Measurement of the Relativistic Doppler Shift in Neon

The relativistic Doppler shift is measured by the counting of the frequency difference between two cw dye lasers. One laser is locked to a two-photon transition in a fast beam of neon, and the other is locked to the same two-photon transition in thermal neon. The experimental result is compared to the prediction of special relativity. The result is in excellent agreement with this theory. An accuracy of 4×${10}^{-5\mathrm{}}$ is obtained, which provides the most accurate direct verification of time dilatation to date.