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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April 30, 1926]



right ascension and declination of the earth’s absolute motion. The right ascension is the sidereal time at which the azimuth (in the simple case) passes from east to west of north; this corresponds to the place where the curve crosses its true axis when passing from a maximum to a minimum. The dotted lines in the lower part of Fig. 6 show that this occurs at 17% hours, which is the right ascension of the apex; or being expressed in degrees it is equal to 262°. The declination of the apex may be determined from the amplitude of the curve taken in connection with the latitude of the observatory; the value thus .obtained is a declination of +65°. The observed velocity of the earth’s motion, projected on the plane of the interferometer, should show a daily variation in magnitude as a result of the rotation of the earth on its axis; this magnitude should drop to its minimum value at a sidereal time which is the right ascension of the apex; and should reach its maximum twelve hours from this time. Considering the latitude of Mount Wilson, 31° 14', and the declination of the apex as just determined from the azimuth of observations, it appears that at the time of maximum the plane of the interferometer makes an angle of less than 8° with the direction of the earth’s motion; thus the projection of the velocity at this time does not differ appreciably from its full value, which is then shown to be equal to ten kilometers per second. The declination of the apex may be determined from the magnitude observa

tions, as well as from those for azimuth, since it determines the ratio of maximum and minimum values of velocity for a given latitude. The agreement of the two right ascensions derived from these independent curves, indicated by dotted lines in Fig. 6, together with an equal concordance of the declinations, is a further very strong confirmation of the argument that the observed effects and the presumed motion are directly related.

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A study of the numerical results shows that the probable error in the determination of the azimuth of the effect is ~ 2°, while the probable error of the observed velocity on the supposition of a maximum value of ten kilometers per second is — 0.6 kilometers per second.

The argument that the direction and magnitude of the observed ether drift is independent of local time and is constant with respect to sidereal time implies that the effect of the earth’s orbital motion is imperceptible in the observations. No effect of this orbital motion has been found in these observations of 1925; this is strictly in accordance with the results obtained by Michelson and Morley in 1887 and by Morley and Miller in 1905. In order to account for this fact it is assumed that the constant motion of the earth in space is more than two hundred kilometers per second, but that for some unexplained reason the relative motion of the earth and the ether in the interferometer at Mount Wilson is reduced to ten kilometers per second; under these conditions a component motion equal to the earth’s orbital motion would produce an effect on the resultant which is just below the limit of



[Vol. LXIII, No. 1635

the smallest quantity which can be measured by the present interferometer. It is for this reason that it is concluded that the velocity of the motion of the solar system is at least two hundred kilometers per second and it may be much greater. The fact that the observed effect is dependent upon sidereal time and is independent of diurnal and seasonal changes of temperature and other terrestrial causes shows that it is a cosmical phenomenon.

The previous observations made at Mount Wilson, while not sufficiently extended to determine curves of the kind just indicated, should, nevertheless, be consistent with these observations. In Fig. 7 the results of the observations for April 15, 1921, are compared with the curves calculated from the observations of 1925, showing a very good agreement.

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The complete study of the ether-drift experiments of 1925, at Mount Wilson, leads to the conclusion that there is a systematic displacement of the interference fringes of the interferometer corresponding to a constant relative motion of the earth and the ether at this observatory of ten kilometers per second ; and that the variations in the direction and magnitude of the indicated motion are exactly such as would be produced by a constant motion of the solar system in space, with a velocity of two hundred kilometers, or more, per second, towards an apex in the constellation Draco, near the pole of the elliptic, which has a right ascension of 262° and a declination of + 65°. In order to account for these effects as the result of an ether drift, it seems necessary to assume that, in effect, the earth drags the ether so that the ap

parent relative motion at the point of observation is reduced from two hundred, or more, to ten kilometers per second, and further that this drag also displaces the apparent azimuth of the motion about 45° to the west of north.

It is evident that the present experiments are no more consistent with the old theories of a stagnant ether than were those of Michelson and Morley of 1887, and of Morley and Miller of 1905; the present work is in no way a contradiction of the earlier results, but is rather a confirmation and extension of them. That a set of six characteristic curves obtained from observations which are wholly independent of each other, and which were made at times of year with extreme differences of weather conditions, so consistently fit curves depending upon the assumed motion, as shown in Figs. 5 and 6, leads irresistibly to the conclusion that the observed effects are related to the presumed cause. One is compelled therefore to consider whether there can be a possible readjustment of the theories of the ether that will account for the reduced velocity and other experimental results.

The values of the quantities defining the absolute motion of the solar system as obtained from these ether-drift observations are in general agreement with the results obtained by other methods. The recent study of proper motions of stars by Ralph Wilson, of the Dudley Observatory, and of the radial motions of the stars by Campbell and Moore, of the Lick Observatory, give the apex of the sun's way in the constellation Hercules with a right ascension of 270° and a declination of about +30°, with a velocity of about nineteen kilometers per second. Dr. G. Stromberg, of the Mount Wilson Observatory, from a study of globular clusters and spiral nebulae, finds evidence of a motion of the solar system towards a point having a right ascension of 307° and a declination of +56°, with a velocity of three hundred kilometers per second. Lundmark, studying the spiral nebulae, finds evidence of a motion having a velocity of four hundred kilometers per second. The various determinations of the motion of the solar system are all in the same general direction and lie within a circle having a radius of 20°. Our assumed velocity of two hundred kilometers per second is simply a lower limit; it might equally well be three hundred or four hundred kilometers per second. The first assumption therefore seems to offer no difficulty. The location of the apex in the constellation Draco, at right ascension 262° and declination + 65°, is within 6° of the pole of the ecliptic, that is, the indicated motion of the solar system is almost perpendicular to the plane of the ecliptic. The sun's axis of rotation points to within 12° of this apex. One can not help wondering whether there may be some dynamic significance in these facts.