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Based on the Wikipedia article for special relativity:

enter image description here

It shows how light is red or blue shifted based on whether the observer is moving or the source is moving.

I'm having a hard time understanding how this doesn't contradict relative motion. We can determine relative motion (toward or away from us) using spectral analysis of stars. But in this case, we can only say there is motion between us, not which one is 'actually' moving. But in transverse Doppler it appears that we can determine who is moving, us or the star.

What am I missing?

John Rennie
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HardlyCurious
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  • Related : Deriving relativistic Doppler shift in terms of wavelength. – Frobenius Dec 12 '21 at 16:10
  • This is a very common misunderstanding about SR. You can always choose one frame to be stationary, usually you need to do this to get the simplest solution. Its state of motion just doesn't have any physical significance. The rule is "all frames are equally valid", not "there are no stationary frames". Once you have picked a stationary frame, all other frames are moving by definition! – m4r35n357 Dec 12 '21 at 16:44
  • The transverse Doppler effect is due to time dilation i.e. an observer sees a moving clock going slower. This means the observer will always see a redshift. The illustration from the Wikipedia article is misleading. An observer is by definition always at rest with regard to himself and the star is moving. Case a) is not what an observer ever would see. – Thomas Dec 12 '21 at 18:00
  • So, in case A, observer is experiencing slower time than the source because the observer is moving relative to the source, and in case b the observer is experiencing faster time because the source is moving. Is that correct? – HardlyCurious Dec 12 '21 at 18:44
  • Duplicate of Relativistic Doppler Effect between rotating bodies. I'll write an answer to that one. – benrg Dec 12 '21 at 19:04
  • First you need to know about this: https://physics.stackexchange.com/questions/682043/why-does-going-close-to-the-speed-of-light-make-almost-everything-look-like-its – stuffu Dec 12 '21 at 21:15

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