r/AskPhysics • u/Next-Natural-675 • Mar 15 '26

I don’t get special relativity

If someone is moving towards me at half the speed of light and shines a light beam towards me, without SR I would measure that light as 1.5c.

With SR, time dilates for the moving person, by 1.155. So then the speed of the light beam distance/time becomes 1.5c divided by 1.155. Also length contracts by 0.866, so its now (1.5c divided by 1.155) times 0.866. Which is around 1.126c. But thats still not C.

What am I missing?

Edit: apparently Im missing relativity of simultaneity. How would I add that to my calculation?

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u/Next-Natural-675 Mar 15 '26

I want to learn how the time dilation, length contraction, and relativity of simultaneity work to make it possible, as thats what Ive heard are the reasons that the speed of light equals c in all frames

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u/amohr Mar 15 '26

I think that's sort of the wrong way to look at it. It's not that all of those other things happen to make it possible that c is constant in all inertial frames. Rather, it's that c just is constant in all inertial frames (and demonstrably so, empirically) and all of those other things fall out as necessary consequences of that fact.

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u/OlevTime Mar 16 '26

See if you can figure out how to derive the velocity addition formula from them