They're talking about what the Copenhagen interpretation predicts, not what actually happens. It's a mathematical question, not an experimental one. Solutions

They're talking about what the Copenhagen interpretation predicts, not what actually happens. It's a mathematical question, not an experimental one. Solutions to the Schrodinger equation all take the form `Psi(t) = U(t)Psi(0)` for some unitary operator `U`. "Collapse" applies a projection operator, which is nonunitary.