The Ether-Drift Experiment and the Determination of the Absolute Motion of the Earth

T ether-drift experiment first suggested by Maxwell in 1878 and made possible by Michelson's invention of the interferometer in 1881, though suitable for the detection of the general absolute motion of the earth, was actually applied for detecting only the known orbital component of the earth's motion. For the first time, in 1925 and 1926, I made observations at Mount Wilson of such extent and completeness that they were sufficient for the determination of the absolute motion of the earth. These observations involved the making of about 200, 000 single readings of the position of the interference fringes. The ether-drift observable in the interferometer, as is well known, is a second order effect; and the observations correctly define the line in which the absolute motion takes place, but they do not determine whether the motion in this line is positive or negative in direction. At the Kansas City meeting of the American Association for the Advancement of Science, in December, 1925, before the completion of the Mount Wilson observations, a report was made showing that the experiment gives evidence of a cosmic motion of the solar system, directed towards a northern apex ; but the effects of the orbital motion were not found, though it seemed that the observations should have been quite sufficient for this purpose. The studies of the proper motions and of the motions in the line of sight of the stars in our galaxy have shown that the solar system is moving, with respect to oiir own cluster^ in the general direction of a northern apex in the eonstellation Hercules. This apex is near that indicated by the ether-drift observations as just reported, and seemed to be confirmatory evidence of its correctness. Probably it was this that caused