The real great achievement of quantum mechanics is not its successful treatment of elementary systems, the basis for which was presented in VII, but its successful description of composite systems. According to VI, D 2.1 the representation of the Galileo group in Hilbert space for composite systems is reducible. In addition, there exist decision effects E ⊂ G(ℋ) which are different from 0 and 1 which are left invariant under transformations of the Galileo group.
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