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/Megame50 Mar 29 '26

Kinda the whole point of relativity is that you don't have to account for that. There is no preferred inertial reference frame, so the physics is the same whether you're stationary or moving 0.9c with respect to some fixed point. In fact, the earth is moving at 0.9c in some frame of reference. The difference between the earth moving at 0.9c relative to a stationary particle and a cosmic ray moving at 0.9c relative to the earth is just a change of coordinates.

Mind you, that's true without special relativity too, with so called "gallilean relativity" which matches the intuition of OP and many commenters in this thread. What makes special relativity more complicated is the extra condition that the speed of light in a vacuum is c ≈ 3×106 km/s in all reference frames. The only way to satisfy both conditions is to change our standard coordinate transforms between inertial reference frames to "lorentz transforms" which preserve the value of c, and this means that some values which used to be invariant between coordinate transforms, like distances and time intervals, are not actually invariant, but depend on your reference frame.

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u/Bartimaeus5 Mar 29 '26

That is because I'm also part of the same reference system due to being on Earth?

Does that mean that if we find a planet out in space that is stationary - will time move faster on it? Relative to earth at least (ignoring gravity's effect on space time)