The Michelson-Morley Experiment Zach Citty Stuart Lagerson Kyle McElvain Jaime Villarreal

Michelson and Morley Albert Abraham Michelson was an American Physicist and Nobel Laureate who first started studying ether after his invention of the interferometer in 1881. Edward Williams Morley was an American Physicist who started collaborating with Michelson in 1887 and helped him with the improved Michelson-Morley Experiment

The Luminiferous Ether As Maxwell’s equations were established in 1861, the wave characteristics of light were well accepted. It was then believed that just as sound waves have a medium they travel through, so must light. This medium was called the “Ether” (or æther). Although the medium was assumed to exist, it had never before been measured. Since light can travel through a vacuum, it was assumed that the Ether was present everywhere and was the absolute reference frame. Previous experiments such as the Fizeau experiment and experiments on the aberration of light suggested that the ether was stationary with respect to the earth. Thus, it was possible to detect the motion of the ether relative to the motion of the Earth with sensitive enough apparatus. Before Michelson’s invention of the interferometer, no instrument had been sensitive enough to be able to detect the ether’s presence.

The Interferometer The Michelson Interferometer was invented to study the “interference” patterns of light. A beam of light is sent from a light source and is split by a mirror and reflected back so the light beams recombine and interfere as shown in the figure. In the presence of the ether certain interference patterns can be expected depending on the orientation of the interferometer. With enough experimental data it was believed that the interferometer could detect the presence of the ether wind relative to the Earth’s orbit.

The Experiment Michelson and Morley created a more advanced version of Michelson’s 1881 experiment as shown in Figure 1 and 2 The apparatus was assembled in a closed room in the basement of a heavy stone dormitory. It was mounted on a large block of sandstone and floated on a annular trough of mercury to eliminate vibrational and thermal effects. The apparatus could be freely rotated on the mercury so the apparatus would be easily aligned and misaligned to the ether wind, providing easy to observe phenomena. The experiment was performed over the course of months from April to July 1887 but the experiment FAILED every single time. The interference patterns observed appeared to be completely independent of the orientation of the apparatus at any given time. ?? Thus, it was assumed that the ether cannot be stationary with respect to the Earth, but must move alongside it, which contradicted other experiments at the time. Michelson and Morley later gave up on their experiment and considered it a failure.

Theoretical Predictions If Tl and Tt are the times it takes for the light to travel in the longitudinal and transverse directions, respectively, then : So the length difference should be: and where is the length difference of the apparatus rotated 90 degrees. Thus, the expected maximal fringe shift n is : = .44

Impact of the Michelson-Morley Experiment The Michelson-Morley Experiment failed attempt to measure the ether, disputed the actual existence of the Luminiferous ether. From the information obtained in the null results, Einstein’s Theory of Special Relativity was given experimental support. In Special Relativity the failure of the Michelson-Morley experiment was explained and the theory of the Luminiferous ether was discarded. The idea of an absolute reference frame was abandoned.

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