I have some doubts regarding my personal interpretation that i was contemplating about in the context of Wigner's friend experiment (also tested in the laboratory).Could it be that a system is always in a superposition, and when we perform a measurement, we obtain a definite value due to the interaction, but after it, the system returns to a superposition? For Wigner, who will check if superposition exists for him after his friend's measurement, he will find the system again in a superposition. If they were to measure at the same time, they would see the same definite result instead. Wigner and his friend might have different measurements, but this wouldn't imply a different reality, only their knowledge of it. Please note that i am not a physicist and i do not intend to propose any theory or interpretation because i certantly don’t have the knowledge required. I am just expressing my curiosity.
3 Answers
It is unclear to me what you think the Wigner's Friend is about so I'm going to describe it and consider the implications of what you have suggested.
Suppose you have a sealed box with an atom, a laboratory and a person who happens to be a friend of Wigner and Wigner is outside the box. By sealed I mean it is completely impossible for any information, fields or matter to get out of the box: no light, no sound, no anything else.
The atom $A$ has two possible states: spin up or spin down each with probability of 1/2, which we write as $\tfrac{1}{\sqrt{2}}(|\uparrow\rangle_A+|\downarrow\rangle_A)$. Wigner's friend $F$ measures the atom and remembers the result and if we assume the joint system of the atom and the friend obeys quantum mechanics the state is then: $$\tfrac{1}{\sqrt{2}}(|\uparrow\rangle_A|\uparrow\rangle_F+|\downarrow\rangle_A|\downarrow\rangle_F).$$
It is common for physicists to say that when a measurement takes place the laws of QM somehow stop operating and only one outcome happens. The operative word here is somehow, as in most physicists can't be bothered to work out the details and assume it doesn't matter. So then Wigner measures the state of the atom and the friend and somehow he sees one of the two possible options and that is the only state. But the question is why we can't treat Wigner $W$ according to the laws of quantum mechanics and just say the state after his measurement is $$\tfrac{1}{\sqrt{2}}(|\uparrow\rangle_A|\uparrow\rangle_F|\uparrow\rangle_W+|\downarrow\rangle_A|\downarrow\rangle_F|\downarrow\rangle_W).$$
You ask
Could it be that a system is always in a superposition, and when we perform a measurement, we obtain a definite value due to the interaction, but after it, the system returns to a superposition?
How would the system you're observing know whether you're looking at it to flip back and forth between those two options?
You further ask:
For Wigner, who will check if superposition exists for him after his friend's measurement, he will find the system again in a superposition.
In the real world, if Wigner observes the friend and the atom, he will find the same result as he found the first time if he repeats the measurement. If the state flipped back to being a superposition when he stopped looking at it what would stop him from seeing the outcome from changing?
It is difficult to come up with a consistent theory that is also testable and correct. It is okay not to have answers to questions like the one I asked as long as you don't pretend to know more than you really do. I have another answer that links to papers about some of the options and how to test them:
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This experiment cannot be done, even in principle, because it is impossible to isolate the lab with the friend. It is always possible to get the relevant information regarding the outcome of the measurement from the outside. Any macroscopic change inside the lab (required for a successful measurement) will be accompanied by a change in the gravitational, electric and magnetic fields associated with that change, fields which could be measured from outside.
Given the above, the superposition ends when the friend performs its measurement for any observer, including Wigner.
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An undisturbed quantum system remains in superposition. When it is observed it collapses to a specific state, whereafter the system evolves according to the time development of the Schrodinger equation, unless of course that it is continually observed.
If Wigner's friend makes an observation at some time $t$ he will find the system in an eigenstate $A$. If Wigner has not yet made an observation then the system is for him, in a state dictated by unitary time development from the state $A$. If at some time $t_1$, after $t$, Wigner decides to make a measurement; Wigner will find the system to be in an eigenstate $B$. If both Wigner and his friend decide to measure together at time $t_1$, then they will both observe the system to be in state $B$.
Who ever disturbs the system destroys the superposition.
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