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you are certainly right that concepts like entanglement or decoherence depend on an arbitrary definition of
(subsystems of the universe). Therefore, branches cannot (and need not) be exactly defined; these branches are merely a matter of convenience.
What IS important is a concept of local observers that is compatible with the assumed global unitarity. If observers are assumed to be organized and strongly coupled subsystems of the universe, they must be pretty local (assuming local dynamics), but as a consequence of unitarity, and in a universe of growing entanglement between its local parts (an important cosmic initial condition!), they must then permanently
into dynamically separate components states (
) – just like Schrödinger’s cat together with its environment. However, each component is coupled only to its own
branch, and so he observes only one outcome of a quantum measurement.
Clearly, we do not know very much about the precise (conscious?) observer states in the brain. (There is much decoherence still going on within the brain.) Nonetheless, we can estimate what superpositions in the world can have coherent effects on such local observers. If they can not any more, their components must separately affect separate components of the observer. Precisely such a coherent effect on potential observers is eliminated by an irreversible decoherence process. Therefore, it is CONVENIENT to consider components separated by decoherence as forming an incoherent ensemble thereafter (an apparent collapse), regardless of whether an observer will ever enter the scene.
I may have missed your essential point. Did I also overlook the definition of your letters DIS?
Regards, Dieter Zeh