Suppose we have two electrons A and B.
My friend measure the spin on electron B the value is +1/2, and he writes on a piece of a paper the value.
Electron A has not been measured, so the spin is in superposition.
Now my friend ask me to guess which of the two electrons has collapsed to a known state, is there a way to do that?
Now my doubt is that if I can't tell which of the two electrons has collapsed means that I am unable to prove that wave function collapse exist or it doesn't affect a different observer.
If am able to check which of two has collapsed means that I can use quantum entaglement to transfer information: As example: I can take 10 electrons on earth numbered from 0 to 9, and other 10 entagled on mars. On Earth I measure electron 3 and on Mars I check them to know if they are collapsed, I find the number 3, so information has been transferred, and as far I know this is not possible.
Since quantum superposition, and wave collapsing theories have been proved, I suppose that since I am a different observer both electrons are still in superposition until I do a measure, only after that I determine the value.
But this involves a paradox, since I don't need to do a measure to know the state, I just need to read the paper note where the friend has written the value.
It's like to admit that the piece of paper is in a quantum superposition until I read its value (very like Schrödinger's cat paradox). But as far as I know for quantum decoherence a macrosopic thing can't be in superposition.
Now my guess is that a differen't observer can't know if the electron has collapsed in a known state, the piece of paper will be in superposition until quantum decoherence has occurred and quantum decoherence propagates at light speed.