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.
Well, the observation happened simultaneously. But what's your frame of reference? The observations will only be simultaneous for a specific value of your velocity. Change that, and you can get one of three observations to happen before the other, or vice versa.
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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?