Fizeau's experiment was designed to determine whether the velocity of light through moving liquid media was different from that through a stationary medium, i.e. whether the motion of the liquid caused a drag on the æther, which it would do if the mechanical law of relativity held for light phenomena, for then the light ray would be in the same position as a swimmer travelling upstream or downstream respectively.[18]
[18]It is well known that it takes a swimmer longer to travel a certain distance up and down stream than to swim across the stream and back an equal distance.
No "ether-drag" was, however, detected; only a fraction of the velocity of the liquid seemed to be added to the velocity of light (
) under ordinary conditions, and this fraction depends on the refractive index of the liquid, and had previously been calculated by Fresnel: for a vacuum this fraction vanishes.
This result seemed to favour the hypothesis of a fixed ether, as was supported by Fresnel and Lorentz. But a fixed ether implies that we should be able to detect absolute motion, that is, motion with respect to the ether.
Arguing from this, let us consider an observer in the liquid moving with it. If there is a fixed ether, he should find a lesser value for the velocity of light (i.e.
) owing to his own velocity in the same direction, or vice versa in the opposite direction.
But we on the earth are in the position of the observer in the liquid since we revolve around the sun at the rate of, approximately, 30 kms. per second (i.e.