When the source of a sound wave is moving in the direction of the sound wave it is a different scenario than if the receiver of the sound wave is moving in the direction where the sound waves are coming from. Why is this not the case in the relativistic Doppler effect, why is it unimportant whether the source or the receiver is moving?
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$\begingroup$ Your first sentence...those are different scenarios? $\endgroup$– DKNguyenCommented Aug 17, 2020 at 21:05
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$\begingroup$ Yes those are indeed different scenarios $\endgroup$– Merkur WeimerCommented Aug 18, 2020 at 19:36
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For sound, when we speak of the observer moving or the source moving we mean moving with respect to the medium through which, and by means of which, the sound travels, so the two cases can be distinguished. The relativistic Doppler effect is for electromagnetic waves, which don't use a medium. So the only source or observer velocity that can be relevant is the relative velocity between source and observer.
[The speed of light is the same in the source's frame of reference (that is the frame in which the source is stationary) as in the observer's frame.]
answered Aug 17, 2020 at 20:10