# Dopler Shift and Photon Number

• cmos
In summary, this seems to be a reasonable explanation for why the amplitude of a light wave does not change when the wavelength is changed.f

#### cmos

Here's something that I recently thought about:

If we assume a Doppler shifted EM wave retains the amplitude of the unshifted wave, then we must be creating/annihilating photons. This seems consistent with the conservation of energy; the removal of several photons removes energy from the system but this energy is conserved by the increase in energy of the remaining photons, as would be the case of a blue shift.

Does this seam reasonable?

I realize that number conservation of photons is never a requirement, but it originally bothered me that photons can just come in and out of existence in the manner described above. My explanation for this is that during a Doppler shift, a single photon is either joining with several others or splitting into several others in a manner that conserves energy.

Does this seem reasonable?

Probably a good idea to look into coherent states and bogoliubov transformations.

The Doppler shift does not change the number of photons.
The frequency, energy and momentum of each photon changes.

The Doppler shift does not change the number of photons.
The frequency, energy and momentum of each photon changes.

If the number of photons does not change, then a change in wavelength will induce a change in amplitude. For the amplitude to remain the same, the number of photons must change - hence my original post.

lbrits, I will be looking into it...

If the number of photons does not change, then a change in wavelength will induce a change in amplitude. For the amplitude to remain the same, the number of photons must change - hence my original post.
lbrits, I will be looking into it...
Why do you assume the amplitude won't change? The E and B fields change in a Lorentz transformation.
Consider light so weak that there is only one photon.
Could a small change in frequency add or subtract a photon?