Collapse of the wave function, aka at which point potentiality becomes an actuality. (But nothing in regards as to, for example, spin up was the result rather than spin down.)I see it a whole ensemble of problems revolving around quantum measurement, so I'm unsure what specific problem you have in mind. Knowing that detection events involve nonlinear EEIs is just one step toward a better understanding. If there is some specific problem which you consider "the measurement problem," I will try to be more specific.