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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The “speed of light” means two different things.
One meaning is it’s the fundamental speed limit imposed on us by the very geometry of space-time. Massless particles travel at this speed in a vacuum, so that’s also the speed of photons (ie, light), in a vacuum.
Another meaning is “here’s some light, how fast is it traveling?” and the fact is, that depends on what it’s traveling through. Whether they’re being absorbed and re-transmitted, or actually traveling slowly, might just end up being an argument over the best words to describe the maths.
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