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Basically, the paradox works like this:

Two entangled particles (A and B) are a light year apart.

1. Measure the X spin of A
2. Measure the Z spin of B

You now know (the paradox claims):

a. B's Z spin, because you just measured it.    
b. B's X spin, as it must be opposite of A's.     

Of course, this turns out not to be true. However-- what if, after measuring B's Z spin, I immediately measure the B's X spin. What is the probability of it being, at that point, the opposite A's X spin (as the paradox claims to be the case)?

If that probability is not 100% (as the paradox's refutation claims), then I can determine that B's X spin is in the same direction as what I had originally measured A's X spin to be.

So then, what would happen if, at that point, I then measured A's X spin (a second time)?

My point -- Is it possible to measure A and observe its X spin, and then flip A's X spin by merely measuring B's Z spin? Has this been recorded experimentally?

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ineedahero,

"Is it possible to measure A and observe its X spin, and then flip A's X spin by merely measuring B's Z spin?"

It is not possible. When you measure the X spin of A the entangled state is destroyed (collapsed), both particles remaining in a X-spin eigenstate (say A is X-up, B is X-down). A subsequent measurement of Z spin would be random, no correlation between Z spins of A and B is expected.

If your proposed experiment would succeed it will enable you to communicate instantly and we know this is no possible (there is a no-signaling theorem).

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  • $\begingroup$ But then if I measure B's Z spin, I will know both its X spin and its Z spin, which is not supposed to be possible $\endgroup$ – ineedahero Dec 14 '19 at 5:33
  • $\begingroup$ So then Suppose A spins positive in X direction. What would be the outcome of these two experiments: 1. Measure B spin in X direction 100 times. [I expect this to be negative all 100 times] 2. Measure B spin in Z direction 100 times. [I expect this to be either negative or positive, but the same for all 100]. Is that right? Or would (2) be 50 positive and 50 negative? $\endgroup$ – ineedahero Dec 14 '19 at 6:32
  • $\begingroup$ " if I measure B's Z spin, I will know both its X spin and its Z spin" - no, once you measure the Z-spin the X-spin becomes undefined again. If you were to measure the X-spin of B again after you measured the Z-spin of B you may find out that the X-spin changed. If you repeat many times the same measurement (say X-spin) you will get the same value. Once you measure the Z-spin the value will be random, and so on. The point here is that the measurement perturbs the particle. $\endgroup$ – Andrei Dec 14 '19 at 12:48
  • $\begingroup$ A measurement forces the particle to align with the magnetic field of the Stern-Gerlach device. The particle that comes out is not identical with the one that entered. So, by performing more measurements you cannot obtain information for more than one spin component at the same time. $\endgroup$ – Andrei Dec 14 '19 at 12:50

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