Abstract

Hugh Everett III describes a quantum measurement as resulting in the “branching” of the quantum state of observer and measured system, with all possible measurement outcomes represented by the ensuing branches of the total quantum state. But Everett does not specify a general rule for decomposing a quantum state into branches, and commentators have long puzzled over how, and even whether, to regard Everett׳s notion of branching states as physically meaningful. It is common today to appeal to decoherence considerations as a way of giving physical content to the Everettian notion of branches, but these appeals to decoherence are often regarded as considerations foreign to Everett׳s own approach. This paper contends that this assessment is only half right: though he does not invoke environmental decoherence, Everett does appeal to decoherence considerations, broadly understood, in his treatment of measurement. Careful consideration of his idealized models of measurement, and of the significance he ascribes to the branching of states corresponding to definite measurement outcomes, reveals that his notion of branching refers to a special physical characteristic of elements of a particular decomposition, namely the absence of interference between these component states as a result of the particular dynamics governing the evolution of the system. Characterizations of branching that appeal to the results of modern decoherence theory should therefore be regarded as a natural development of Everett׳s own physically meaningful conception of branching.