How and why the wave function collapses after a measurement
  We explain the collapse of the wavefunction with the notion that, in a measurement, the system observed nucleates a first order phase transition in the measuring device. The possible final states differ by the values of macroscopic observables, and their relative phase is therefore unobservable. The process is irreversible, but needs no separate postulate. 
Massimo Fioroni and Giorgio Immirzi