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

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parts were all new, and nothing was used from the original apparatus excepting the mercury tank and its wooden float.

Such an instrument with a base made of planks of pine wood was used at Cleveland, in 1902, 1903, and 1904, for the purpose of directly testing the Lorentz-FitzGerald effect, but the changes in the wooden frame due to the variations in humidity and temperature made it difficult to obtain accurate observations. A new supporting frame was designed by Professor F. H. Neff, of the department of civil engineering of the Case School of Applied Science, the purpose being to secure both symmetry and rigidity. This frame, or base, was made of structural steel and was so arranged that the optical dimensions could be made to depend upon distance-pieces of wood, or upon the steel frame itself. Observations were made with this apparatus in 1904. The procedure was based upon the effect to be expected from the combination of the diurnal and annual motions of the earth, together with the presumed motion of the solar system toward the constellation Hercules with a velocity of 19 km/sec. On the dates chosen for the observations there were two times of the day when the resultant of these motions would lie in the plane of the interferometer, about 11:30 a.m. and 9:00 p.m. The calculated azimuths of the motion would be different for these two times. The observations at these two times were, therefore, combined in such a way that the presumed azimuth for the morning observations coincided with that for the evening. The observations for the two times of day gave results having positive magnitudes but nearly opposite phases; when these were combined, the result was nearly zero. The result, therefore, was opposed to the theory then under consideration; but according to the ideas which will be set forth later in this address it now seems that the superposition of the two sets of observations of different phases was based upon an erroneous hypothesis and that the positive results then obtained are in accordance with a new hypothesis as to the solar motion. Our report of these experiments published in the Philosophical Magazine for May, 1905, concludes with the following statement: “Some have thought that this experiment only proves that the ether in a certain basement room is carried along with it. We desire therefore to place the apparatus on a hill to see if an effect can be there detected/’

As an important factor I may mention the state of mind in



which we then performed the experiment. It was proposed to look for a certain effect in order to check a certain theory. We had definite pictures in our minds as to what should happen. We calculated the magnitude and azimuth of the effect from the theory and discussed our experimental results in relation to these specific expectations. In every case we found that the result was negative as to these expectations. But it was never numerically zero, not even in the original Michelson and Morley experiment. It was zero in so far as the motion of the earth in its orbit is concerned. The remaining effect, however, was large enough to be measured. Experiments were performed to prove that it was not due to magnetic deformation of the frame, nor to temperature disturbances, since the effect was systematic. It was suggested that the ether might be entrained differently inside and outside of a masonry building.

In the autumn of 1905, Morley and Miller removed the interferometer from the laboratory basement to a site on Euclid Heights, Cleveland, free from obstruction by buildings, and having an altitude of about three hundred feet above Lake Erie and about eight hundred and seventy feet above sea-level. The house was purposely of very light construction, and was transparent (glass windows) in the direction of expected drift. Five sets of observations were made in 1905-1906, which give a definite positive effect of about one-tenth of the then-expected drift. Professor Morley retired from active work in 1906, and it devolved upon the present speaker to continue the experiments. It seemed desirable that further observations should be carried out at a much higher altitude, but numerous causes prevented the resumption of observations.

The deflection of the light from the stars by the sun, as predicted by the theory of relativity, was put to test at the solar eclipse of 1919. The results were widely accepted as confirming the theory. This revived the writer’s interest in the ether-drift experiments, the interpretation of which had never been acceptable to him. Through the kindness of President Merriam and Directors Hale and Adams, a site was provided at the Mount Wilson Observatory on the top of Mount Wilson, at an elevation of about six thousand feet. The ether-drift interferometer was set up here in February, 1921, and observations were carried on during the succeeding five years.