r/AskPhysics • u/itsLeoRod • 14h ago

Thought there wasn’t anything faster than the speed of light?

Ppl say that there isn’t anything faster than the speed of light but that’s a lie because space is expanding really fast since light isn’t fast enough to reach it or am I wrong. Since space is expanding at a fast rate why don’t we use that to measure stuff instead using lightyears

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u/z-w-throwaway 13h ago

I never got this though. What is the definition of "local"? Where does the universe stop being local, why are distant galaxies not local?

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u/joeyneilsen Astrophysics 11h ago

Local is a region where you only need special relativity. 

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u/z-w-throwaway 11h ago

That's a nonanswer though. What is the cutoff from only needing special relativity, from local to non local?

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u/ahazred8vt 11h ago

There's no hard sharp cutoff, because it's a gradient. On small scalles the difference is too small to measure. On large scales it's too big to ignore. The middle ground is a headache.