  Quantum measurement finds the observed system in a collapsed state, rather
than in the state predicted by the Schr\"odinger equation. Yet there is a
relatively spread opinion that the wavefunction collapse can be explained by
unitary evolution (for instance in the decoherence approach, if we take into
account the environment).
  In this article it is proven a mathematical result which severely restricts
the initial conditions for which measurements have definite outcomes, if pure
unitary evolution is assumed. This no-go theorem remains true even if we take
the environment into account. The result does not forbid a unitary description
of the measurement process, it only shows that such a description is possible
only for very restricted initial conditions.
  The existence of such restrictions of the initial conditions can be
understood in the four-dimensional block universe perspective, as a requirement
of global self-consistency of the solutions of the Schr\"odinger equation.
