Ask Ethan: Does light always move at the same speed?
Ethan Siegel

light is the simplest particle in the Universe. Even though it always moves at the speed of light, it doesn’t always move through completely empty space. As long as there’s matter in the Universe that’s transparent to light, you won’t be able to avoid slowing it down. But as soon as that light heads back into empty space again, it’s back to the speed of light in vacuum, with every photon moving as though it had never moved at any other speed at all!

Thanks again Ethan. Here’s my dumbass question of the day (after all, I was a liberal arts major, so forgive me).

I understand that space is a near vacuum. But as I understand it space is not quite, anywhere, a perfect vacuum. Almost nothing in nature is perfect, so there is no perfectly “empty space.” Even the near perfect vacuum of space does have some widely scattered stuff in it.

I understand it may not mean much in practical terms, I’m going theoretical here in terms the absolute speed of light requiring an “infinite vacuum” even though no such vacuum (to the best of my knowledge) actually exists.

I guess my question is “does the speed of light ever really travel at the absolute, actual speed of light we have been taught?” It seems to me that no matter how un-dense space is that even the lowest such density is never a perfect zero. Does not the even the sparse presence of matter in “space” slow down light (no matter how infinitesimally)? Given this, is there anywhere in “space” where light actually travels at “the speed of light”?

Forgive me, I was liberal arts major. I have a BS in psychology (which I realize is redundant). I have some non-science advanced degrees in Health Administration and Law. All that creates an inquisitive mind, not necessarily an informed one.

So an answer to my dumbass question would be appreciated. Thank you, should you choose to address this question.

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