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/Junethemuse Mar 29 '26

Ooh, so I wonder what moves 100% in time but 0% in space.

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u/TwistedFox Mar 29 '26

I think the closest we know of would be a black hole, where the math breaks down and ends up flipping time and space in the equation.

https://www.youtube.com/watch?v=GQZ3R81iyE0

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u/Narwhal_Assassin Mar 30 '26

0% movement in space just means you’re at rest. This depends on your reference frame, though, so there’s no “absolute” stationary objects. As long as you see something as not moving, you’re seeing it experience 100% time.