We describe a simple realization of Fizeau’s “aether-drag” experiment. Using an inexpensive setup, we measure the phase shift induced by moving water in a laser interferometer and find good agreement with the relativistic prediction or, in the terms of 19th century physics, with Fresnel’s partial-drag theory. This appealing experiment, particularly suited for an undergraduate laboratory project, not only allows a quantitative measurement of a relativistic effect on a macroscopic system but also constitutes a practical application of important concepts of optics, data acquisition and processing, and fluid mechanics.

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This linearity can be checked by using a powermeter (or a calibrated photodiode) and a set of neutral density filters to attenuate the laser beam. One then readily verifies that for a proper setting of the webcam’s gamma, the pixel counts are proportional to the light intensity on the detector.
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Corrections due to the compressibility of air are on the order of (v/vs)2, where ν is the flow velocity and νs340  m/s the speed of sound in air. For our parameters, compressibility thus amounts to an error of about 1% at most, and is completely negligible with respect to other sources of uncertainty.
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