Editing Michelson–Morley experiment (section)

=== Michelson experiment (1881) ===
[[File:Michelson1881c.png|thumb|300px|Michelson's 1881 [[interferometer]]. Although ultimately it proved incapable of distinguishing between differing theories of [[luminiferous aether|aether]]-dragging, its construction provided important lessons for the design of Michelson and Morley's 1887 instrument.<ref group=note>Among other lessons was the need to control for vibration. Michelson (1881) wrote: "...&nbsp;owing to the extreme sensitiveness of the instrument to vibrations, the work could not be carried on during the day. Next, the experiment was tried at night. When the mirrors were placed half-way on the arms the fringes were visible, but their position could not be measured till after twelve o'clock, and then only at intervals. When the mirrors were moved out to the ends of the arms, the fringes were only occasionally visible. It thus appeared that the experiments could not be performed in Berlin, and the apparatus was accordingly removed to the Astrophysicalisches Observatorium in Potsdam&nbsp;... Here, the fringes under ordinary circumstances were sufficiently quiet to measure, but so extraordinarily sensitive was the instrument that the stamping of the pavement, about 100 meters from the observatory, made the fringes disappear entirely!"</ref>]]
{{Wikisource|The Relative Motion of the Earth and the Luminiferous Ether|The Relative Motion of the Earth and the Luminiferous Ether (1881)}}
Michelson had a solution to the problem of how to construct a device sufficiently accurate to detect aether flow. In 1877, while teaching at his alma mater, the [[United States Naval Academy]] in Annapolis, Michelson conducted his first known light speed experiments as a part of a classroom demonstration. In 1881, he left active U.S. Naval service while in Germany concluding his studies. In that year, Michelson used a prototype experimental device to make several more measurements.

The device he designed, later known as a [[Michelson interferometer]], sent [[yellow]] light from a [[sodium]] flame (for alignment), or [[white]] light (for the actual observations), through a [[beam splitter|half-silvered mirror]] that was used to split it into two beams traveling at right angles to one another. After leaving the splitter, the beams traveled out to the ends of long arms where they were reflected back into the middle by small mirrors. They then recombined on the far side of the splitter in an eyepiece, producing a pattern of constructive and destructive [[Interference (wave propagation)|interference]] whose transverse displacement would depend on the relative time it takes light to transit the longitudinal ''vs.'' the transverse arms. If the Earth is traveling through an aether medium, a light beam traveling parallel to the flow of that aether will take longer to reflect back and forth than would a beam traveling perpendicular to the aether, because the increase in elapsed time from traveling against the aether wind is more than the time saved by traveling with the aether wind.  Michelson expected that the Earth's motion would produce a [[fringe shift]] equal to 0.04 fringes—that is, of the separation between areas of the same intensity. He did not observe the expected shift; the greatest average deviation that he measured (in the northwest direction) was only 0.018 fringes; most of his measurements were much less. His conclusion was that Fresnel's hypothesis of a stationary aether with partial aether dragging would have to be rejected, and thus he confirmed Stokes' hypothesis of complete aether dragging.<ref name=michel1/>

However, [[Alfred Potier]] (and later [[Hendrik Lorentz]]) pointed out to Michelson that he had made an error of calculation, and that the expected fringe shift should have been only 0.02 fringes.  Michelson's apparatus was subject to experimental errors far too large to say anything conclusive about the aether wind. Definitive measurement of the aether wind would require an experiment with greater accuracy and better controls than the original. Nevertheless, the prototype was successful in demonstrating that the basic method was feasible.<ref group=A name=Jan /><ref group=A name=AIMiller />