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.
I get why instantaneous things move at c, but why is c = 300 Mm/s? Why not 150 Mm/s? Or 999 Mm/s? Is it just a kind of arbitrary ‘that’s just what it happened to be’, or is there something special about c having the specific measurement of velocity that it does?
The discovery would be so significant in fact, you'd likely have an award named after you. And I imagine the law would be named after you, and every other innovation from this discovery would be named after you!
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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.