I was looking for an intuitive explanation as to why the Doppler effect happens. I haven't found any, but this is what I thought:

-Waves emitted travel at a constant speed

-The source emits a wave

-If the source remained still, then in a given time, there would a large distance between the emitted wavefront and the stationary source

-But if the source starts moving, the speed of the source relative to the wavefront is higher.

-This means in a given time, the distance between the emitted wavefront and the source will be lower

-So when the source (while moving) emits a new wavefront, the distance between wavefronts will also be lower

Is this explanation wrong? It suggests that the Doppler effect with light will be barely noticeable, because even if a source starts moving, because the speed of light is so high, the source speed will still be small relatively, so there will be a small (basically insignificant) decrease in distance between wavefronts.

So how can red-shift and blue-shift be so noticeable, unless recession speed is extremely high (so high that it is actually significant relative to the speed of light)?


You are correct, that is where the Doppler effect comes from, and indeed the speed of the waves relative to the speed of the source or detector is very important.

I would simplify your breakdown slightly. The number of wavefronts that have reached the detector is equal to the number of wavefronts emitted, minus the number that are still in-transit.

If the distance between the two things is increasing the number of wavefronts that are in-transit is increasing, so fewer are arriving than are being emitted.

"So how can red-shift and blue-shift be so noticeable, unless recession speed is extremely high (so high that it is actually significant relative to the speed of light)?".

The doppler effect with light is NOT really noticeable at all. Cars, trains and other everyday objects never appear to change colour, for the exact reason you state - they are basically not moving at all relative to the speed of light. In order for the redshift/blue shift of light to be significant incredible speeds are needed (which sometimes arise in cosmological problems). Often the redshift is detected even on things going relatively slowly (stars moving) but this is because the instruments are very sensitive to tiny changes in the stars colour.

  • $\begingroup$ so how would you address the red-shift question? $\endgroup$ – XXb8 Jul 15 '20 at 14:07
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    $\begingroup$ @XXb8. Edited to include that. Hope it helps. $\endgroup$ – Dast Jul 16 '20 at 10:50
  • $\begingroup$ would it not also be because of the cosmological Doppler effect? $\endgroup$ – XXb8 Jul 17 '20 at 13:31
  • $\begingroup$ The Cosmological Doppler effect, from what I understand, comes from space getting bigger. So the distance between you and some far-off galaxy is growing (which red-shifts for the same reason moving away from each other would). I am afraid I don't myself understand the difference between the space between them expanding and the two objects just being in relative motion away from one another. $\endgroup$ – Dast Jul 21 '20 at 9:52

You are correct that for slow-moving objects this effect is small. But for 'fast'-moving objects that are moving at a significant fraction of light speed, remember that objects emitting light with mass can only approach it, and create a lensing effect with directional 'curvature'.


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