[Robcat (OP)]

A ship sails from a to b like a photon but the ship can be subjected the high seas and slow down but
Still arrive at b as if on a straight line.
Does the photon get perturbed on its travels or is its speed constant?

[evan_au]

Does the photon get perturbed on its travels or is its speed constant?

When a photon enters glass (or anything denser than a vacuum), its speed slows down.

When you measure the speed of light in a vacuum, in your laboratory, it always travels at c.

However, as I understand it, if light travels through a deep gravitational well, far from your laboratory, it will seem to be traveling slower than c. Similarly, if you are in a deep gravitational well, and you observe the effects of light far outside the gravitational well, it will seem to be traveling faster than c.

[alancalverd]

However, as I understand it, if light travels through a deep gravitational well, far from your laboratory, it will seem to be traveling slower than c. Similarly, if you are in a deep gravitational well, and you observe the effects of light far outside the gravitational well, it will seem to be traveling faster than c.

I think this is stretching the meaning of "seems to be", as the measured speed is constant. What happens is the perceived frequency is altered by the relative gravitational potential of the source and observer - hence "red shift" and "blue shift". Maxwell's equations resolve to c = 1/√ε₀μ₀ which is necessarily a constant.