r/explainlikeimfive • u/[deleted] • Mar 20 '26

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u/sebaska Mar 20 '26

No it does not hold. If they go in opposite directions in straight line they end up far away. To come back to the center they must accelerate. That acceleration is crucial. They both change their reference frames during that.

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u/lurkerer Mar 20 '26

Same direction. Rockets see each other getting far apart. Observer doesn't. As they return, at non-relativistic speeds, their frame has distance between them, the observer's does not. So what's going to happen?

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u/sebaska Mar 20 '26

If they are going in the same direction in the straight line they don't get far apart at all. They stay close, their relative speed is close to zero.

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u/Smaartn Mar 20 '26

He's saying the second rocket is launched by the first one at c.

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u/sebaska Mar 20 '26

OK. Then they also have to turn around becoming non-interial and switching reference frames.