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Q & A: red shift

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Most recent answer: 10/22/2007
Q:
I know that I’m asking a lot of questions for one guy but I don’t understand the principles of Red Shift. It seems very similar to the Doppler Effect so I can’t 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
A:
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 other.

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.

Mike W.

(published on 10/22/2007)

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