# Is the fine structure constant the phase change when a photon is emitted?

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Feynman beautifully explained how QED works.

Reading his book, it seems that the fine structure constant $\alpha$ can be seen as that change of phase angle of an electron that occurs whenever a photon is emitted.

Is this correct? Where can I read more about it?

asked Dec 13, 2019

It is not correct. Alpha is just a number, but the probability of emitting a photon depends also on some other dimensionless variable combinations, say, on $\hbar\omega/mc^2$ or so; thus $\alpha$ is not a clue for understanding QED.

Vladimir, I have not asked about or even mentioned the probability of emitting photons. I also have not talked about understanding QED. Why is the sentence "not correct"? What other, corrected sentence is correct then?

When a real photon is emitted, the electron changes its momentum. This change is a quantitative thing. The momentum is involved in the "pases" of the electron wave function: $\psi(x,t)\propto\text{e}^{ipx}$. If the final momentum is $p'$, the electron phase has changed, but it is not solely $\alpha$ who determines $\Delta p=p'-p$.

Emission of photon occurs at a single instant of time. The change in momentum has no effect on the phase during one instant!

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