[GregoryFul]

I am confused. Here we have a theory that says that a quantum parameter is indefinite until measured. But in a pair production, measuring the quantum parameter in particle 1 fixes the value in 2, no matter what distance separates them. How are we to know that the measured quantum parameter is not actually fixed in both (but unknown) at the time of particle pair birth?

[Alamo-Girl]

Thanks for your reply!

Please let me know if the following sheds any light on the question: How is spin in quantum systems a direct result of special relativity

[Doctor Stochastic]

I am confused. Here we have a theory that says that a quantum parameter is indefinite until measured. But in a pair production, measuring the quantum parameter in particle 1 fixes the value in 2, no matter what distance separates them. How are we to know that the measured quantum parameter is not actually fixed in both (but unknown) at the time of particle pair birth?

Fixed-but-unknown would violate other things that can be measured. QM is weird. Much of the reason for the weirdness is that the basic objects in QM are not the macroscopic things we normally deal with, but the wave function. Physicist can explain things much better than I can.