How do we know how fast light is?

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How do we know how fast light is?

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We measure it. Either by measuring something [very far away](https://www.amnh.org/learn-teach/curriculum-collections/cosmic-horizons-book/ole-roemer-speed-of-light#:~:text=The%20speed%20of%20light%20could,is%20186%2C000%20miles%20per%20second.) (TL:DR: he measured the eclipses of Jupiter’s moons at one time of year and at a different time of year, and used the difference between when he measured the eclipses to calculate the speed of light.)

Or by using something a bit [closer to home](https://wonders.physics.wisc.edu/measure-the-speed-of-light/#:~:text=Note%3A%20MHz%20%3D%2010%5E6,10%5E10%20cm%2Fs.)

The most precise measurements of c come from the fact that we actually defined the meter to be based on c, so c is actually exactly known.

Another reason we know what speed c is has nothing to do with light; light simply happens to be able to travel at the speed limit. c stands for causality, it’s the speed at which things can be acted upon from an outside perspective. That speed has been calculated and theorized, and we measured whether light travels at that speed as predicted of our models.

From the light point of view, travel is instantaneous due to time being warped as you go faster. From the point of view of others, it goes at speed c, effecting empty space and anything it goes through or gets reflected/refracted on to travel on.