  The 'collapse' of the wave function in a general measuring process is
analyzed by a pure quantum mechanical (QM) approach. The problem of the delayed
choice and Welcher-Weg (WW) experiments is analyzed for Mach-Zehnder (MZ)
interferometer. The WW effect is related to complementarity principle and
orthogonality of wavefunctions although it produces small momentum changes
between the electromagnetic (EM) field and the beam-splitters (BS's). For QM
states for which we have a superposition of states and interference effects
there is no reality before measurement, but by excluding such effects the
system gets a real meaning already before measurement.

Quantum Physics