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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.

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  • $\begingroup$ "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?" I'm not sure what you mean. Of course you can make a random guess. But if you want to actually know anything about the electrons you have to perform measurements on them. $\endgroup$
    – PM 2Ring
    Commented Dec 20, 2019 at 10:03
  • $\begingroup$ @PM 2Ring, but if you do measurement, you cause a collapse, and you can't know if the value has already been determined before. in fact it's like ti say: I don't know if collapse really occurs or not. $\endgroup$
    – Stefano Balzarotti
    Commented Dec 20, 2019 at 10:11
  • $\begingroup$ That's exactly correct. $\endgroup$
    – PM 2Ring
    Commented Dec 20, 2019 at 10:15
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    $\begingroup$ Collapse isn't a physical phenomenon or process. It only exists in the Copenhagen interpretation, which is just one interpretation of quantum mechanics. $\endgroup$
    – user4552
    Commented Dec 20, 2019 at 14:50
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    $\begingroup$ Collapse is not physically real. In reality you have two electrons that are either correlated or not correlated. $\endgroup$
    – Bill Alsept
    Commented Dec 20, 2019 at 15:02

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A study was done on a very similar question. The question was whether a person who was flipping coins, and the state of the coin, could be in superposition for someone else who hadn't observed the first person and the coin flip. According to the study which used sets of photons, the answer is yes. You can find a layman's article and the link to the original study here:

https://phys.org/news/2019-11-quantum-physics-reality-doesnt.html

So if this study holds, the answer to your question should be which electron got measured should be in superposition for you, WHILE it is not in superposition for the person who measured the electron.

Note: This means that there is no way directly for YOU to tell which electron got measured until you look at the paper or until the first person tells you.

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