Red Shift Energy

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I find it difficult to conceptualize red shift when thinking of light as a stream of photons. In thinking of it as a wave phenomena, I can see it as a matter of a given energy concentration/area. But why should a photon lose energy as a result of the velocity of the emitter?
 

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  • #2
phinds
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I find it difficult to conceptualize red shift when thinking of light as a stream of photons ...
That's good because light is not a stream of photons. Photons are things you get when an electromagnetic wave interacts with atoms, or to say it another way, photons are excitations of the electromagnetic field
 
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I find it difficult to conceptualize red shift when thinking of light as a stream of photons. In thinking of it as a wave phenomena, I can see it as a matter of a given energy concentration/area. But why should a photon lose energy as a result of the velocity of the emitter?
The EM field carries momentum k=λ for a mode with wavelength λ. As with bodies with mass the observed momentum depends on relative velocity and light obeys this so that a different wavelength is observed for different relative velocities.

As @phinds has said you don't need to think of photons - just a field that has momentum but is massless.
 
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But why should a photon lose energy as a result of the velocity of the emitter?
Because energy of photon is proportional to its momentum, and momentum depends on the velocity of the frame of reference.
 
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That's good because light is not a stream of photons.
That's interesting, as Feynman said in QED that ".....every phenomenon about light that has been observed in detail can be explained by the theory of quantum electrodynamics." And later he explained that the reverse was not true. That wave theory could not explain some phenomenon that QED could. So what would be the basis for believing that light is not a stream of photon 'particles'? And why wouldn't red shift be explainable by QED?
 
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That's interesting, as Feynman said in QED that ".....every phenomenon about light that has been observed in detail can be explained by the theory of quantum electrodynamics."
Yes, but..
So what would be the basis for believing that light is not a stream of photon 'particles'?
That's not what QED says, at least not if you're understanding "stream of photon particles" to be something like the way that a flowing river is a stream of water molecules. We have many threads that try to explain the relationship between photons and classical electromagnetic radiation - this one would be a good start https://www.physicsforums.com/threads/what-is-a-photon.879128/#post-5522356.
 

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