In Shankars "Principle of Quantum Mechanics" in Chapter 4, page 122, he explains what the "Collapse of the State Vector" means. I get that upon measurement, the wave function can be written as a linear combination of the eigenvectors belonging to a operator which corresponds to the measurement (because the operators are hermitian and their eigenvectors form a basis). But then he goes on to say, that the measurement forces the state vector [itex]|\psi>[/itex] into the eigenstate [itex]|\omega>[/itex]. But HOW does the operator do that? I can't find a single equation which describes the collapse of the wave function. So my question is, what happens mathematically behind the scenes?