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A particle with zero rest energy/mass must always be at $c$ in all referentials, even why, if you could get to its referential it would have zero total energy, effectively not existing in that referential.

But, in a medium that speed can be lower and then nothing stops you from being in the same referential it is. Which also means it can't have zero rest energy in there.

So, what exactly do you 'see' then?

What is a photon in a medium for someone who's moving together with it?

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    $\begingroup$ An individual photon still moves at $c$ in a medium. It is only the EM wave that is slower than the speed if light. $\endgroup$
    – jng224
    Commented May 4, 2021 at 18:31
  • $\begingroup$ @Jonas It would be better to put that in an answer. Comments should only be used to suggest edits or to ask for clarification from the OP. $\endgroup$
    – BioPhysicist
    Commented May 4, 2021 at 18:33
  • $\begingroup$ Related: Photon energy - momentum in matter $\endgroup$
    – Chiral Anomaly
    Commented May 5, 2021 at 0:35

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A photon always moves at the speed of light $c$, so one can never be in the same frame of reference. What causes the speed of light to be slower in a medium is the interaction with this medium. This is explained in more detail here:

As the wave moves through a medium, it intersects with (usually) the electrons, causing them to vibrate. That vibration does not exactly follow the wave: the E field causes a force, which causes an acceleration, which builds to velocity. That motion of charge causes the reradiation of another, weaker, delayed wave. The combination of the original wave and the reradiated one results in the overall wave being a bit delayed. The more material traversed, the more it’s delayed. In that sense, it’s showing a slower velocity: the more material it goes through, the longer it takes to get there.

(from Bob Jacobsen's answer)

I am by far no expert, but I like the way it is explained above – basically (and probably very simplified), a photon/the electromagnetic wave will travel at $c$ between the individual atoms, but gets absorbed and re-emitted when interacting with the atoms. The time delay between absorption and emission is what causes the overall decrease in velocity.

For more information, you may want to refer to What is the mechanism behind the slowdown of light/photons in a transparent medium? and What really causes light/photons to appear slower in media?

So to answer your question, one cannot make any statements of one would see if the relative velocity of a photon was $0$ because that is not possible (since a photon has no rest frame)

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  • $\begingroup$ The photons being "absorbed and re-emitted" in a transparent medium doesn't much sense, as this would mean having to promote electrons of particular atoms to an excited state, which would then need to return to ground state emitting that same photon in the same direction again, a coherent emission, which doesn't easily happen. I suspect that's not quite what's happening from the quantum mechanical standpoint. $\endgroup$
    – user2934303
    Commented May 4, 2021 at 21:51

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