Einstein’s theory of relativity is founded on the **speed of light being constant**. However, there are postings and scientific discussions where there is mention of “**light slowing down traveling through materials”**. Does it really slow down in the material or is the entrance/exit delay explained by something else?
For example, would it instead be explained that the photons are absorbed and then re-generated on the other side of atoms as they make their way through water, glass, etc? The “delay” is then actually a measure of the time spent between absorption and emission?
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Light slows down in a medium. Basically, the electric fields of the atoms slow down the light’s propagation.
What special and general relativity rely on is the speed of causality to be the same for all observers. The speed of causality is the fastest possible speed that any two things can interact at. Light moves at this speed in a vacuum because there are no electric fields to slow it down, and it has no mass, so it moves at the fastest possible speed, the speed of causality.
Gravitational waves also move at the speed of causality because they, too, have no mass and therefore nothing to slow them down.
We often use the speed of light and speed or causality interchangeably, but that’s because in most situations, they are the same speed.
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