 Solution

The measurement problem arises from applying quantum dynamics to both a measured system and a physical apparatus. If $|0⟩|M_{\ast }⟩\to |0⟩|M_0⟩$ and $|1⟩|M_{\ast }⟩\to |1⟩|M_1⟩$, linearity maps a superposition to $\alpha |0,M_0⟩+\beta |1,M_1⟩$. That is generally not one selected apparatus record, although ordinary experience and the usual stochastic outcome rule assign a definite record in each run.

The conflict concerns the combination of a complete wavefunction description, universal unitary evolution and a unique ordinary outcome. They cannot all be used naively in the indicated example. Declaring that a measurement collapses the state can be a practical predictive rule, but a complete physical account must say when that nonunitary rule applies and why the apparatus is exempt from the system's ordinary equation.

Decoherence helps by correlating records with a large environment and suppressing observable interference between them. It explains approximate stability of record bases and why coherent reversals are hard, but the reduced density matrix is an improper mixture arising from an entangled state. Suppression of off-diagonal terms alone does not select one global component.

Physicists can make excellent laboratory predictions while leaving this issue open. It still matters when extending the theory to universal systems, precision tests of collapse, or claims about what the state represents. Responses include new configurations and guidance, modified stochastic dynamics or branch-relative outcomes in a unitary theory. These differ in what they assert physically and sometimes in additional testable effects. The problem is therefore not simply ignorance of which detector clicked, but the relation between a purportedly universal law and the definite evidence used to test it.