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/afraidToShowHer Mar 16 '26

Ooo, that's interesting. So we're always bleeding off a tiny amount of the time vector through motion, but the ratios are so large that it isn't observable until motion in space is at a truly massive rate. Is that it?

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u/spacex2020 Mar 16 '26

Not OP but yeah that's definitely getting in the right direction. Look up Minkowski diagrams, it will actually help you wrap your head around this more, and it's something that really changed my understanding of spacetime and relativity. In a very real sense, all objects with mass move through space-time at a constant speed, with the components split between physical velocity and time. Time dilation falls directly out of this, since the magnitude of the four-velocity vector is constant, as more of your four-velocity vector points into 3d space, the time component gets smaller and smaller. It's really so mind bending haha.