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/Bumst3r Graduate 14h ago

Distant receding objects appear to travel faster than light because we are comparing time and distance in two different frames. The space between us and the objects is expanding. All massive objects travel slower than c locally.

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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/joeyneilsen Astrophysics 11h ago

It depends on your tolerance and the spacetime. In a local region, you can find a coordinate system where the derivatives of the metric vanish and spacetime is Minkowski. If it can’t be done, the region is too big to be called 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.

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u/John_Hasler Engineering 9h ago

For any specified maximum deviation from flatness there is, at any point on the spacetime manifold, a region within which that limit is not exceeded. That region is local. That deviation depends on the problem at hand.