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I know that Im asking a lot of questions for one guy but I dont understand the principles of Red Shift. It seems very similar to the Doppler Effect so I cant understand how physicists say it can work. Light moves completely differently to sound so how can the same effect occur in both?
- Muhammed (age 17)
Sir George Monoux College, London
At least for standard small red shifts, you're right that you can think
of the red shift as a Doppler shift just like for sound. It doesn't
matter that light and sound are different types of waves. If you're
rushing towards the source, you will encounter the wave fronts more
often (at higher frequency) than if you and the source are at a fixed
distance. The same happens if the source rushes toward you. Of course
the opposite happens if you and the source are moving away from each
For big speeds, of course it does matter that light and sound are
different. For sound waves, there's a medium (say air) and it matters
how you and the source are moving through it, not just your relative
motion. For light, there's no medium so only relative motion matters,
and one needs to use Special Relativity to calculate the red shift. But
so long as the speeds involved are small compared to the speed of the
wave, the light and sound Doppler shifts look basically the same.
(republished on 07/29/06)
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