  The postulate of the collapse of the wave function stands between the
microscopic, quantum world, and the macroscopic world. Because of this
intermediate position, the collapse process cannot be examined with the
formalism of the quantum mechanics (QM), neither with that of classical
mechanics. This fact makes some physicists to propose interpretations of QM,
which avoid this postulate. However, the common procedure used in that, is
making assumptions incompatible with the QM formalism. The present work
discusses the most popular interpretations. It is shown that because of such
assumptions those interpretations fail, i.e. predict for some experiments
results which differ from the QM predictions. Despite of that, special
attention is called to a proposal of S. Gao, the only one which addresses and
tries to solve an obvious and major contradiction. A couple of theorems are
proved for showing that the collapse postulate is necessary in the QM. Although
nonexplainable with the quantum formalism this postulate cannot be denied,
otherwise one comes to conclusions which disagree with the QM. It is also
proved here that the idea of collapse at a distance is problematic especially
in relativistic cases, and is a misunderstanding. Namely, in an entanglement of
two quantum systems, assuming that the measurement of one of the systems
(accompanied by collapse of that system on one of its states) collapses the
other system too without the second system being measured, leads to a
contradiction.
