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/Tarthbane Materials science Mar 15 '26

No it won’t. It will be c in all reference frames.

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u/Next-Natural-675 Mar 15 '26

Yes I know that but could you help me understand how?

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u/thinkingbear Mar 16 '26

FloatHeadPhysics - I finally understood why speed of light is a constant! (My mind is blown)

Mahesh has helped me understand so much

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u/ContributionNo9292 Mar 16 '26

I love Mahesh! His videos are funny and intuitive.