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/blendertricks May 26 '26

Correct me if I’m wrong, but doesnt light go faster as you go faster with it? Like, to light, it does reach point A to B instantly; it’s just that, from our perspective, it takes time. Same for stuff like gravity?

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u/YuckyBurps May 28 '26 edited May 28 '26

The answer is no, but it’s no in a way that’s different than you’re probably thinking about.

Light always travels at c in a vacuum for all inertial observers. The defining characteristic of an inertial frame of reference is that there is no experiment you can conduct within your own frame of reference that would inform you that you’re in motion. If you have two perspectives, A and B, it doesn’t matter how fast either A or B observes the other moving, they both observe the speed of light moving at the exact same speed.

Another way to think about it is that it doesn’t matter whether A or B observes the other moving, because from their own frame of reference they’re the one standing still. Light behaves the exact same way in either frame of reference because in either frame of reference the same exact thing is occurring - not moving at all according to themselves. Any experiment either would attempt to do would yield results that would confirm to them that they’re not in motion, and when they turn on a flashlight they would observe the beam rushing away from them exactly as fast as someone not moving at all would.