Michelson-Morley experiment

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.

Luminiferous aether
"medium" of light

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.

If the line between the starting and ending points were at right angles to the current, on the other hand, then, in both directions, the swimmer would have to compensate by swimming at a slight angle to his desired goal.

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 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.

Michelson-Morley experiment
Performed in the basement of a
stone building close to sea-level

Michelson and Morley set up what has come to be known as an Michelson interferometer. The Michelson-Morley experiment was performed in the basement of a stone building close to sea-level. 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 by a mirror and 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.

To account for possible imperfections in the construction of the equipment, 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 "the most famous failed experiment of all time". 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 provided a complete explanation that did not require an aether, while consistent with the results of the experiment.

Michelson 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. 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.

External Links

• ScienceWorld Michelson-Morley Experiment
• Virginia University Michelson-Morley Experiment

Related Papers

• Michelson-Morley Experiments Revisited and the Cosmic Background Radiation Preferred Frame (arxiv.org)
• Modern Michelson-Morley experiments and gravitationally-induced anisotropy of c (arxiv.org)
• Ether-Drift Experiment and the Determination of the Absolute Motion of the Earth
• Evaluation of Brane World Mach Principles and the Michelson-Morley Experiments relation

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