Fizeau experiment
The Fizeau experiment was an 1851 interferometric test of how flowing water affects the propagation of light. Hippolyte Fizeau sent light in opposite directions through moving water and measured a shift in the interference fringes. The result was inconsistent with simple classical addition of the water's speed to light's speed and agreed approximately with Augustin Fresnel's partial-drag prediction. In 1907, Max von Laue showed how the result follows from relativistic velocity transformation, making the experiment an important precursor to experimental tests of special relativity.
Experimental question and arrangement
In nineteenth-century physics, competing models made different predictions about light in moving transparent matter. In 1851, French physicist Hippolyte Fizeau devised a sensitive test using water flowing through tubes. Light was split into beams that traveled in opposite directions along paths through the moving water, then recombined to produce interference fringes. Reversing or changing the water flow altered the relative travel times and shifted the fringes. This differential measurement could detect a small effect that would have been difficult to measure by timing a single light pulse.
What Fizeau observed
Fizeau found that the moving water affected the interference pattern, but not as much as would be expected if the water simply carried light along at its full speed. The shift was broadly consistent with Fresnel's earlier proposal that a moving transparent medium only partially dragged the luminiferous ether, with a coefficient depending on the medium's refractive index. The experiment therefore challenged simple models of complete ether drag and gave an important quantitative result for the physics of moving media. The historic interpretation invoked ether; that interpretation is not part of modern physics.
Later explanation and replication
After special relativity was published in 1905, Max von Laue derived Fresnel's drag coefficient from relativistic velocity addition in 1907, without requiring a mechanically dragged ether. Michelson and Morley repeated Fizeau's experiment in 1886 and confirmed the original finding. Modern laboratory versions use laser interferometers and moving water; a 2012 undergraduate-laboratory study reported measurements in good agreement with the relativistic prediction, while discussing experimental uncertainties such as flow calibration and velocity profiles.
Scientific significance
The experiment did not directly measure the speed of light in a vacuum or independently establish every part of special relativity. Its significance lies in testing how light propagation combines with the motion of a material medium. The observed partial-drag effect is explained in modern terms by relativistic kinematics; when dispersion is relevant, the refractive index's dependence on frequency also affects the detailed prediction. Fizeau's method remains a clear example of how an interferometer can reveal small changes in optical path length.
Key facts
- Year
- Fizeau performed the moving-water experiment in 1851.
- Method
- Counter-propagating light beams traversed moving water and were recombined to measure a shift in interference fringes.
- Finding
- The fringe shift was smaller than the prediction from simple full addition of the water velocity to the light velocity and was broadly consistent with Fresnel's partial-drag coefficient.
- Modern account
- In 1907, Max von Laue derived Fresnel's coefficient using relativistic velocity addition.
- Replication
- Michelson and Morley repeated the experiment in 1886; later laboratory demonstrations have used laser interferometers.
Timeline
1818
Augustin Fresnel proposed a partial-drag coefficient for light in a moving transparent medium.
1851
Hippolyte Fizeau reported his moving-water interferometer experiment.
1886
Albert A. Michelson and Edward W. Morley repeated the moving-medium experiment.
1905
Albert Einstein published the special theory of relativity.
1907
Max von Laue derived Fresnel's drag coefficient from relativistic velocity addition.
2012
Thierry Lahaye, Pierre Labastie and Renaud Mathevet reported a laser-based laboratory realization of the experiment.
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References
- 1.Fizeau’s “aether-drag” experiment in the undergraduate laboratory
American Journal of Physics; arXiv · other · reliability: unverified · retrieved 26 October 2026
Supports: Peer-reviewed instructional paper describing the experiment's history, relativistic prediction, apparatus and modern laboratory measurements.
- 2.Speed of Light Experiment
American Physical Society (APS News) · other · reliability: unverified · retrieved 26 October 2026
Supports: Historical overview of Fizeau's experiments and the later interpretation of light propagation through moving liquids.
- 3.Armand-Hippolyte-Louis Fizeau
Encyclopaedia Britannica · other · reliability: unverified · retrieved 26 October 2026
Supports: Biographical overview noting Fizeau's 1851 ether-detection experiments and their role in the later abandonment of ether theory.
- 4.The Fizeau Experiment: Experimental Investigations of the Relativistic Doppler Effect
The African Review of Physics; Cornell University · other · reliability: unverified · retrieved 26 October 2026
Supports: Describes the original experiment, Michelson–Morley and Zeeman follow-up, and a modern reproduction; its alternative interpretation is not adopted here.
References are suggested by the research engine and may be incomplete. Unverified URLs are omitted rather than guessed.
Article history
- Version 126 October 2026 · EditorCreated by Zapia