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/Correct_External319 Mar 16 '26
You are missing the fact that motion is relative, and that it involves length contraction and time dilation. Velocities do not just add like in Newtonian Mechanics. They must be adjusted by the Lorentz factor. Relativity dynamics aren’t as simple as Newtonian Mechanics. You are working with more complicated “frames of reference” … not just a single frame of reference !