Fizeau experiment — experiment measuring the speed of light in moving water
The 1851 Fizeau experiment provided the first empirical measurement of light-dragging in a moving medium, later explained by Einstein's special relativity.
Key Facts
- Experimenter
- Hippolyte Fizeau
- Year conducted
- 1851
- Medium tested
- Moving water (and air as control)
- Hypothesis supported
- Partial aether-drag (Fresnel)
- Later explanation
- Einstein's special relativity (relativistic velocity addition)
Location
Cause → Event → Consequence
Prevailing 19th-century theories predicted that light passing through a moving medium would be fully dragged along by it, resulting in a simple additive combination of the medium's speed and the speed of light. Physicists sought an empirical test of this prediction, particularly in relation to the aether-drag hypothesis proposed by Augustin-Jean Fresnel.
In 1851, Hippolyte Fizeau used a precision interferometer to measure the speed of light passing through water flowing in opposite directions. He detected a dragging effect, but its magnitude was far smaller than full-drag theories predicted. A control test using air in place of water produced no measurable effect.
Fizeau's results gave partial support to Fresnel's aether-drag coefficient but left physicists without a satisfying physical explanation for over fifty years. The anomaly was eventually resolved by Albert Einstein's 1905 theory of special relativity, which showed the result corresponds directly to the relativistic velocity-addition formula applied at low velocities.