The Role of Decoherence in Quantum Mechanics

Guido Bacciagaluppi

Research output: Chapter in Book/Report/Conference proceedingEntry for encyclopedia/dictionary


Interference phenomena are a well-known and crucial aspect of quantum mechanics, famously exemplified by the two-slit experiment. There are situations, however, in which interference effects are artificially or spontaneously suppressed. The theory of decoherence is precisely the study of (spontaneous) interactions between a system and its environment that lead to such suppression of interference. We shall make more precise what we mean by this in Section 1, which discusses the concept of suppression of interference and gives a simplified survey of the theory, emphasising features that will be relevant to the following discussion. In fact, the term decoherence refers to two largely overlapping areas of research. The characteristic feature of the first (often called ‘dynamical’ or ‘environmental’ decoherence) is the study of concrete models of (spontaneous) interactions between a system and its environment that lead to suppression of interference effects. That of the second (the theory of ‘decoherent histories’ or ‘consistent histories’) is an abstract (and in fact more general) formalism that captures the essential features of the phenomenon of decoherence. The two are obviously closely related, and will both be reviewed in turn in Section 1.

Decoherence is relevant (or is claimed to be relevant) to a variety of questions ranging from the measurement problem to the arrow of time, and in particular to the question of whether and how the ‘classical world’ may emerge from quantum mechanics. This entry mainly deals with the role of decoherence in relation to the main problems and approaches in the foundations of quantum mechanics. Specifically, Section 2 analyses the claim that decoherence solves the measurement problem. It also discusses the exacerbation of the problem through the inclusion of environmental interactions, the idea of emergence of classicality, and the motivation for discussing decoherence together with approaches to the foundations of quantum mechanics. Section 3 then reviews the relation of decoherence to some of the main foundational approaches. Finally, in Section 4 we mention suggested applications that would push the role of decoherence even further.

Suppression of interference has of course featured in many papers since the beginning of quantum mechanics, such as Mott's (1929) analysis of alpha-particle tracks. The modern foundation of decoherence as a subject in its own right was laid by H. D. Zeh in the early 1970s (Zeh 1970; 1973). Equally influential were the papers by W. Zurek from the early 1980s (Zurek 1981; 1982). Some of these earlier examples of decoherence (e.g., suppression of interference between left-handed and right-handed states of a molecule) are mathematically more accessible than more recent ones. A concise and readable introduction to the theory is provided by Zurek in Physics Today (1991). (This article was followed by publication of several letters with Zurek's replies (1993), which highlight controversial issues.) More recent surveys are the ones by Zeh (1995), which devotes much space to the interpretation of decoherence, Zurek (2003), and the books on decoherence by Giulini et al. (1996) and Schlosshauer (2007).
Original languageEnglish
Title of host publicationStanford Encyclopedia of Philosophy
Place of PublicationStanford
PublisherCenter for the Study of Language and Information, Stanford University
Publication statusPublished - 2012

Bibliographical note

Major revision of an entry originally published in 2003.


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