The Michelson-Morley experiment
An optical experiment in which an interferometer with two perpendicular arms was meant to detect an 'ether wind' from the Earth's motion by a shift of the interference fringes.
The experiment was set up by the Americans Albert Michelson and Edward Morley; neither in 1881 nor in 1887 was a shift found. The authors concluded that the relative motion of the Earth and the ether, if any, is small.
Michelson himself continued to regard the ether as real, and the physicists George FitzGerald (1889) and Hendrik Lorentz (1892) rescued the ether with the length-contraction hypothesis.
Why this date
Michelson's first version 1881, Potsdam. exact
What it grew from
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Maxwell's electromagnetic theory of light
influenced · Confidence: high
Maxwell's remark on measuring motion relative to the ether (letter to Todd, 1879) was the immediate motive for Michelson's 1881 experiment.
Link sources
- Michelson 1881, Am. J. Sci. 22 (1881)
Clerk Maxwell pointed out that [the velocity difference] could be calculated by measuring the velocity of light by means of the eclipses of Jupiter's satellites ... but that for this purpose the observations of these eclipses must greatly exceed in accuracy those which have thus far been obtained.
- Haubold, Astronomische Nachrichten (arXiv:2111.12176) (1982) Verified 20 Sep 2026
The incitement for undertaking such a crucial experiment Michelson had found in ideas of J.C. Maxwell.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
- Michelson 1881, Am. J. Sci. 22 (1881)
What grew out of it
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Special theory of relativity ("Zur Elektrodynamik bewegter Körper")
influenced · Confidence: medium
The Michelson-Morley null result was one of the preconditions of special relativity; the degree of its influence on Einstein is disputed.
Dispute: Two positions. (1) The "textbook" version (Britannica and most textbooks): the null result led to special relativity. (2) Holton (Isis, 1969), on the basis of Einstein's letters (1942, 1952, 1954) and conversations with Shankland (1950s), showed that the experiment's role was "limited and indirect": Einstein started from general considerations (the asymmetry of Maxwell's electrodynamics, aberration, Fizeau's experiment), and the 1905 paper does not name Michelson. Against absolutizing Holton: van Dongen (2009) published the 1921 Chicago speech in which Einstein says he knew Michelson's experiment as a student; the 1922 Kyoto lecture (Ishiwara's notes) also credits the experiment with a role in abandoning the ether; in the 1907 review (Jahrbuch der Radioaktivität) Einstein already discusses the Michelson-Morley experiment explicitly as an argument. Weinstein (2012) notes that Fizeau's experiment, which Einstein called the main one, is not mentioned in the 1905 paper either, so the absence of a citation does not prove ignorance. Bottom line: there was an influence, but it cannot be called decisive.
Link sources
- Britannica «Michelson-Morley experiment» (2024) Verified 24 Sep 2026
This null result seriously discredited the ether theories and ultimately led to the proposal by Albert Einstein in 1905 that the speed of light is a universal constant.
- Einstein, 14.12.1922 (1922)
Soon I came to the conclusion that our idea about the motion of the Earth with respect to the ether is incorrect, if we admit Michelson's null result as a fact.
- Einstein, 1921 (1921) Verified 20 Sep 2026
But when I was a student, I saw that experiments of this kind had already been made, in particular by your compatriot, Michelson.
- Einstein, 1931 (1931) Verified 20 Sep 2026
It was you who led the physicists into new paths, and through your marvelous experimental work paved the way for the development of the theory of relativity.
- Einstein, F. G. Davenport 9.02.1954 (1954) Verified 20 Sep 2026
In my own development Michelson's result has not had a considerable influence. I even do not remember if I knew of it at all when I wrote my first paper on the subject (1905).
- Shankland R. S. «Conversations with Albert Einstein», Am. J. Phys. 31, 47-57 (1963) Verified 20 Sep 2026, quotation assembled from fragments
he had become aware of it through the writings of Lorentz, but only after 1905 had it come to his attention. 'Otherwise', he said, 'I would have mentioned it in my paper'. ... the experimental results which had influenced him most were stellar aberration and Fizeau's water tube experiment. 'They were enough'.
- Holton G., Isis 60(2), 133-197 (1969)
the Michelson-Morley experiment had played a limited and indirect part
- van Dongen J., Arch. Hist. Exact Sci. 63 (2009) Verified 20 Sep 2026
good reasons to accord greater credence to his earlier words, spoken in Chicago
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.
- Britannica «Michelson-Morley experiment» (2024) Verified 24 Sep 2026
Other links
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Wave optics of Young and Fresnel and the luminiferous ether
refuted by · Confidence: high
The null result refuted the specific model of Fresnel's stationary ether that the experiment was designed to test.
Dispute: The experiment refuted not "the ether as such" but Fresnel's stationary-ether hypothesis; Lorentz and FitzGerald kept the ether via contraction, and Michelson himself, after repeating Fizeau's experiment (1886), held Fresnel's theory to be correct (Wikipedia "History of special relativity": "Michelson was now of the opinion that Fresnel's stationary aether theory was correct").
Link sources
- Michelson & Morley 1887, Am. J. Sci. 34 (1887) Verified 20 Sep 2026, quotation assembled from fragments
It appears, from all that precedes, reasonably certain that if there be any relative motion between the earth and the luminiferous ether, it must be small...
- Michelson 1881 (1881) Verified 20 Sep 2026
The result of the hypothesis of a stationary ether is thus shown to be incorrect, and the necessary conclusion follows that the hypothesis is erroneous.
- Britannica «Michelson-Morley experiment» (2024) Verified 24 Sep 2026
This null result seriously discredited the ether theories
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.
- Michelson & Morley 1887, Am. J. Sci. 34 (1887) Verified 20 Sep 2026, quotation assembled from fragments
Links under review
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The FitzGerald-Lorentz contraction hypothesis
influenced · Confidence: high
The first half of this link is supported verbatim by FitzGerald's letter of 1889. Lorentz's independent arrival in 1892 comes from a summary in the Wikipedia article; a verbatim quotation is needed from Lorentz H. A., «De relatieve beweging van de aarde en den aether», 1892, or from Miller A. I., «Albert Einstein's Special Theory of Relativity», 1981.
The contraction hypothesis was put forward specifically to explain the null result.
Link sources
- FitzGerald G. F. «The Ether and the Earth's Atmosphere», Science 13 (328), 390 (1889) Verified 20 Sep 2026
I have read with much interest Messrs. Michelson and Morley's wonderfully delicate experiment attempting to decide the important question as to how far the ether is carried along by the earth. Their result seems opposed to other experiments showing that the ether in the air can be carried along only to an inappreciable extent. I would suggest that almost the only hypothesis that can reconcile this opposition is that the length of material bodies changes, according as they are moving through the ether or across it, by an amount depending on the square of the ratio of their velocity to that of light.
- Wikipedia «History of special relativity» (2024) Verified 20 Sep 2026, quotation assembled from fragments
FitzGerald (1889) offered another explanation of the negative result of the Michelson-Morley experiment ... material bodies would contract in the line of motion (length contraction). ... Also Lorentz (1892b) proposed length contraction independently from FitzGerald in order to explain the Michelson-Morley experiment.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.
- FitzGerald G. F. «The Ether and the Earth's Atmosphere», Science 13 (328), 390 (1889) Verified 20 Sep 2026
Node sources
- Michelson A. A., Morley E. W. «On the Relative Motion of the Earth and the Luminiferous Ether», Am. J. Sci. 34, 333-345 (1887) Verified 20 Sep 2026
It appears, from all that precedes, reasonably certain that if there be any relative motion between the earth and the luminiferous ether, it must be small; quite small enough entirely to refute Fresnel's explanation of aberration.
- Michelson A. A., Morley E. W. «On the Relative Motion of the Earth and the Luminiferous Ether», Am. J. Sci. 34, 333-345 (1887) Verified 20 Sep 2026
the relative velocity of the earth and the ether is probably less than one sixth the earth's orbital velocity, and certainly less than one-fourth
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
Further reading
Not evidence: these carry no quotation and support nothing we wrote. They are books and scans to read next.