Which 1938 experiment is a classic test of the transverse Doppler effect?

The 1938 Ives–Stilwell experiment is a classic test of the transverse Doppler effect.

Herbert E. Ives and G. R. Stilwell examined the light emitted by fast-moving hydrogen atoms in canal rays. Their measurements compared Doppler-shifted spectral lines from ions moving toward and away from the observer. The large first-order, or longitudinal, Doppler shifts were arranged so that their effects could be separated from the much smaller second-order shift associated with relativistic time dilation.

Einstein predicted the relevant relativistic Doppler behavior in 1905, and in 1907 suggested testing it with moving ions. Ives and Stilwell initially considered a direct transverse observation, but their 1938 setup used longitudinal observations and extracted the transverse contribution mathematically. The experiment therefore tested the same relativistic effect without requiring a beam viewed exactly at right angles.

The experiment is often described as a direct quantitative confirmation of time dilation, although it is not identical to every modern transverse-Doppler test. It is also commonly confused with the Michelson–Morley experiment, which tested the proposed luminiferous ether, or with the Stern–Gerlach experiment, which demonstrated space quantization of angular momentum.