressed the conviction that the experiment should be repeated with a more sensitive apparatus. 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 224 feet, equal to about 150,000,000 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 1.5 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
2 Michelson and Morley: “Relative motion of the earth and the luminiferous ether, ” Am. Jl. of Sci., 34, 333 (1887); Phil. Mag., 24, 449 (1887); Jl. de Phys., 7, 444 (1888).