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Hugh Everett III proposed treating an observer and a measured quantum system within one unitary quantum dynamics. For an ideal quantum measurement, $\alpha |0⟩|R⟩+\beta |1⟩|R⟩$ evolves into $\alpha |0⟩|A_0⟩+\beta |1⟩|A_1⟩$: the apparatus records become correlated with the alternatives. A relative state describes the measured quantum system conditional on a particular record, while the joint quantum state remains entangled. His 1957 formulation sought to account for records within this common dynamics, without adding a separate dynamical wave function collapse postulate.^[ref]

A physicist who proposed a unitary relative-state account of quantum measurement, developed in the many-worlds interpretation.

Wave function collapse describes the conditional change of a quantum state associated with a measurement outcome. For an ideal projective measurement with projector $P_a$, the outcome probability is $p_a=\mathrm{Tr}(P_a\rho )$ and the conditional state is $P_a\rho P_a/p_a$ when $p_a>0$, according to the Lüders rule.

If the outcome is ignored, the state becomes ${\sum }_a{P}_a\rho P_a$, which is generally a mixture rather than any one conditional state. These are different information conditions. General measurement instruments can produce other state changes with the same outcome probabilities, so the ideal projection rule is a specified measurement model. Its physical interpretation is part of the quantum measurement problem.