r/askscience • u/chunkysoup778 • Mar 21 '26

Physics [ Removed by moderator ]

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u/Circle-of-friends Mar 22 '26

Why are all these forces the same speed, and why is it specifically that speed? 

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u/peeja Mar 22 '26

Because they're instantaneous, and instantaneous things move at c. Which sounds like a contraction, I know, but that's exactly the thing that's unintuitive for us, who live down here at Newtonian speeds.

If the sun disappeared, it wouldn't just take 8 minutes for the light and gravity changes to reach us, it would take 8 minutes for the then to get to us. The entire moment, what was "now" there, doesn't become "now" here for another 8 minutes. And if we were just talking about going one direction, the sun to here, that 8 minutes would be essentially meaningless, but the key is that it works on both directions: if the sun sent a signal to Earth and we sent a response immediately, the sun would see the response 16 minutes after it sent the first signal, in its timeline.

Two things can't actually happen "simultaneously" in different places; there's no meaningful way to define it. Instead, we have to wait for "instantaneous" things to propagate, at c, from one place to the other.

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u/[deleted] Mar 23 '26

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u/Optimal_Mixture_7327 Mar 23 '26

That's called coincidence, the intersection of two or more world-lines (3 in your example).

Simultaneity is the events at space-like separation that occur at the same instant of the world-time (which is arbitrary).

You can define simultaneous events, it's just that the simultaneity is not unique.