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Weinberg mentions the quote by Eugene Wigner regarding the importance of consciousness in the section on instrumentalists. However, I see it is being fundamental to realist's interpretation also. I subscribe the the realist approach, which I view as meaning nothing special happens when a human makes an observation - it is just quantum mechanics as usual. This has interesting implications. The key is that the observer is not outside the system, he is a part of it. His wave function becomes entangled with the system when the measurement is made. But what makes this difficult to analyze is that we don't know what it means for the observer to say "I see the result was *". What physical process allows the observer to be aware of the measurement?

We can make a simplifying and unsatisfying assumption - the observer has a register in his head which records the result of the measurement. Here, it is easy to imagine this register is part of the wave function just like the object being measured. As mentioned above, the wave function for this register is entangled with the object being measured so the measurement and register value are correlated. In our model this corresponds to the person independently thinking each of the possible results, which some people call multiple universes.

But this simple register is not what goes on inside the brain. Not knowing this makes it difficult to accurately model the measurement process.

EDIT: for clarity

In the realist interpretation, even without people, you wind up in entangled states after measurement of a spin (where by "measurement" I mean: allow a giant classical system to interact with a spin and tilt a pointer on some gauge to / wind up with some register holding + or -). It's perfectly possible to formulate the realist ideas without any consciousness or insistence that the measurements register in a brain or observer.

Yes, I agree you can formulate the realist ideas, because the observer is nothing special at all. And I also think this is how the world works. But from an experimental verification point of view I think you really need a model for what the brain does to say you understand how a measurement is made. What is a theory without verification?

Granted his quote used the word formulate, but I interpret the comment as referring to not just writing some rules but also having some justification for them.

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