# Wigner's Friend in the many worlds?

## Background

In the thought experiment of Wigner's friend the many-worlders claim:

"Everett claims to solve the Wigner's Friend paradox by only allowing a continuous unitary time evolution of the wave function of the universe. Measurements are modelled as interactions between subsystems of the universe and manifest themselves as a branching of the universal state."

## Question

This makes it sound me that one should (rightfully) question whether any initial state (let us assume is an eigenvector) could and most likely is already actually a superposition. However, it's okay to model them the way we do as they manifest themselves as a branching of the universal state.

Is there a paper or derivation that drives this point home?

## My naive understanding (which is probably wrong):

$$|0 \rangle + |1 \rangle$$

Is a valid superposition (ignoring normalization).

However, when the intial state is an eigenvector $$|0 \rangle$$ or $$|1 \rangle$$ . Then they are also in a valid superposition (just which happen to be in different worlds) and could might as well be the below superposition:

$$|0 \rangle + |1 \rangle$$

However, the catch is they have branched in different worlds (so the experimentalist cannot distinguish eigenvector and superposition in this case):

Once, again: Is there a paper or derivation that drives this point home?

• If you're asking whether every state is a superposition, then of course the answer is yes. – WillO Sep 11 at 0:10
• @WillO Do you also agree with the your above comment in light of the edit? I agree every state is a superposition however there are clearly basis where that superposition is an eigenvector ... And superposition usually refers to a sum of more than one eigenvector? – More Anonymous Sep 11 at 0:13
• try to work out better the phrasing your question, is not clear what you are asking – lurscher Sep 11 at 0:21
• @lurscher I guess what I'm asking might as well be my own inadequacy in my understanding of many worlds. Which I might phrase as: Why is situation $1$ (a superposition in the my understanding section) different from situation $2$ (an eigenvector which seems to be in superposition as well but in different branches)? Or what is it that the process does that allows these 2 kind of superpositions to be distinguishable? I do not find it satisfactory that this just postulated. I feel the many worlders seem to imply their interpretation is more than replacing "superposition" with "world". – More Anonymous Sep 11 at 0:27
• Maybe if one can answer a related question what is the cardinality of the set of universes from which one universe (of which it originated from in the many worlds). Then would major progress (atleast for me) ? – More Anonymous Sep 11 at 0:44