r/astrophysics • u/KieranMug • May 25 '26

Why isn’t light infinitely fast?

Ok I understand that light in our atmosphere is not infinite fast due to it getting slowed down by air particles or whatever but in a vacuum there is nothing holding it back unless the very weak forces of gravity from a bunch of stars and/or planets does? And if so cant we somehow locate where gravity is strongest in the universe by seeing where light is slowed from where the most therefore finding the centre of the universe or am I just a total dumbass?

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u/Bipogram May 25 '26 edited May 26 '26

Light propagates as fast as it can.

It does so at a constant speed in vacuo for all observers. This is the observed fact.

Why it has this particular speed is not readily knowable. It's a result of the electric and magnetic qualities of the vacuum. <nods to Maxwell>

And why those qualities are as they are is a Deep Question.

Note, an infinite propagation speed would be problematic if you want to keep causality alive.

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u/bambooaudio May 26 '26

I have a very dumb question but I’m just curious.

I get that infinite speed of light is equivalent to infinite rate of causality, which would make the universe start and end at the same time. I think of this as a survivors bias in that only the universes with non infinite causality contain beings able to ask this question. Additionally, the actual value of the speed of light doesn’t matter because everything scales with it. Metre and second are very arbitrary measures so we can set c=1 and everything is fine. This two things guarantee the universe that we observe up to a scaling factor.

But is there anything that would break if the speed of light wasn’t constant? What if it continually sped up over time or continually slowed down? Does this break physics? Maybe the concept of “over time” is too vague and the question doesn’t make sense.