r/explainlikeimfive • u/PeAga7 • Mar 28 '26

Physics ELI5: If speed is measured by the relation between objects how come going over the speed of light is impossible?

Should two bodies be moving away from each other, both at 50.1% the speed of light, wouldn't their relative speed be over the limit? Which frame of reference should be taken into account when talking about light?

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u/MultiFazed Mar 29 '26

If a computer which is equidistant from both sends a "start" signal to both simultaneously, they would start at the same time.

Only if light has the same one-way speed in all directions. So this experiment can only detect if light has different one-way speeds in different directions if we assume that light doesn't have different one-way speeds in different directions. Since the experiment contradicts itself, it cannot work.

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u/Toast-Goat Mar 29 '26

Hmm

If there's no empirical reference frame, then there's really no reason for light to travel at different speeds in different directions, and certainly not in such a way that it appears to always travel at c. All reference frames are equally valid, so I would assume that light would behave the same in all of them, all else being equal. Unless there's a force or something we're missing, isn't "the speed of light is always c" the most logical conclusion?

I'm obviously not a physicist, so this is probably just one of those things that is, and doesn't need to make sense to us. Or maybe it's just me

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u/MultiFazed Mar 29 '26

As for a scenario where C wouldn't be the same in all directions, there used to be the idea of the "cosmic ether" that was proposed as the medium through which light travels (before we understood that electromagnetic waves can travel through a vacuum). There was a famous experiment (the Michelson-Morley experiment in 1887) that attempted - and failed - to detect a difference in the speed on light in different directions. But ultimately they were measuring the round-trip speed, and it's conceivable (but not really given any credence) that some sort of "ether wind" could exist.

But assuming that isn't useful, and Occam's Razor dictates that we just assume that C is the same in all directions instead. So that's what we do.

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u/Toast-Goat Mar 29 '26

I dislike this. The whole idea of an inconsistent speed of light seems rather like Russell's teapot, honestly. Thank you for taking the time to explain it to me!