frequency standard (not useful for ordinary timekeeping) based on the extremely sharp frequency of the gamma (gamma ray) emission (electromagnetic radiation arising from radioactive decay) and absorption in certain atomic nuclei, such as iron-57, that exhibit the Mössbauer effect. The aggregate of atoms that emit the gamma radiation of precise frequency may be called the emitter clock; the group of atoms that absorb this radiation is the absorber clock. The two clocks remain tuned, or synchronous, only as long as the intrinsic frequency of the individual pulses of gamma radiation (photons) emitted remains the same as that which can be absorbed. A slight motion of the emitter clock relative to the absorber clock produces enough frequency shift to destroy resonance or detune the pair, so absorption cannot occur. This allows for a thorough study at very low velocities of the Doppler effect (the change in the observed frequency of a vibration because of relative motion between the observer and the source of the vibration). Gamma photons from an emitter placed several stories above an absorber show a slight increase in energy, the gravitational shift toward shorter wavelength and higher frequency predicted by general relativity theory. Some pairs of these nuclear clocks can detect energy changes of one part in 10¹⁴, being about 1,000 times more sensitive than the best atomic clock.