I recall that whenever an electron in a higher-than-normal orbit falls back to its normal orbit and produces photon. So shouldn't the photon accelerate from 0?
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Qmechanic
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1What does "should" mean? – WillO Sep 30 '22 at 18:36
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1Even on a pre-quantum intuition whereby the photon's energy continuously increases over a brief period, the relativistic energy-momentum relation for massless particles predicts they'd always be at speed $c$, and their energy and momentum would grow viz. the proportionality relation $E=pc$. – J.G. Sep 30 '22 at 18:49
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2Does this answer your question? Does a photon instantaneously gain $c$ speed when emitted from an electron? – John Rennie Sep 30 '22 at 19:13
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No. Photons cannot travel at any velocity but $c$ when in a vacuum. When traveling through transparent matter, you can get a lower speed according to the index of refraction, but the photon comes into existence traveling at that speed, and any photons absorbed while traveling in that medium will disappear while traveling at that speed.
Sean E. Lake
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Photons don't travel at all. That's just a semi-classical model that is virtually useless in quantum mechanical situations. A photon is an irreversible energy transfer between the electromagnetic field and an external system. It is the amount of energy that is being transferred while the angular momentum of the field changes by one Planck unit. The free electromagnetic field is not "made up" of photons. It's much more complicated than that because of the superposition property of quantum systems. – FlatterMann Sep 30 '22 at 23:22