Fizeau and Airy Experiments

Light can also propagate through transparent dielectric media, such as air and water, but does so at the reduced phase velocity, $c/n$, where $c$ is the speed of light in vacuum, and $n$ is the medium's refractive index. Prior to the mid-nineteenth century, it was suppose that the aether was entrained by a moving medium, so that if $v$ is the speed of the medium then the phase velocity of light is

$$u_+ = \frac{c}{n}+ v$$ (3.55)

when it propagates in the same direction as the medium, and

$$u_-=\frac{c}{n}-v$$ (3.56)

when it propagates in the opposite direction [c.f. Equations (3.4) and (3.5).] However, in 1951, Hippolyte Fizeau measured the speed of light in moving water, and found that

$$u_+ = \frac{c}{n}+ v\left(1-\frac{1}{n^2}\right),$$ (3.57)

$$u_- = \frac{c}{n}-v\left(1-\frac{1}{n^2}\right).$$ (3.58)

He concluded that the aether is only partially dragged by a moving medium. In particular, air, which has a refractive index of 1.0003, hardly drags the aether at all. This is a good thing because a strong aether drag through air would contradict Bradley's explanation of stellar aberration. On the other hand, water, which has a refractive index of 1.33, drags the aether by $43\%$ of its velocity. Unfortunately, in 1871, George Airy demonstrated that measured stellar aberration is the same when the observing telescope is filled with water as when it is filled with air. This completely contradicts the partial aether drag hypothesis.