Fig. 2. The Michelson-Morley interferometer of 1887.
centimeters square and 30 centimeters thick. In order to obtain the necessary sensitivity, the effective light path was increased by reflecting the light back and forth so that it traversed the diagonal of the square stone block eight times, giving the effect of an interferometer with an arm about 1100 centimeters in length. The expected displacement of the fringes due to a velocity equal to that of the earth in its orbit was 0.4 of a fringe width.
Michelson and Morley performed the historic experiment in the northwest room of the basement of the Main Building of Adelbert College in Cleveland in 1887; their entire series of observations was of six hours duration, one hour at noon on each day of July 8, 9 and 11, and one hour in the evening of July 8, 9 and 12 and consisted of thirty-six turns of the interferometer, readings being made at each of sixteen equidistant points in each turn. The method of observation was arranged to detect the preconceived effect of the motion of the earth toward a known point in space with a given velocity, and hence no general series of observations was made. The brief series of observations was sufficient to show clearly that the effect did not have the anticipated magnitude. However, and this fact must be emphasized, the indicated effect was not zero; the sensitivity of the apparatus was such that the conclusion, published4 in 1887, stated that the observed relative motion of the earth and ether did not exceed one-fourth of the earths orbital velocity. This is quite different from a null effect now so frequently imputed to this experiment by writers on Relativity. It also seems necessary to call
attention to another historical fact: Michelson and Morley made only the one series of observations, in July, 1887, and never repeated the ether-drift experiment at any other time, notwithstanding many printed statements to the contrary.
In the original account of their experiment, Michelson and Morley give the actual readings for the position of the interference fringes in the six sets of observations. The upper one of the two long curves in Fig. 3, shows the average
Fig. 3. Fringe displacements of the original Michelson-Morley experiments of 1887.
of the three sets of readings taken at noon, and the lower long curve is the average for the three sets taken in the evening. These curves show the fringe displacements for a full turn of the interferometer, while the ether-drift effect being sought is periodic in each half turn. To find the latter effect, the second half of the long curve is superimposed on the first half by addition, which
cancels the full-period effect and all odd harmonics, giving the shorter curve which is the desired half-period effect (together with any higher even harmonics which may be present). Inspection shows clearly that these curves are not of zero value, nor are the observed points scattered at random; there is a positive, systematic effect. These full-period curves have been analyzed by the mechanical harmonic analyzer, which determines the true value of the half-period effect; this, being converted into its corresponding value for the velocity of relative motion of the earth and ether, gives a velocity of 8.8 kilometers per second for the noon observations, and 8.0 kilometers per second for the evening observations. In Fig. 4, the smooth
Fig. 4. Velocity of ether drift observed by Michelson and Morley in 1887, and by Morley and Miller in 1902, 1904 and 1905, compared with the velocity obtained by Miller in 1925.
curve shows the value of the ether-drift throughout the day for the latitude of Cleveland, as determined by the specifications of the drift which are derived later in this report from the observations made at Mount Wilson. The two circles on this chart show the magnitude of drift actually obtained by Michelson and Morley for the noon and evening observations, indicating a result wholly consistent with the later work here reported.
The fact that the result obtained by Michelson and Morley was not negligibly small was very fully set forth by Professor Hicks of University College, Sheffield, in 1902, in his important theoretical examination of the original experiment.2 Hicks also called attention to the presence of a full-period, first-order effect, which has never been sufficiently investigated; this first-order effect will be considered later.
The Lorentz-Fitzgerald Hypothesis
The Michelson-Morley experiment, which indicated that the theory of the ether was either incomplete or incorrect, attracted world-wide attention because of its fundamental character and because the result was wholly unexpected. Professor FitzGerald of Dublin, in 1891, offered an explanation for the small effect on the hypothesis that the forces binding the molecules of a solid might be modified by the motion of the solid through the ether in such a way that the dimension of the stone base of the interferometer would be shortened in the direction of motion and that this contraction might be such as to neutralize the optical effect sought in the Michelson-Morley experiment. FitzGerald did not publish this theory in a scientific journal but he expounded it in his lectures. This hypothesis was given publicity by Sir Oliver Lodge in his address on Aberration Problems and New Ether Experiments, presented to the Royal Society on March 31, 1892, which address was published in the Philosophical Transactions for the year 1893.5 Lodge has given further details of this historical fact in his recently issued autobiography.6 In 1895 Professor Lorentz of Leyden developed the theory in a systematic manner, on the supposition that the particles of all solids are held together by electrical forces; and that a motion of the body as a whole would superpose upon the electrostatic forces between the atoms a magnetic effect due to the motion. There would result a contraction of the body in the direction of motion which is proportional to the square of the ratio of the velocities of translation and of light and which would have a magnitude such as to annul the effect of ether-drift in the Michelson-Morley interferometer.7 If the contraction depends upon the physical properties of the solid, it was suggested by others that while the expected effect might be annulled in one apparatus, it might in an apparatus of different material give place to an effect other than zero, perhaps with a contrary sign.
5 G. F. FitzGerald, see O. J. Lodge, Aberration Problems, Phil. Trans. Roy. Soc. 184, 749 (1894).
6 Sir Oliver Lodge, Past Years, 204 (1932).
7 H. A. Lorentz, Versuch einer Theorie der electrischen und optischen Erscheinungen in bewegten Körpern (Leyden, 1895); Theory of the Electron, 195 (1909).
