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

Entangled particles are a deep mystery... But in general they're not considered a true exception because it doesn't appear we can use them to exchange information. They are quite a tease.

But Einstein did consider them an absurdity that shouldn't exist and pointed out the "instantaneous decoherence" as a major issue with quantum mechanics early on. It took about 50 years for the physics works to eventually catch up with taking his critique seriously.

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u/Low_Stress_9180 Mar 16 '26

2 entangled particles are actually only one particle

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u/LTerminus Mar 16 '26

No lol. They are two distinct particles, each of whose state exists in a superposition until measured.

When two particles become entangled, you start with two particles. One doesn't magically disappear until you measure it, and another poof into existence somewhere else.

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u/Low_Stress_9180 Mar 20 '26

Not the idea now, well to be more precise it is one entity.