It's not that simple. When a measurement is done on a pure state (where the knowledge of the state is already as good as it gets) the observers are not just upd

It's not that simple. When a measurement is done on a pure state (where the knowledge of the state is already as good as it gets) the observers are not just updating their beliefs, they are changing the system. On the other extreme, in some cases one may have measurements that improve our (imperfect) knowledge of the physical state without disturbing it. "Quantum measurement is nothing more, and nothing less, than a refinement and a readjustment of one’s initial state of belief. [...] Let us look at two limiting cases of efficient measurements. In the first, we imagine an observer whose initial belief structure ρ = |ψ⟩⟨ψ| is a maximally sharp state of belief. By this account, no measurement whatsoever can refine it. [...] The only state change that can come about from a measurement must be purely of the mental-readjustment sort: We learn nothing new; we just change what we can predict as a consequence of the side effects of our experimental intervention. That is to say, there is a sense in which the measurement is solely disturbance." https://www.perimeterinstitute.ca/personal/cfuchs/Oviedo.pdf https://www.perimeterinstitute.ca/personal/cfuchs/Oviedo.pdf