Quantum measurement problem

In classical mechanics a measurement process can be represented, in principle, as an interaction between two systems, a measuring instrument M and a measured system S, during which the classical states of M and S evolve dynamically, according to the equations of motion of the theory, in such a way that the ‘pointer’ or indicator quantity of M becomes correlated with the measured quantity of S. If a similar representation is attempted in quantum mechanics, it can be shown that, for certain initial quantum states of M and S, the interaction will result in a quantum state for the combined system in which neither the pointer quantity of M nor the measured quantity of S has a determinate value. On the orthodox interpretation of the theory, propositions assigning ranges of values to these quantities are neither true nor false. Since we require that the pointer readings of M are determinate after a measurement, and presumably also the values of the correlated S-quantities measured by M, it appears that the orthodox interpretation cannot accommodate the dynamical representation of measurement processes. The problem of how to do so is the quantum measurement problem.