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

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No displacement is observed. By assuming a partial entrainment of the ether, FresnePs coefficient p may readily be determined from this experiment. It may also be found in a very simple and direct fashion with help of the Lorentz transformation.

FresnePs result was accepted universally by investigators of his time, including Maxwell, who pointed out that, while there could be no first-order effects, there might, nevertheless, be second-order effects (proportional to u?/c2). Now with o km/sec. for the motion of the earth, w/c = io-4, and w2/c2 = io~8, a quantity too small to be measured, according to Maxwell.

It seemed to me, however, that by making use of light-waves, one might devise an adequate arrangement for measuring such second-

Fig. 2 Fig. 3

order effects. Consider an apparatus, including mirrors, moving with the velocity w through the ether. Suppose two light-beams to travel back and forth in the apparatus, one parallel to w, the other at right angles to w. According to the classical theory, the change in light-path resulting from w should be different for the two beams and produce an appreciable shift of the interference fringes. The first device tried for the measurement of second-order effects is indicated in Figure 2. This arrangement, however, involved very great difficulties and was soon abandoned; and fortunately, because it led to the construction of the interferometer, which has proved of great value in many subsequent experiments.

The interferometer (Fig. 3) is known to all of you. A set of fringes is obtained by superposition of the two beams traveling from the source to a glass plate and then to mirrors 1 and 2, respectively,



and back. If white light is used, the central fringe will be white and the side fringes will be colored. A motion of the apparatus with the velocity w through the ether should have much the same effect on the light as a stream of water would have on a boat trying to go once forth and back across the stream, and once down and then back up the stream. The time for getting forth and back a given distance will be different for the two cases. This is easily seen, because, however swift the current, the boat in its transverse journey could always return to the bank from which it started, whereas, in the second case, it might be unable to get back up stream against the current.

I tried the experiment at Berlin in Helmholtz’ laboratory, but the vibrations of the city traffic made it impossible to get steady fringes. The apparatus was transferred to the observatory at Potsdam. I have forgotten the name of the director (I think it was Vogel), but I remember with pleasure that he was immediately interested in my experiment. Though he had never seen me before, he put the whole observatory with its staff at my disposition. I got a zero result in Potsdam. The accuracy was not very high, because I had a light-path of only about i m. Still it is interesting that the results were quite good. Coming back to America, I had in Cleveland the good fortune to secure the co-operation of Professor Morley. The apparatus then used was the same in principle as that used in Berlin, although the light-path was made longer by introducing a number of reflections instead of a single one. The path was in fact about io-ii m long, which should have yielded a displacement of half a fringe, due to the orbital motion of the earth. But no displacement was found. The shift of fringes was certainly less than 1/20 and may be even 1/40 of that predicted by the theory. The result could be accounted for by the assumption that the earth drags the ether along nearly at its full speed, so that the relative velocity between the ether and the earth at the surface is zero or very small. This assumption, however, is a very dubious one because it contradicts some other important theoretical considerations. Lorentz then suggested another explanation (Lorentz contraction) which in its final form yielded as a result the famous Lorentz transformation equations. These contain the gist of the whole relativity theory. The Michelson-Morley experiment was continued by Morley and