might be expected to shift slightly. Other controls were performed to address various objections of critics of the original experiment. The net result of all of this attention to detail was the complete verification of Ives and Stilwell's 1938 results and the extension of these results to higher speeds. Mössbauer rotor experiments Relativistic Doppler effect A more precise confirmation of the relativistic Doppler effect was achieved by the Mössbauer rotor experiments. From a source in the middle of a rotating disk, gamma rays are sent to an absorber at the rim (in some variations this scheme was reversed), and a stationary counter was placed beyond the absorber. According to relativity, the characteristic resonance absorption frequency of the moving absorber at the rim should decrease due to time dilation, so the transmission of gamma rays through the absorber increases, which is subsequently measured by the stationary counter beyond the absorber. This effect was actually observed using the Mössbauer effect. The maximal deviation from time dilation was 10−5, thus the precision was much higher than that (10−2) of the Ives–Stilwell experiments. Such experiments were performed by Hay et al. (1960), Champeney et al. (1963, 1965), and Kündig (1963). Isotropy of the speed of light Mössbauer rotor experiments were also used to measure a possible anisotropy of the speed of light. That is, a possible aether wind should exert a disturbing influence on the absorption frequency. However, as in all other aether drift experiments (Michelson–Morley experiment), the result was negative, putting an upper limit to aether drift of 2.0 cm/s. Experiments of that kind were performed by Champeney & Moon (1961), Champeney et al. (1963), Turner & Hill (1964), and Preikschat supervised by Isaak (1968). Modern experiments Fast moving clocks A considerably higher precision has been achieved in modern variations of Ives–Stilwell experiments. In heavy-ion storage rings, as the TSR at the MPIK or ESR at the GSI Helmholtz Centre for Heavy Ion Research, the Doppler shift of lithium ions traveling at high speed is evaluated by using saturated spectroscopy or optical–optical double resonance. Due to their frequencies emitted, these ions can be considered as optical atomic clocks of high precision. Using the framework of Mansouri–Sexl a possible deviation from special relativity can be quantified by with as frequency of the laser beam propagating anti-parallel to the ion beam and as frequency of the laser beam propagating parallel to the ion beam. and