Conference on the Michelson-Morley experiment held at the Mount Wilson observatory Pasadena, California February 4 and 5, 1927

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Let us discuss now the half-period effect. After having seen the different diagrams I think there can hardly exist any doubt that there is an actual displacement of the fringes with Mr. Miller’s setup. There arises, then, the question as to its possible cause. Mr. Miller himself has offered some suggestions which are very interesting. His conclusion is that the effect found corresponds to an absolute velocity of 10 km/sec. and for a definite sidereal time is the same throughout the year. It is certainly not connected with the orbital motion of the earth, but indicates a motion of the solar system relative to the stellar system of the same kind as found by Mr. Strömberg from a quite different point of view. The velocity of this motion is estimated to be at least 200 km/sec. For some reason or other, the full relative velocity between ether and earth

does not come into action. Otherwise, one cannot account for the lack of an effect related to the orbital motion of the earth. There is, however, the following point to be mentioned. One could assume as Mr. Miller does that the entrainment is only partial, because the earth, for instance, is not completely impermeable to the ether.' But then the following consideration would have to be taken into account. Suppose w to be the velocity of the earth relative to the ether (which is at rest at C). Then if the ether behaves as an ideal fluid, there will be a relative velocity in it at A with respect to B amounting to w/z. Miller accounts for the daily variation in amplitude of his effect, in a way which is immediately understood from Figure 20. According to the consideration given above, however, w could not be considered as a vector of constant length but would itself vary during one day. This would of course make the interpretation of the drift more complicated.

Fig. 19

my vacation Fig. 20

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As to the average displacement of the azimuth to the west (50°), it seems to be very hard to explain. Fortunately, however, it also changes about this position periodically with sidereal time. Otherwise, one could hardly escape the suspicion that the whole effect might be due to some cause in the laboratory.

Now some words about Piccard’s experiment: I saw the fringes in his laboratory, and they were extremely nice indeed. As a matter of fact, Piccard considers his experiments only preliminary ones, which he hopes still to improve considerably. He worked the first time under very unfavorable conditions, as the night of his first ascent was unusually warm. I might mention, just for general interest, that such observations as Piccard’s are very exhausting. Those of Mr. Miller are, too, of course. Piccard told me that he did not notice any physiological effect in the turning balloon due to centrifugal force; but motions in the vertical direction, nodding the head, for instance, were very painful, because of the effect of Corioli’s force.

Professor A. A. Michelson: There are one or two questions which I should like to ask. Did Mr. Miller put his results together with the intention of finding an orbital effect (effect due to the motion of the earth in its orbit around the sun) ?

Professor D. C. Miller: Certainly. It was for this purpose that the observations were made at four epochs, approximately at intervals of three months; so that the direction of the orbital component of motion changes about 90° from epoch to epoch. The observations for each epoch have been reduced to determine the actual resultant motion for that epoch. The apex of the motion indicated by all the observations is near the pole of the ecliptic, and hence the orbital motion would manifest itself in a change in the position of this apex from epoch to epoch; that is, it would produce a sort of annual aberration in the apex. A comparison of the results for the four epochs fails to show conclusive evidence of this effect. I hope, however, that when several sets of observations for each epoch are available, the effect of the orbital motion may be evident. The positive effect actually obtained corresponds to a relative motion of the earth and ether of about 10 km/sec., with a probable error of I km/sec. It follows that the effect of the orbital motion on the observed resultant velocity must therefore be less than \ km/sec.