(To Moderator: Sorry for reposting. I wanted to correct a spelling error.)

" If two inertial observers exchange light signals they will observe the
same Doppler effect."

This is sometimes used in special relativity derivations. I don't doubt it
but I wondered if there has ever been an experiment to directly test this
and perhaps put an upper limit on any deviation. What experimental evidence
could I cite for it?

(I'm not talking about symmetry of Doppler broadened spectral lines. I'm
asking about the much simpler question of two observers in relative motion
exchanging light signals and measuring the Doppler shifts.)

David Park

http://home.earthlink.net/~djmp/