  Different attempts to solve the measurement problem of the quantum mechanics
(QM) by denying the collapse principle, and replacing it with changes in the
quantum formalism, failed because the changes in the formalism lead to
contradictions with QM predictions. To the difference, Ghirardi, Rimini and
Weber took the collapse as a real phenomenon, and proposed a calculus by which
the wave-function should undergo a sudden localization. Later on, Ghirardi,
Pearle and Rimini came with a change of this calculus into the CSL (continuous
spontaneous localization) model of collapse. Both these proposals rely on the
experimental fact that the reduction of the wave-function occurs when the
microscopic system encounters a macroscopic object and involves a big amount of
its particles. Both of them also change the quantum formalism by introducing in
the Schrodinger equation additional terms with noisy behavior. However, these
terms have practically no influence as long as the studied system contains only
one or a few components. Only when the amount of components is very big, these
terms become significant and lead to the reduction of the wave-function to one
of its components. The present work has two purposes: 1) proving that the
collapse postulate is unavoidable; 2) applying the CSL model to the process in
a detector and showing step by step the modification of the wave-function,
until reduction. As a side detail, it is argued here that the noise cannot
originate in some classical field, contrary to the thought/hope of some
physicists, because no classical field is tailored by the wave-functions of
entanglements.
