The Sagnac effect and the measurement of rotation

Experiments that looked for a linear ether wind found nothing. RSI accepts these null results and explains them: rulers and clocks are built of knots in the medium and adjust to uniform motion through it in exactly the proportion that hides it. Rotation is different, and it is not hidden.

In the Sagnac effect, light sent around a rotating loop takes longer in the direction of rotation than against it. The difference is 4AΩ/c², where A is the area the loop encloses and Ω its rate of rotation. RSI reads this simply: the light moves at c relative to the non-rotating medium while the apparatus turns beneath it. Georges Sagnac measured the effect in 1913, and in 1925 Michelson, Gale and Pearson detected the Earth's own rotation this way.

The effect is a difference in travel time, not a change in how fast clocks tick. Global positioning systems correct their signals for it, and ring-laser gyroscopes in aircraft navigate by it. Special relativity predicts the same formula, so the Sagnac effect does not by itself separate RSI from relativity. What RSI adds is a physical medium against which the rotation is measured.