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For waves which do not require a medium, such as light or gravity in special relativity, only the relative difference in velocity between the observer and the source needs to be considered.

I am perplexed as to how one can have the notion of whether the source or the receiver is moving. If there is no third body to relate to, then whether the source or the emitter is moving should, as far as I understand relativity, be indistinguishable.

...sorry, you cannot view external links. To see them, please REGISTER or LOGINIn it it discusses the two scenarios: 1)either the source is moving (towards/away from) the receiver, and the receiver is stationary, or 2)the receiver is moving (towards/away from) the stationary source.So, I wonder, what if both are moving (again, w.r.t. the ground)? What's the appropriate formula to compute the shift then?

another_anyone gives an illustrative example in which a tiny Hz shift results from a 1GHz transmitted signal. Which is what I'm wondering: Does today's technology able to discern such a tiny difference in frequency, given all the interference and noise in the radio environment? How accurate are the commercial products using the Doppler shift? Anybody with any industry specs?

thanks. I re-read the Wikipedia post and realized that indeed, when the emitted wave is electromagnetic, it doesn't matter whether the source is moving and the receiver stationary, or vice versa, or both are moving. It's only when the emitted wave is, e.g., sound, would there be some differences in calculating the Doppler shift that has to involve specify the mobility/stationarity of the receiver and the source. Am I right?

The presentation of the Wikipedia post (the General and Analysis sections) was a little misleading because the Analysis section appears to treat only sound waves, though, and I thought it was for both sound and electromagnetic... Also, I saw that post claims that "These can be generalized into a single equation with both the source and receiver moving," which is what I've been wondering (although admittedly, I was thinking about electromagnetic waves). But perhaps someone can enlighten me on how this generalization can be formulated?

I think you should take the product of the two equations:f = f_{0}(1 - v_{0}/v)*[1 - v_{s}/(v + v_{s})]v = sound's speed relative to the mediumv_{s} = source's speedv_{0} = observer's speed.For electromagnetic waves, instead:f' = f*Sqrt[(c+v)/(c-v)] in the case of source approaching the observer (or viceversa)f' = f*Sqrt[(c-v)/(c+v)] in the case of source going away of the observer (or viceversa)f = light's frequency when the observer is stationary with respect to the sourcef' = new light's frequency seen when they move with respect to each otherv = relative speedc = light's speed.(I imagine you could understand the reason for those two colours I gave to the 2 equations) []

Quote from: lightarrow on 09/08/2007 15:48:45I think you should take the product of the two equations:f = f_{0}(1 - v_{0}/v)*[1 - v_{s}/(v + v_{s})]v = sound's speed relative to the mediumv_{s} = source's speedv_{0} = observer's speed.For electromagnetic waves, instead:f' = f*Sqrt[(c+v)/(c-v)] in the case of source approaching the observer (or viceversa)f' = f*Sqrt[(c-v)/(c+v)] in the case of source going away of the observer (or viceversa)f = light's frequency when the observer is stationary with respect to the sourcef' = new light's frequency seen when they move with respect to each otherv = relative speedc = light's speed.(I imagine you could understand the reason for those two colours I gave to the 2 equations) [] I am sorry, I cannot see how this can be right?If v is the speed relative to the medium, and the medium is light, then no matter what speed the observer and source are travelling to each other, the speed relative to the speed of light will always be the same, it will be the speed of light (i.e. light travels at the same speed for all observers).

Actually, lightarrow, your color-coding the two equations inspires me to ask another question on astronomy (or is it cosmology?), something I'm not an expert of. From our observation of the light we know the universe is expanding (due to the so-called redshift). Presumably, the light that we've observed was originally emitted some billions of years ago, right? If that's the case, we can claim that billions of years ago, the universe was in an expansion mode. How can we say it's still expanding today? Why could it not be contracting mode now?If, on the other hand, the light that we observed and used to determine the universe is expanding emitted only a few hundred years ago, that implies that the source of the light is only a few hundred lightyears away from Earth, which is quite a local area on the cosmological scale. Then we could only claim that this area of the universe is expanding currently. How then can we apply this claim to the universe as a whole?I know I must have missed something here. I would have moved this question to the Astronomy section if I knew how. So, I appreciate your answer or anyone else's. And feel free to treat me as an astronomy novice and explain in layman's terms.