The Ether-Drift Experiment and the Determination of the Absolute Motion
of the Earth
Dayton C. Miller, Case School of Applied Science
CONTENTS
The Ether-Drift Experiment, Historical, 1878-1881 ............................ 204
The Michelson-Morley Experiments, Cleveland, 1887......................... 205
The Lorentz-Fitzgerald Hypothesis........................................... 207
The Morley-Miller Experiments, Cleveland, 1902-1906........................ 208
The interferometer of wood, 1902............................................ 208
Description of the new steel interferometer.................................... 208
Adjustment of the interferometer............................................ 210
Method of using the interferometer........................................... 211
Reduction of the interferometer observations.................................. 213
Stability of the interferometer............................................... 214
Observations by Morley and Miller in 1904.................................... 215
Observations by Morley and Miller in 1905.................................... 217
The Inception of the Theory of Relativity, 1905.............................. 217
The Mount Wilson Experiments, 1921......................................... 217
Observations of April, 1921. Steel interferometer............................... 217
Observations of December, 1921 Concrete interferometer....................... 218
Laboratory Tests of the Interferometer, Cleveland, 1922-1924................ 219
The Mount Wilson Experiments, 1924......................................... 220
General Analysis of the Ether-Drift Problem................................ 222
The various component motions involved..................................... 222
Solution for the absolute motion of the solar system........................... 224
The apex of the absolute motion determined from the magnitude of the ether-drift
effect................................................................... 225
The apex of the absolute motion determined from the azimuth of the ether-drift effect. 225
Harmonic analysis of the fringe displacements................................. 226
The Ether-Drift Observations Made at Mount Wilson in 1925-1926............ 228
General program of observation.............................................. 228
Data of observation........................................................ 228
Final results of observation.................................................. 230
Absolute Motion of the Solar System and the Earths Orbital Motion Determined.................................................................. 231
Northern apex of the solar motion rejected.................................... 231
Southern apex of the solar motion adopted.................................... 232
Reduced velocity and displaced azimuth are unexplained........................ 234
Validity of the solution..................................................... 235
Probable error............................................................. 238
Full-Period Effect........................................................... 238
The Entrained Ether Hypothesis............................................. 239
Other Recent Ether-Drift Experiments....................................... 239
Other Evidences of Cosmic Motion............................................ 241
Acknowledgments............................................................ 241
The Ether-Drift Experiment, Historical 1878-1881
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. 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 ether was 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 moving bodies such as the earth has been considered since the early days of the wave theory. 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. Fresnel proposed an explanation based on the wave theory, which has been generally accepted, which presumes first that the ether is at rest in free space; and, second, that the ether density is different in different substances and that the velocity of propagation of light in any substance varies inversely as the square root of the ether density. 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, has always been in doubt.
The first suggestion of a method for measuring the relative motion between the earth and the ether by means of an optical experiment was made by James Clerk Maxwell in the article on Ether, which he contributed to Vol. VIII of the 9th Edition of the Encyclopaedia Britannica, published in 1878. 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 with respect to the ether and that the earth in its motion in space passes freely through the ether without disturbing it. The
experiment is based upon the argument that the apparent velocity of light would be different according to whether the observer is carried by the earth in the line in which the light is travelling or at right angles to this line. It would thus be possible to detect a relative motion between the moving earth and the stationary ether, that is to observe an ether drift. The orbital motion of the earth has a velocity of thirty kilometers per second, while the velocity of light is ten thousand times as great, three hundred thousand kilometers per second. If it were possible to measure the direct effect of the earths orbital motion on the apparent velocity of light, then the velocity measured in the line of motion should differ from the velocity at right angles to this line by thirty kilometers per second, that is by one part in ten thousand. This would be a first-order effect. Maxwell explains that, since all practicable methods require that the light shall travel from one station to another and back again to the first station, a positive effect of the earths motion on the ray going outward would be neutralized by a negative effect on the returning ray, except that on account of the motion of the observer during the time the light is travelling the neutralization would not be quite complete, and a second-order effect, proportional to the square of the ratio of the velocity of the earth to the velocity of light, would be observable. Maxwell concludes with the statement, The change in the time of transmission of the light on account of a relative velocity of the aether equal to that of the earth in its orbit would be only one hundred-millionth part of the whole time of transmission, and would therefore be quite insensible.
The late Professor Albert A. Michelson accepted the challenge of Maxwells suggestion and while attending the University of Berlin in 1880-1881, he devised the remarkable instrument universally known as the Michelson interferometer which was especially adapted to the ether-drift experiment.1, 2 In the interferometer a
1 A. A. Michelson, Phil. Mag. [5] 13, 236 (1882); Am. J. Sci. 23, 395 (1882); H. A. Lorentz, Astrophys. J. 68, 345 (1928); Thos. Preston, Theory of Light, 5th ed., 229, 566 (1928); R. W. Wood, Physical Optics, 2nd ed. 265, 672 (1911).
2 W. M. Hicks, Phil. Mag. [6] 3, 9, 256, 555 (1902);
