text: The relative energy shift in the Kundig experiment is proportional to this quantity squared for theabsorber.
full_question: The relative energy shift in the Kundig experiment is proportional to this quantity squared for theabsorber. This quantity was sinusoidally varied by a loudspeaker for the radiation source in the PoundRebka experiment. For a galaxy, the total value for this quantity equals its "peculiar" form plus H-nought times distance. This quantity multiplies the cosine of the angle between the emitter and receiver in a (*) transverse effect predicted by special relativity. The change in frequency equals the emitted frequency times a ratio of this quantity. If the radial form of this quantity is increasing, then a redshift is seen. For 10 points, name this quantity which causes a Doppler shift if it's different for the source and emitter, and equals the product of wavelength and frequency of a wave.
first_sentence: The relative energy shift in the Kundig experiment is proportional to this quantity squared for theabsorber.
raw_answer:  wave velocity [or source velocity; or radial velocity; or speed] 