I think you’ve got to be a little careful how you say what you mean here:
In light’s own reference frame, this is true-ish from a pure special relativity perspective. Velocity is sort of undefined in that case because at c, Lorentz transformations bring all distances to zero, meaning that the photon is everywhere at the SAME time. Or said another way, it’s everywhere on its own simultaneity curve. Maybe this is splitting hairs on the definition of “undefined” because, mathematically yeah you’re right, but a rock also moves zero distance in zero time. Its more like it’s velocity doesnt make sense to compute.
From the outside though (as in a non photon frame) this is not true at all. Using laws of refraction you can compute, and even photograph and verify a real, defined speed for a photon in a medium.
I think you’ve got to be a little careful how you say what you mean here:
In light’s own reference frame, this is true-ish from a pure special relativity perspective. Velocity is sort of undefined in that case because at c, Lorentz transformations bring all distances to zero, meaning that the photon is everywhere at the SAME time. Or said another way, it’s everywhere on its own simultaneity curve. Maybe this is splitting hairs on the definition of “undefined” because, mathematically yeah you’re right, but a rock also moves zero distance in zero time. Its more like it’s velocity doesnt make sense to compute.
From the outside though (as in a non photon frame) this is not true at all. Using laws of refraction you can compute, and even photograph and verify a real, defined speed for a photon in a medium.