Editing Michelson–Morley experiment (section)

=== Michelson–Morley experiment (1887) ===
{{Wikisource|On the Relative Motion of the Earth and the Luminiferous Ether|On the Relative Motion of the Earth and the Luminiferous Ether (1887)}}
[[Image:On the Relative Motion of the Earth and the Luminiferous Ether - Fig 4.png|300px|thumb|right|This diagram illustrates the folded light path used in the Michelson–Morley interferometer that enabled a path length of 11 m. ''a'' is the light source, an [[oil lamp]]. ''b'' is a [[beam splitter]]. ''c'' is a compensating plate so that both the reflected and transmitted beams travel through the same amount of glass (important since experiments were run with white light which has an extremely short [[coherence length]] requiring precise matching of optical path lengths for [[interference fringe|fringes]] to be visible; monochromatic sodium light was used only for initial alignment<ref name=michel1/><ref group=note>Michelson (1881) wrote: "...&nbsp;a sodium flame placed at ''a'' produced at once the interference bands. These could then be altered in width, position, or direction, by a slight movement of the plate ''b'', and when they were of convenient width and of maximum sharpness, the sodium flame was removed and the lamp again substituted. The screw ''m'' was then slowly turned till the bands reappeared. They were then of course colored, except the central band, which was nearly black."</ref>). ''d'', ''d' '' and ''e'' are mirrors. ''e' '' is a fine adjustment mirror. ''f'' is a [[telescope]].]]

In 1885, Michelson began a collaboration with [[Edward Morley]], spending considerable time and money to confirm with higher accuracy [[Fizeau experiment|Fizeau's 1851 experiment]] on Fresnel's drag coefficient,<ref name=michel1a /> to improve on Michelson's 1881 experiment,<ref name=michel2 /> and to establish the wavelength of light as a [[Length measurement|standard of length]].<ref name=michel3 /><ref name=michel4 /> John Brashear made the high-quality optics for the Interferometer in his [[Allegheny Observatory|Allegheny-Observatory]]-affiliated shop. At this time Michelson was professor of physics at the Case School of Applied Science, and Morley was professor of chemistry at Western Reserve University (WRU), which shared a campus with the Case School on the eastern edge of Cleveland. Michelson suffered a mental health crisis in September 1885, from which he recovered by October 1885. Morley ascribed this breakdown to the intense work of Michelson during the preparation of the experiments. In 1886, Michelson and Morley successfully confirmed Fresnel's drag coefficient – this result was also considered as a confirmation of the stationary aether concept.<ref group=A name=staley />

This result strengthened their hope of finding the aether wind. Michelson and Morley created an improved version of the Michelson experiment with more than enough accuracy to detect this hypothetical effect. The experiment was performed in several periods of concentrated observations between April and July 1887, in the basement of Adelbert Dormitory of WRU (later renamed Pierce Hall, demolished in 1962).<ref group=A name=Fickinger /><ref group=A name=hamerla />

As shown in the diagram to the right, the light was repeatedly reflected back and forth along the arms of the interferometer, increasing the path length to {{Convert|11|m|abbr = on}}. At this length, the drift would be about 0.4 fringes. To make that easily detectable, the apparatus was assembled in a closed room in the basement of the heavy stone dormitory, eliminating most thermal and vibrational effects. Vibrations were further reduced by building the apparatus on top of a large block of sandstone (Fig.&nbsp;1), about a foot thick and {{convert|5|ft|spell=in}} square, which was then floated in a circular trough of mercury. They estimated that effects of about 0.01 fringe would be detectable.

[[File:MichelsonCoinAirLumiereBlanche.JPG|thumb|[[Interference fringe|Fringe pattern]] produced with a Michelson interferometer using [[white light]]. As configured here, the central fringe is white rather than black.]]
Michelson and Morley and other early experimentalists using interferometric techniques in an attempt to measure the properties of the luminiferous aether, used (partially) monochromatic light only for initially setting up their equipment, always switching to white light for the actual measurements. The reason is that measurements were recorded visually. Purely monochromatic light would result in a uniform fringe pattern. Lacking modern means of [[air conditioning|environmental temperature control]], experimentalists struggled with continual fringe drift even when the interferometer was set up in a basement. Because the fringes would occasionally disappear due to vibrations caused by passing horse traffic, distant thunderstorms and the like, an observer could easily "get lost" when the fringes returned to visibility. The advantages of white light, which produced a distinctive colored fringe pattern, far outweighed the difficulties of aligning the apparatus due to its low [[coherence length]]. As [[Dayton Miller]] wrote, "White light fringes were chosen for the observations because they consist of a small group of fringes having a central, sharply defined black fringe which forms a permanent zero reference mark for all readings."<ref group=A name=Miller1933/><ref group=note>If one uses a half-silvered mirror as the beam splitter, the reflected beam will undergo a different number of front-surface reflections than the transmitted beam. At each front-surface reflection, the light will undergo a phase inversion. Because the two beams undergo a different number of phase inversions, when the path lengths of the two beams match or differ by an integral number of wavelengths (e.g. 0, 1, 2&nbsp;...), there will be destructive interference and a weak signal at the detector. If the path lengths of the beams differ by a half-integral number of wavelengths (e.g., 0.5, 1.5, 2.5&nbsp;...), constructive interference will yield a strong signal. The results are opposite if a cube beam-splitter is used, because a cube beam-splitter makes no distinction between a front- and rear-surface reflection.</ref> Use of partially monochromatic light (yellow sodium light) during initial alignment enabled the researchers to locate the position of equal path length, more or less easily, before switching to white light.<ref group=note>Sodium light produces a fringe pattern that displays cycles of fuzziness and sharpness that repeat every several hundred fringes over a distance of approximately a millimeter. This pattern is due to the yellow sodium D line being actually a doublet, the individual lines of which have a limited [[coherence length]]. After aligning the interferometer to display the centermost portion of the sharpest set of fringes, the researcher would switch to white light.</ref>

The mercury trough allowed the device to turn with close to zero friction, so that once having given the sandstone block a single push it would slowly rotate through the entire range of possible angles to the "aether wind", while measurements were continuously observed by looking through the eyepiece. The hypothesis of aether drift implies that because one of the arms would inevitably turn into the direction of the wind at the same time that another arm was turning perpendicularly to the wind, an effect should be noticeable even over a period of minutes.

The expectation was that the effect would be graphable as a sine wave with two peaks and two troughs per rotation of the device. This result could have been expected because during each full rotation, each arm would be parallel to the wind twice (facing into and away from the wind giving identical readings) and perpendicular to the wind twice. Additionally, due to the Earth's rotation, the wind would be expected to show periodic changes in direction and magnitude during the course of a [[sidereal day]].

Because of the motion of the Earth around the Sun, the measured data were also expected to show annual variations.