Projection Postulate

In standard quantum theory, the projection postulate plays a crucial but controversial role: crucial, because standard quantum theory makes contact with physics and the results of experiments via the measurement axioms of quantum theory, the most important of which is the projection postulate; and controversial, because the projection postulate appears to conflict with Schrödinger’s equation. This apparent conflict is the notorious measurement problem of quantum mechanics, or, what amounts to the same thing, the paradox of ►Schrödinger’s cat. See also ►Bohmian mechanics; Measurement theory; Metaphysics in Quantum Mechanics; Modal Interpretation; Objectification.

A variety of proposals have been put forward for resolving the measurement problem. For many of these, whether in fact they do solve the problem remains highly controversial. Two proposals that clearly resolve the measurement problem are the ►GRW theory and the ►pilot-wave formulation of quantum mechanics (►Bohmian mechanics). In the former, collapse of the wave function during measurement is achieved, and the projection postulate recovered, by a stochastic modification of the Schrödinger dynamics on the microscopic level [3]. (See Consistent histories, Ignorance interpretation, Ithaca Interpretation, Many Worlds Interpretation, Modal Interpretation, Orthodox Interpretation, Transactional Interpretation).