Special & General Relativity Questions and Answers
Does gravity produce a Doppler shift?
Yes it does, but we do not call it a Doppler Shift because the process is a very different one that the Doppler mechanism. The ordinary Doppler effect comes about when one object emitting a signal ( siren on a fire truck) moves rapidly towards or away from you compared to the speed of sound or the speed of light. The Doppler shift is a frequency shift caused by this motion. The gravitational redshift does not involve motion at all. All that matters is the DIFFERENCE in the strength of the gravitational field where you are and where the source of radiation is located. Because clocks 'run slower' in stronger gravitational fields, the effect is that a light signal leaving the surface will gradually have its frequency ( period of oscillation) extended so that the signal arrives at a longer wavelength than it had when sent. You can also think of this as the photons loosing energy as they work their out of the strong gravitational field. As they loose energy, their frequency must decrease because their speed must always remain that of light.
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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.