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The '''Michelson interferometer''' is an optical instrument that splits a single beam of light into two paths, sends them along separate arms to mirrors, recombines them, and reads the resulting interference fringes. Because the fringe pattern responds to differences in the ''optical path'' travelled by the two beams, the instrument converts a change far smaller than a wavelength of light into a visible shift — which is what has made it, for more than a century, the most sensitive general-purpose measuring device in physics.

==How it works==

A half-silvered beam splitter divides the incoming light. One part travels to a mirror at the end of arm 1 and returns; the other travels to a mirror at the end of arm 2 and returns. The two returning beams recombine at the splitter and are observed together. If the two round trips take exactly the same time the beams arrive in phase; any difference produces a proportional displacement of the fringes. Rotating the whole apparatus, changing an arm length, changing the medium in an arm, or changing the speed of the instrument through whatever the light propagates in will all, on one theory or another, alter the fringe position.

[[Albert A. Michelson]] built the first version in 1881 in Potsdam and, with [[Edward Morley]], a far more sensitive one mounted on a stone slab floating in mercury for the 1887 [[Michelson–Morley experiment|Michelson–Morley experiment]] at the Case School of Applied Science in Cleveland. That instrument had an effective arm length of about 11 metres, achieved by multiple reflections, and was expected to show a fringe shift of roughly 0.4 of a fringe as it was turned.

The design has outlived the question it was built to answer. Fourier-transform infrared spectrometers are Michelson interferometers with one moving mirror. Michelson himself used the principle to measure stellar diameters — the first was Betelgeuse in 1920 — and to define the metre in terms of the wavelength of cadmium light. The LIGO and Virgo gravitational-wave detectors are Michelson interferometers with kilometre-scale arms and Fabry–Pérot cavities; the first detection, GW150914, was made on 14 September 2015. The related '''Michelson–Gale–Pearson''' ring interferometer of 1925 detected the Earth's rotation as a fringe shift, a [[Sagnac Effect|Sagnac]]-type result.

==On this wiki==

The instrument itself, rather than the 1887 result, is a recurring subject here. Discussion of the 1887 experiment and its interpretation is gathered at [[Michelson–Morley experiment|the Michelson–Morley experiment]] and in the "On this wiki" section of [[Albert A. Michelson]]; what follows concerns the apparatus and its later descendants.

'''Gas-mode versus vacuum-mode.''' The single most consequential technical claim made by researchers here is [[Reginald T Cahill]]'s: that a Michelson interferometer operated with air or another gas in the light paths and one operated in vacuum are ''not the same instrument''. On his analysis the fringe sensitivity to absolute motion carries a factor depending on the refractive index of the medium, so that it vanishes as ''n'' approaches 1. If that is right, the historical gas-mode instruments of Michelson, Morley, [[Dayton C Miller]] and Illingworth could in principle register a signal that a modern evacuated resonator cannot, and the two classes of experiment are not in contradiction. See "[[Michelson-Morley Experiments Revisited and the Cosmic Background Radiation Preferred Frame]]" and "[[The Detection of Absolute Motion: From 1887-2005]]". This is a testable structural claim about the instrument, and it is the reason the gas/vacuum distinction is insisted on so often in this collection.

'''Instrument-level reanalysis.''' [[Wolfgang Engelhardt]] examines what an interferometer actually registers in "[[Phase and Frequency Shift in a Michelson Interferometer]]". [[Hector A Munera]] built a ''stationary'' Michelson interferometer near the equator in Bogotá — mounted so that the Earth's own rotation, rather than a turntable, carries the arms through their orientations — and reported correlations between fringe shift and Earth motion in "[[Observation of Highly Significant Correlations Between Earth Motion and Fringe-Shifts in a Stationary Michelson-Morley Experiment During the Period 2003-2005]]"; his reanalysis of the historical runs is "[[Michelson-Morley Experiments Revisited: Systematic Errors, Consistency Among Different Experiments, and Compatibility with Absolute Space]]". Alternative geometries appear too — see "[[A Rotating "Mach-Zehnder" Interferometer for Measuring the Earth's Absolute Velocity]]", which uses a Mach–Zehnder rather than a Michelson layout, and proposals such as "[[Proposal for a New Michelson-Morley Experiment Using a Single Whispering Spherical Mode Resonator]]".

'''Classical fringe calculations.''' Several contributors here argue that the expected fringe shift was mis-derived rather than mis-measured: [[Victor Nikolayevich Cochetkov]], "[[Explanation of the Results of the Michelson Experiments Using Classical Mechanics]]", and [[Randy Reukauf]], "[[A Particle Explanation of the Michelson-Morley and Kennedy-Thorndike Experiments]]".

It should be recorded plainly that modern interferometric isotropy tests using cryogenic optical resonators report null results at the level of parts in 10<sup>18</sup>, and that these are the strongest constraints in existence. Whether they test the same quantity as the gas-mode instruments is precisely what is in dispute here, and readers should follow Cahill's argument and its critics rather than take either side on assertion.

==See also==

* [[Michelson–Morley experiment]]
* [[Kennedy-Thorndike experiment]]
* [[Albert A. Michelson]]
* [[Edward Morley]]
* [[Sagnac Effect]]
* [[Speed of Light]]

[[Category:Light]]
[[Category:Aether]]
[[Category:Relativity]]

Michelson interferometer - Revision history

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