Redshifting Help - Understand the Doppler Effect of Light Waves

[JesseM]

Section 11 of http://arxiv.org/abs/physics/0605204" , cited in the Wikipedia article "Relativistic Doppler Shift", includes the following:

"In deriving the mass-energy relation we show, as a by-product, that the energy of radiation suffers a Doppler shift too, without having to resort to the energy-frequency relation of elementary quantum mechanics. This will allow us to introduce the “four-momentum” of a particle in a quite natural way."

Thanks for the link. It looks to me like they are saying that they can derive the difference in energy for an electromagnetic wave in two frames, not the traditional Doppler shift equation which gives the difference in frequency/wavelength in two frames...even if one chooses to call this a "Doppler shift in energy", they are deriving a relativistic formula E' = E((1+ \beta )/(1- \beta ))^{1/2}, whereas you were saying that some sort of classical Doppler shift equation could be derived from energy/momentum considerations. Also note that they specifically avoid using the quantum rules relating momentum and wavelength which you had invoked at certain points. I wasn't arguing that it was definitely impossible to find some derivation of a Doppler shift from energy considerations (I wasn't sure either way, and I'm still not sure whether one could derive the frequency/wavelength relation from energy/momentum in relativistic electromagnetism), I was just arguing that you didn't seem to have a derivation yourself so your confident statements that it was possible were unjustified, and that some of your stabs in the direction of a relation between the two were based on incorrectly combining quantum formulas with classical ones.

 

[nutgeb]

I had read Moriconi's paper long before I made my first post in this thread. Took me a while to remember the source and refresh on the details.

 

[JesseM]

I had read Moriconi's paper long before I made my first post in this thread. Took me a while to remember the source and refresh on the details.

And as I said, I think what the paper was saying was fairly different from what you were saying...you seemed to be arguing we could get the classical Doppler shift as opposed to the relativistic one from energy/momentum considerations, and you also seemed to be making use of quantum rules involving frequency/wavelength like p = h / \lambda as a basis for your statement that the momentum would be reduced by a factor of (1+v/c) in the observer frame (whereas the Moriconi discussion avoided invoking any quantum rules, and in fact had nothing to do with frequency or wavelength whatsoever). As always, if this is a misunderstanding of what you meant please clarify.

 

[nutgeb]

A relativistic formula can only be used with respect to relativistic Doppler shift.

I think we've beat this topic to death. Thanks for the thoughts about QM and relativity, which are consistent with Moriconi.

 

[JesseM]

A relativistic formula can only be used with respect to relativistic Doppler shift.

Of course. But you seemed to be saying earlier that the classical (non-relativistic) Doppler shift can be derived from energy/momentum considerations, which is not supported by the Moriconi paper. Anyway, if you're done with this topic then I'm fine letting it be.

 

[Evolver]

Of course. But you seemed to be saying earlier that the classical (non-relativistic) Doppler shift can be derived from energy/momentum considerations, which is not supported by the Moriconi paper. Anyway, if you're done with this topic then I'm fine letting it be.

Phew! That was quite the debate there. After all that, my head is spinning, do you think you could do me a favor and have another pass at it's description please? I sort of became tangled up in the discussion and lost the straight and forward description.

Thanks!

 

[sylas]

Phew! That was quite the debate there. After all that, my head is spinning, do you think you could do me a favor and have another pass at it's description please? I sort of became tangled up in the discussion and lost the straight and forward description.

Thanks!

I recommend you simply do not worry about all the back and forth there. It is all a bit beyond your original question. It is also a nutgeb exchange, and without meaning to give offense, I would suggest that a beginner is best to ignore that. Nutgeb's proposed answers are often a little bit ... unusual, and frequently unclear, and (IMO) often seem to include misconceptions or at least can lead you off in the directions where misconceptions lie. Definitely not recommended for a beginner. Some folks might take up the an exchange to try and sort out the problems and issues, but what you want is a simple answer to the original post.

At the risk of tooting my own horn, the response [post=2333168]msg #3[/post] is more what you are after; and you shouldn't really worry about the rest of the thread where Jesse and nutgeb try to figure out what precisely they are talking about.

I don't think message #3 is the best possible answer to your original post; if you get an answer from someone like George Jones, then you should ignore my posts also, and just focus on his answer.

Cheers -- sylas