However, a few scientists rejected the second basic postulate of relativity: the constancy of the speed of light in all inertial frames.
All of them can explain the negative outcome of the Michelson–Morley experiment, since the speed of light is constant with respect to the interferometer in all frames of reference.
Numerous terrestrial experiments have been performed, over very short distances, where no "light dragging" or extinction effects could come into play, and again the results confirm that light speed is independent of the speed of the source, conclusively ruling out emission theories.
However, similar observations have been made more recently in the x-ray spectrum by Brecher (1977), which have a long enough extinction distance that it should not affect the results.
[2] Hans Thirring argued in 1924, that an atom which is accelerated during the emission process by thermal collisions in the sun, is emitting light rays having different velocities at their start- and endpoints.
[15] Such experiments include that of Sadeh (1963) who used a time-of-flight technique to measure velocity differences of photons traveling in opposite direction, which were produced by positron annihilation.
[16] Another experiment was conducted by Alväger et al. (1963), who compared the time of flight of gamma rays from moving and resting sources.
Data collected from various detector-target distances were consistent with there being no dependence of the speed of light on the velocity of the source, and were inconsistent with modeled behavior assuming c ± v both with and without extinction.
Continuing their previous investigations, Alväger et al. (1964) observed π0-mesons which decay into photons at 99.9% light speed.
The experiment showed that the photons didn't attain the velocity of their sources and still traveled at the speed of light, with
The investigation of the media which were crossed by the photons showed that the extinction shift was not sufficient to distort the result significantly.
[20][21] The predictions of Ritz's version of emission theory were consistent with almost all terrestrial interferometric tests save those involving the propagation of light in moving media, and Ritz did not consider the difficulties presented by tests such as the Fizeau experiment to be insurmountable.
Tolman, however, noted that a Michelson–Morley experiment using an extraterrestrial light source could provide a decisive test of the Ritz hypothesis.
[22] Babcock and Bergman (1964) placed rotating glass plates between the mirrors of a common-path interferometer set up in a static Sagnac configuration.
[24] Majorana analyzed the light from moving sources and mirrors using an unequal arm Michelson interferometer that was extremely sensitive to wavelength changes.
[25][26] Beckmann and Mandics (1965)[27] repeated the Michelson (1913) and Majorana (1918) moving mirror experiments in high vacuum, finding k to be less than 0.09.