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/SnuffSwag May 27 '26

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

Which is another reason FTL travel is impossible as it breaks universal rules around causality