[trenchgun]

Why from all people?

What if there is a person in an lab which is isolated from the rest of the people?

[kgwgk]

Wigner’s friend?

[akvadrako]

Then he will certainly be able to say a measurement happened.

That’s something only conscious beings can do.

[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?