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I have started to read about Rovelli's Relational interpretation in Wikipedia.

http://en.wikipedia.org/wiki/Relational_quantum_mechanics#All_systems_are_quantum_systems

In the section labeled "All systems are Quantum Systems" in the article it says:

It seems to me that even though O' does can't assume a total collapse, the establishment of entanglement between O and S implies a non-unitary (projective) transformation.

Any ideas about this?

http://en.wikipedia.org/wiki/Relational_quantum_mechanics#All_systems_are_quantum_systems

In the section labeled "All systems are Quantum Systems" in the article it says:

In the above paragraph it is assumed that both S , O and O' are two-state systems. O measures S and some time later O' will measure (S+O). I will label them "+" and "-". What I don't understand is the claim that when O measures S, their state after measurement can be seen as a unitary evolution from the state before measurement. I understand that to O' the composite system will be in a superposition |O+,S+> and |O-,S-> before it decides to measure it. But this superposition means a reduction of the product Hilbert space because now the states |O+,S-> and |O-,S+> can't be observed by O'.Taking the model system discussed above, if O' has full information on the system, he will know the Hamiltonians of both S and O, including the interaction Hamiltonian. Thus, the system will evolve entirely unitarily (without any form of collapse) relative to O', if O measures S. The only reason that O will perceive a "collapse" is because he has incomplete information on the system (specifically, he does not know his own Hamiltonian, and the interaction Hamiltonian for the measurement).

It seems to me that even though O' does can't assume a total collapse, the establishment of entanglement between O and S implies a non-unitary (projective) transformation.

Any ideas about this?

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