Light theoretically would travel at c in a vacuum, but I think a vacuum is practically impossible. So light doesn’t ever travel at lightspeed? And therefore it does “experience” a tiny amount of time between emission and absorption/reflection?
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If the average density of the medium is low enough, and the wavelength of the light is short enough compared to the average distance between electrons that make up the plasma of the interstellar/intergalactic medium… it shouldn’t provide any overall impedance that would lower the speed of light. You would probably model it over a long distance and take the occasional interaction into account, but between charges the light should be moving at c or *very* close to it.
The second part of your question is more direct, no, light is always moving at the speed of light, even when that speed isn’t c for a given medium. If you want to think about something nonphysical like a photon’s perspective, it would always be the same.
It doesn’t have to be a perfect vacuum. Light is only slowed down if it actually hits a particle before reaching its destination. If there are just a few particles in an area, there’s a decent chance that the light will pass through without hitting any of them.
Light always travels at light speed. It seeming to slow down in a medium is it just getting absorbed a bunch of times.
No, light always moves at `c`. But it may not move in a straight line.
When the light moves in some material, it has to avoid atoms, so it moves in zigzag pattern, making the total path longer. So it appears that light moves slower, while in actuality it does not.
You may ask: “if the light moves in zigzags, why doesn’t it scatter in all directions?” That’s because of a quantum mechanics. In quantum mechanics, light moves in all possible paths simultaneously, but two paths can also cancel each other.
In transparent materials, all scattering paths cancel each other, only those who return on the refraction line don’t. In other materials, the light does scatter. The exact behavior depends on the placement of atoms relative to each other.
What do you consider a practically possible vacuum? The vast majority of the universe is not just interstellar, not just intergalactic, but actually enormous voids between galactic clusters. The average density of atoms in the universe is really low (one google result says “around 1 atom per 100 cubic meters” which seems about right to me.) A crude estimate is that a photon must come within about 1 wavelength of an atom to interact with it. If I did my math right, that translates into about 1 light year on average between interactions, assuming all the matter in the universe were spread out evenly, which it’s not.
And note that those interactions aren’t always, or even usually absorption or reflection. I responded to another commenter on this topic — optical refraction (which is the phenomenon that leads people to say that “light slows down in materials”) is a very complex process where the EM field of the moving light causes electrons in nearby atoms to shift a bit, which in turn generates transient EM fields that combine and distort the original field. But this isn’t absorption. A crude analogy would be if you play a pure musical note near a harp – the harp strings vibrate and the result is a sound that isn’t exactly the note you generated, but the harp didn’t completely absorb it and re-emit it.
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