In quantum mechanics we have the projection postulate saying that during a measurement the wave function of an observable collapse into a definite state. However, a friend of mine is convinced that there is no collapse, only linearity. After a measurement the wave function of an observable is still present, but is (for unresolved reasons) shifted towards a definite value. Hence, other outcomes become insignificant. Are there any findings that suggest that it really is such a thing as a definite state?