How do we know Q entanglement exists if particles stop being entangled as soon as we observe them?
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A single pair measured could be a coincidence, but if we measure many many pairs in many different ways we can get to the point where the measurements being the result of coincidence is extremely unlikely.
Lets understand the theory a bit better.
Particles have some kind of angular momentum, we call it their spin. They aren’t spinning but we lack a better word. Angular momentum, without a doubt, is conserved. So if we look at a particle that has 0 spin and we split (or it decays) it into two particles where each of them have some non zero spin like ½, our conservation law states that they have to sum up to 0.
So particle A has ½ spin since I measured it and now I know that particle B has to have -½.
But these states are not predetermined for either particle. There is a 50-50 chance I measure ½ or -½ for particle A. And you can experimentally show that it’s in fact not predetermined. So we have this probabilistic nature for measuring states, this is experimentally proven, and we also have the conservation of angular momentum. Entanglement logically follows from this.
So entanglement is a consequence of superposition and the conservation of angular momentum. Of course we need to show that superposition exists and we don’t have hidden variables that predetermin the spin of our particles. This video explains just that: https://youtu.be/hiyKxhETXd8
The way we run these experiments is we prepare particles in a certain state (entangled or unentangled), allow them to do something (move through magnetic fields or arrays of mirrors or anything else we might think of), and then observe the final result. We consistently see that if we prepare entangled particles, the final result is different than if we prepare unentangled particles. Even though we can’t see the entangled state directly, we know that it is distinct in some way from unentangled states.
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