  We derive the projection hypothesis in projective quantum measurement by
restricting the set of observables. This projection hypothesis accompanies a
bipartite system with the von Neumann-type interaction, which consists of a
quantum mechanical system, with a meter variable to be measured, and a quantum
field theoretically macroscopic extended object, that is, a spatiotemporally
inhomogeneous Bose-Einstein condensate in quantum field theory with the quantum
coordinate, that is, the zero-energy Goldstone mode(s) of the spontaneously
broken global spatial translational symmetry. The key steps in the derivation
are the return of the symmetry translation of this quantum coordinate to the
inverse translation of the c-number spatial coordinate in quantum field theory
and the reduction of quantum fluctuations to classical fluctuations with
respect to the Goldstone mode(s) due to a superselection rule.
