A history of entanglement: Decoherence and the interpretation problem☆
Introduction
The problem of explaining how the essentially ‘classical’ features of the macro-world emerge from a quantum theory of the micro-world has been one of the central problems for the interpretation of quantum mechanics since the 1930s. Yet recent developments in the study of the entanglement between quantum systems and their environments have shed new light on the emergence of ‘classicality’ in quantum mechanics. As Erich Joos explains: “The interaction with the environment which is in principle present for all systems seems to play a decisive role for an understanding of classical properties in the framework of quantum theory” (Joos, 1986, p. 12). This process, known as ‘environment-induced decoherence’ or simply ‘decoherence’, has been the subject of much recent interest and provides a new insight into explaining why quantum superpositions effectively disappear in the macroscopic world—in other words why we do not encounter ‘Schrödinger cats’. While there continues to be much debate and disagreement about the extent to which decoherence can explain the appearance of the ‘classical’ world, the destruction of interference resulting from quantum entanglement between a local subsystem and its environment is now an experimentally well-confirmed process (Brune et al., 1996; Munro, Meekhof, King, & Wineland, 1996; Raimond, Brune, & Haroche (1997), Raimond, Brune, & Haroche (2001); Chiorescu, Nakamura, Harmans, & Mooij, 2003; Il’ichev et al., 2003).
Decoherence is merely an application of standard quantum mechanics and therefore strictly speaking it should not be regarded as providing a new interpretation or a modification of the existing theory. However, the historical development and reception of decoherence has been closely connected with the debates over the interpretation of quantum theory. In this paper I examine the way in which two of the principal figures in the theoretical development of decoherence—H. Dieter Zeh and Wojciech Zurek—have understood this new development to bear upon the problem of interpretation.
In 1970 Zeh proposed that it is precisely the openness of quantum systems, i.e. their interaction with the environment, which is essential to explaining why in a unitary dynamics of the Schrödinger equation, the interference terms approximately vanish, in other words, why the ‘superposition’ of states effectively disappears in the macroscopic world. Zeh's work on the dynamics of quantum entanglement began from the assumption of the universality of the wave function, which invariably led him to an Everett-like relative-state interpretation. Thus, decoherence, as it would later become known, was for Zeh, from its inception an integral part of the Everett picture. As will become clear, this was one of the reasons that Zeh's early papers remained largely ignored by physicists for over a decade. In the early 1980s Wojciech Zurek published two important papers in which he derived the preferred basis and the superselection rules on the basis on environment-induced decoherence. While Zurek's work arose independently of Zeh's and differed in its formal approach, it represented a renewed effort to engage in some of the key physical ideas which had appeared in Zeh's early papers. In spite of the similarities in their approaches to decoherence, Zurek's programmatic aims for decoherence and its relationship to interpretational issues have diverged in certain crucial respects from those of Zeh.
One of the key aims of this paper is to show that the different views on the interpretation of quantum mechanics in the work of Zeh and Zurek can be traced to the different origins, motivations and philosophical perspectives, that guided their early work on decoherence. By contrast with Zeh, Zurek never seems to have been directly motivated by ‘interpretational’ issues—at least not to the same extent. By the 1990s Zurek would defend the idea that decoherence does not compel one to adopt the many worlds interpretation, but rather it can be seen as bringing about something of a rapprochement between Everett's and Bohr's views, which he termed the ‘existential interpretation’. While a critical analysis of the interpretational commitments of Zeh and Zurek is beyond the scope of this paper, it is my contention that Zurek's work contributed in no small part to the view, which began to win ascent among both theoretical and experimental physicists in the 1990s, that decoherence provides a ‘completion’ or even a ‘justification’ of the original Copenhagen interpretation. Thus, in contrast to Zeh's views that decoherence forces a radical departure from the Copenhagen orthodoxy, Zurek advanced the view that decoherence is compatible with, or even provides a confirmation of, many of Bohr's insights into quantum mechanics. To this extent, Zurek's work on decoherence can be understood as an attempt to provide a reconstruction of the original ‘Copenhagen interpretation’—reminiscent of Wheeler's attempt in the late 1950s—which serves to blur the traditional battle lines between ‘orthodox’ and ‘heterodox’ interpretations of quantum mechanics. Here we see the way in which different understandings of the Copenhagen interpretation have continued to play a role in debates over the interpretation of quantum mechanics.1 Finally, I argue that it is only by attending to the different interpretive frameworks in which Zeh and Zurek presented their work on decoherence that we can better understand why Zeh's work was largely ignored, while Zurek's work attracted considerable interest. Put simply, my claim is that in the case of decoherence, interpretation influenced reception.
It should be emphasised that it is not my aim to present an overview of the history of decoherence. Nor do I attempt to give a technical exposition of decoherence and its relationship to the measurement problem or the range of different interpretations of quantum mechanics that currently proliferate.2 A more exhaustive historical study of decoherence and its role in the debate over the interpretation of quantum mechanics would certainly need to go beyond the contributions of the two physicists treated in this paper. In particular the work of Anthony Leggett and Amir Caldeira should be acknowledged as having played a key role in the development of decoherence during the 1980s. Their papers on dissipation in quantum systems—in which they formulated the Caldeira–Leggett model—formed the basis for much of the later work on the dynamics of quantum entanglement, and have been extensively cited in the physics literature (Caldeira & Leggett (1981), Caldeira & Leggett (1983), Caldeira & Leggett (1985); Leggett, 1996). While I have paid only scant attention to their contributions here, it should be emphasised that their work was of decisive importance not only for the mathematical treatment and conceptual development of environment-induced decoherence, but also for the interpretational context. Here it is worth noting that Leggett's views on the consequences of decoherence for the interpretation problem, in particular his doubts over the validity of the quantum superposition principle in the macroscopic realm, stand in sharp contrast to the views expressed by both Zeh and Zurek (Leggett, 1984; Leggett & Garg, 1985; Whitaker, 1996, pp. 318–322). Notwithstanding the contributions of Leggett and others, the works of Zeh and Zurek provide an important point of departure for those interested in the history of decoherence and its relevance for the interpretation problem.
Section snippets
H. Dieter Zeh and the origins of decoherence
One can find anticipations of certain key ideas in decoherence, such as the suppression of interference effects, in the work of physicists going back to late 1920s. However, as Guido Bacciagaluppi (2007) points out: “The modern beginnings of decoherence as a subject in its own right are arguably the papers by H.D. Zeh of the early 1970s”. H. Dieter Zeh completed his Ph.D. on nuclear physics at Heidelberg University, and after spending time at Berkeley, Cal Tech, and La Jolla, he returned to
Wojciech Zurek and environment-induced decoherence
The 1970s saw little interest in Zeh's work. Indeed he recently labelled this period ‘the dark ages of decoherence’ (Zeh, 2006, p. 10). This period only came to end in the early 1980s with the publication of two important papers by Wojciech Zurek in Physical Review, which treated the problem of the preferred basis and the derivation of superselection rules from the perspective of environment-induced decoherence (Zurek (1981), Zurek (1982)). As Janssen explains, the claim that a preferred basis,
The dynamics of decoherence and the emergence of the classical world
The 1980s witnessed a renewed interest in the dynamics of entanglement and its consequences for decoherence of quantum systems. In particular, the papers of Caldeira and Leggett were instrumental in the development of new models based on quantum Brownian motion, for the study of the effect of the environment in dissipating quantum interference (Caldeira & Leggett (1981), Caldeira & Leggett (1983), Caldeira & Leggett (1985); Leggett, 1996).19
Zurek's ‘existential interpretation’: towards a rapprochement of Everett and Bohr
While Zeh remains resolutely ‘anti-Copenhagen’, defending an Everett interpretation,22 Zurek takes the view that decoherence brings certain aspects of the Everett interpretation into harmony with the Copenhagen interpretation. This is a view which finds its clearest
How a heterodoxy becomes an orthodoxy
By the 1990s the idea that decoherence can be regarded as a justification or completion of the Copenhagen interpretation began to find favour among a number of physicists. In his book The Interpretation of Quantum Mechanics published in 1994 Roland Omnès presented the newly emerging views on decoherence as “simply a modernized version of interpretation first proposed by Bohr in the early days of quantum mechanics” (Omnès, 1994, p. 498). This is presented in terms of a historical reconstruction
Acknowledgements
I would especially like to thank H. Dieter Zeh and Wojciech Zurek for their time and assistance in answering my questions through our email correspondence and for allowing me to quote from our correspondence. I would also like to thank H. Dieter Zeh and Fábio Freitas for granting me permission to use the transcript of the interview conducted in July 2008 in preparing this paper. Thanks must also go to Olival Freire for allowing me to use correspondence he obtained from the Wheeler papers at the
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An early version of this paper was presented at the second international conference of the History of Quantum Physics in Utrecht on 17 July 2008.