I am trying to read Hugh Everetts thesis about the universal wave function, which is now called the many worlds interpretation of QM. At page 53 ff. he discusses measurements. He talks about a system composed by two state functions, saying that the two subsystems can be considered as measuring each other, using two operators A(t) and B(t) representing those measurements. The he writes (p. 54): I don't understand this at all. To me, a measurement is instantaneous. You apply a measuring apparatus, read it, and you are done. The short time this procedure takes is negligible, says my intuition. Yet, Everett is talking about a process where the time tends to ∞. What is he talking about???