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: 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'. 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?