Quote:
Originally Posted by dhd40
Does that mean that the frequency shift happens already during emission? If so, how does the emitter know about the relative velocities between emitter and detector? And there are trillions of detectors with trillions of different relative velocities.
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The emitter doesn't know or care what the detectors are doing. It just does its thing, which is to emit light that has a characteristic wavelength as measured by a detector which is stationary relative to the emitter. Detectors which are moving at different velocities get different values when they measure that wavelength.