[rbanffy]

But then isn't Wigner's friend also in a superposition until Wigner talks to them?

[akvadrako]

Traditional QM does not allow Wigner’s friend to be in superposition.

[rbanffy]

Until he talks to them, how can Wigner tell? ;-)

[akvadrako]

Wigner can’t tell, but the friend knows. If he thought he was in a superposition he would believe in many worlds.

[rbanffy]

But shouldn't every friend in every possible state observe a single state that's consistent with their own internal state?

The friend only exists in many worlds from Winger's perspective, and only as long as their state has no causal effect on Wigner's own state.

[ithkuil]

What's wrong with many worlds?

[akvadrako]

It’s not what this thread is about.

[ithkuil]

I guess I got confused at "If he thought he was in a superposition he would believe in many worlds."

Do you mean "he can either think he can be in superposition or he cannot be, if he thinks he can be then he would believe in many worlds, which is incompatible with 'traditional' QM, which thus implies 'Traditional QM does not allow Wigner’s friend to be in superposition.'" ?

I don't think the Traditional QM ever explicitly disallowed a person to be in superposition; being in superposition it's just not something "decent people" do, I suppose.

Thus we needed to find other ways to come to terms with what we observe. But the math is the same. The rest is only metaphors that help us reconcile what we observe with how we feel inside.

Thus, many worlds is not antithetic to traditional QM.