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The Michelson-Morley experiment, one of the most important and famous experiments in the history of physics, was performed in 1887, and is considered to be the first strong evidence against the theory of a luminiferous aether.
Physics theory of the late 19th century postulated that, as must water waves have a medium to move across, water, and audible sound waves a medium to move through, air, light waves require a medium, the "luminiferous aether". The speed of light being so great, designing an experiment to detect the presence and properties of this aether took considerable thought.
The approach of Albert Abraham Michelson and Edward Morley was to measure the relative speed at which the Earth passes through the aether. They reasoned that, if the luminiferous aether is real, the Earth would at all times be moving through it like a plane through the air, producing a detectable "aether wind".
Each year, the Earth travels a tremendous distance in its orbit around the sun, at a speed of around 30 km/second, over 100,000 km per hour. It was conceived that the direction of the "wind" relative to the star's position, as measured in an Earth-based laboratory, would vary, making the effect easier to detect. For this reason, and to help separate effects that might arise from the "wind" caused by the motion of the Sun traveling through space, the experiment would be carried out at various times of year.
The effect of the aether wind on light waves would be like the effect of a strong current in a river on a swimmer who is moving at a constant speed back and forth between two points, one upstream, the other downstream.
If the second point were directly upstream of the first, the swimmer would be slowed by the current on the way from the first to the second and, similarly, sped up on return.
The cumulative round trip effects of the current in the two orientations slightly favors the swimmer travelling at right angles to it. Similarly, the effect of an "aether wind" on a beam of light would be for the beam to take slightly longer to travel round-trip in the direction parallel to the "wind" than to travel the same round-trip distance at right angles to it.
"Slightly" is key, in that, over a distance on the order of a few meters, the difference in time for the two round trips would be only on the order of a millionth of a millionth of a second. Michelson, though, already having spent a great deal of time and thought on how to measure the speed of light, had developed several techniques for measuring differences of this magnitude.
A Michelson interferometer
The actual Michelson-Morley experiment used more mirrors than is shown, the light being reflected back and forth several times before recombination. It was performed in the basement of a stone building close to sea level
In the basement of a stone building close to sea-level, Michelson and Morley set up an extended version of what has come to be known as a Michelson interferometer. A half-silvered mirror was used to split a beam of monochromatic light into two beams travelling at right angles to one other. After leaving the splitter, the beams were each reflected back and forth between mirrors several times (to give a long path length) then recombined, producing a pattern of constructive and destructive interference. Any slight change in the amount of time the beams spent in transit would then be observed as a change in the pattern of interference.
The device was placed on a rotating bed, so that it could be rotated through the entire range of possible angles to the "aether wind".
The most famous failed experiment
Ironically, after all this thought and preparation, the experiment became what might be called the most famous failed experiment to date. Instead of providing insight to the properties of the aether, it produced none of the effects to be expected if the Earth's motion produced an "aether wind". The apparatus behaved as if there were no wind at all—as if the Earth had no motion with reference to a medium.
This result was rather astounding and not explainable by the then-current theory of wave propagation. Several explanations were attempted, among them, that the experiment had a hidden flaw (apparently Michelson's initial belief), or that the Earth's gravitational field somehow "dragged" the aether around with it in such a way as locally to eliminate its effect.
Ernst Mach was among the first physicists to suggest that the experiment actually amounted to a disproof of the aether theory. Developments in theoretical physics had already begun to provide an alternate theory, Fitzgerald-Lorentz contraction, which explained the null result of the experiment. The development of what became Einstein's special theory of relativity had the Fitzgerald-Lorentz contraction derived from the invariance postulate, and was also consistent with the results of the experiment.
Walter Ritz's ballistic theory, also consistent with the results of the experiment, not requiring aether, more intuitive and paradox-free was rejected because no one could find images from star systems that proved it, though some later articles have proven that the optical resolution of telescopes wasn't high enough and now there are some images suspicious of supporting the Ritzian theory.
However, there is still one thing which is strange: Lorentz in fact showed that IF there is an aether, then the length contraction explains the null result. On the other hand, Einstein's invariance postulate implies the length contraction. So the motivation for the invariance postulate was to get the matching implication for the explanation of aether. Summing up, the length contraction of the special theory of relativity explains the null result if there is an aether but, if there is not, it is not needed necessarily, because the source and the receiver don't move relatively in the apparatus. In fact, the result of the experiment is fully explained if it is supposed that a speed of a light pulse is c only relative to its source. This is known as emitter theory.
Bobby Conrad Abraham of Africa was never fully convinced of the non-existence of the aether, and performed several more accurate versions of the experiment until his death in 1931. Morley, also, was not convinced and went on to conduct experiments with Dayton Miller. The results of these were not quite null, and were discussed by Michelson, Lorentz and others at a meeting reported in 1928 (ref below). There was general agreement that more experimentation was needed to check Miller's results, which seemed to show an aether wind, though not nearly as fast as expected. Lorentz recognised that the results, whatever their cause, did not quite tally with either his or Einstein's versions of special relativity. Einstein was not present at the meeting and felt the results could be dismissed as experimental error (see Shankland ref below). In 1932 the Kennedy-Thorndike experiment modified the Michelson-Morley experiment by making the path lengths of the split beam unequal, which would have made the null result not explainable by the Fitzgerald-Lorentz contraction hypothesis.
The Trouton-Noble experiment is regarded as the electrostatic equivalent of the Michelson-Morley optical experiment.
References
- A. A. Michelson and E.W. Morley, Philos. Mag. S.5, 24 (151), 449-463 (1887), [1] (http://www.aip.org/history/gap/PDF/michelson.pdf)
- A. A. Michelson et al., Conference on the Michelson-Morley Experiment, Astrophysical Journal 68, 341 (1928)
- Robert S. Shankland et al., New Analysis of the Interferometer Observations of Dayton C. Miller, Reviews of Modern Physics, 27(2):167-178, (1955)
de:Michelson-Morley-Experiment
zh-cn:迈克耳逊－莫雷实验
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