Niels Bohr famously insisted on the indispensability of what he termed “classical concepts.” In the context of the decoherence program, on the other hand, it has become fashionable to talk about the “dynamical emergence of classicality” from the quantum formalism alone. Does this mean that decoherence challenges Bohr’s dictum—for example, that classical concepts do not need to be assumed but can be derived? In this paper we’ll try to shed some light down the murky waters where formalism and philosophy cohabitate. To begin, we’ll clarify the notion of classicality in the decoherence description. We’ll then discuss Bohr’s and Heisenberg’s take on the quantum—classical problem and reflect on different meanings of the terms “classicality” and “classical concepts” in the writings of Bohr and his followers. This analysis will allow us to put forward some tentative suggestions for how we may better understand the relation between decoherence‐induced classicality and Bohr’s classical concepts.
Niels Bohr famously insisted on the indispensability of what he termed “classical concepts.” In the context of the decoherence program, on the other hand, it has become fashionable to talk about the “dynamical emergence of classicality” from the quantum formalism alone. Does this mean that decoherence challenges Bohr’s dictum—for example, that classical concepts do not need to be assumed but can be derived? In this paper we’ll try to shed some light down the murky waters where formalism and philosophy cohabitate. To begin, we’ll clarify the notion of classicality in the decoherence description. We’ll then discuss Bohr’s and Heisenberg’s take on the quantum—classical problem and reflect on different meanings of the terms “classicality” and “classical concepts” in the writings of Bohr and his followers. This analysis will allow us to put forward some tentative suggestions for how we may better understand the relation between decoherence‐induced classicality and Bohr’s classical concepts.
Niels Bohr famously insisted on the indispensability of what he termed "classical concepts." In the context of the decoherence program, on the other hand, it has become fashionable to talk about the "dynamical emergence of classicality" from the quantum formalism alone. Does this mean that decoherence challenges Bohr's dictum -- for example, that classical concepts do not need to be assumed but can be derived? In this paper, we'll try to shed some light down the murky waters where formalism and philosophy mingle. To begin, we'll clarify the notion of classicality in the decoherence description. We'll then discuss Bohr's and Heisenberg's takes on the quantum-classical problem and reflect on the different meanings of the terms "classicality" and "classical concepts" in the writings of Bohr and his followers. This analysis will allow us to put forward some tentative suggestions for how we may better understand the relation between decoherence-induced classicality and Bohr's classical concepts.
It is now widely accepted that environmental entanglement and the resulting decoherence processes play a crucial role in the quantum-to-classical transition and the emergence of "classicality" from quantum mechanics. To this extent, decoherence is often understood as signifying a break with the Copenhagen interpretation, and in particular with Bohr's view of the indispensability of classical concepts. This paper analyzes the relationship between Bohr's understanding of the quantum-classical divide and his doctrine of classical concepts and the decoherence-based program of emergent classicality. By drawing on Howard's reconstruction of Bohr's doctrine of classical concepts, and by paying careful attention to a hitherto overlooked disagreement between Heisenberg and Bohr in the 1930s about the placement of the quantum-classical "cut," we show that Bohr's view of the quantum-classical divide can be physically justified by appealing to decoherence. We also discuss early anticipations of the role of the environment in the quantum-classical problem in Heisenberg's writings. Finally, we distinguish four different formulations of the doctrine of classical concepts in an effort to present a more nuanced assessment of the relationship between Bohr's views and decoherence that challenges oversimplified statements frequently found in the literature.
Although microscopic systems are described by means of quantum mechanics, the outcomes obtained when performing measurements on such systems “appear” to us as being “classical” (Giulini et al. 2003). This situation is puzzling. How does the classical appearance of these measurement outcomes emerge (problem A)? And why, after all, couldn’t we describe these measurement outcomes by means of quantummechanics (problem B)? Problem A can be tackled within the domain of physics. As a matter of fact, it has been partly solved by means of decoherence theory (Zurek 1981, 1982, 1991; Blanchard et al. 2000; Joos et al. 2003). Problem B falls under the competency of epistemology, as it deals with the respective role of classical physics and quantum mechanics. Concerning this topic of the quantum- classical cut, the contribution of Bohr and especially his thesis of the necessary use of classical concepts have been very influential during the twentieth century. Many scholars in the past have examined Bohr’s writings concerning this thesis (Folse 1985; Honner 1987; Murdoch 1987; Faye 1991; Chevalley 1991; Scheibe 1993). Recently, this thesis has been discussed in relation to decoherence theory. More precisely, Camilleri and Schlosshauer (2015) asked if Bohr’s thesis is challenged by decoherence theory. Their inquiry leads them to conclude that this is not the case. In other words, Bohr’s viewis still defendable. This chapter aims at showing that Bohr’s view is not only defendable but can throw light on the problems associated with the quantum- classical cut. First, I will recall that Bohr’s way of dealing with this cut is not ontological but epistemological. I will then put forward four conceptual distinctions and argue that those are essential to understand and discuss Bohr’s thesis of the necessary use of classical concepts. Finally, on the basis of this clarification, the relation between Bohr’s thesis and decoherence theory will be discussed.
Niels Bohr's philosophical view of quantum mechanics has been the subject of extensive scholarship for the better part of five decades. Yet Bohr’s writings have remained obscure, as evidenced by the variety of different scholarly interpretations of his work. In this chapter, I review the historiography of Bohr scholarship, arguing that his meaning has remained elusive because his central preoccupations lay not so much with an interpretation of the quantum-mechanical formalism, which many commentators see as the problem of quantum theory, but rather with the epistemological question of how we can acquire empirical knowledge of quantum objects by means of experiment. Bohr’s doctrine of classical concepts, I argue, is therefore best understood as a philosophy of experiment.
Conceptual and formal problems Many of the problems surrounding the interpretation of quantum mechanics stem from the absence of a sufficiently general and physically transparent analysis of the connection with the domain in which classical concepts are assumed to apply. It is the purpose of this chapter to describe with the aid of a few examples how the quantum potential approach provides a context within which the conceptual and formal issues raised by this problem may be clarified. A key question is to explain how the state or context dependence which is so characteristic of the quantum domain turns into state independence at a certain level, characterized as ‘classical’. To illustrate one aspect of the conceptual problem we recall that in Bohr's interpretation the validity of classical concepts is already presupposed since, it is suggested, it is only in terms of these that one can unambiguously communicate the results of experiments in the quantum domain. Thus, classical physics must be considered as prior to quantum mechanics and the latter is a generalization of the former in that it provides a new set of laws governing the application of classical concepts. According to this view, any procedure by which classical mechanics is recovered from quantum mechanics as a mathematical limit can only be a demonstration of consistency with the already postulated epistemological relation between the two theories and not as a ‘derivation’ of classical mechanics from quantum mechanics (George, Prigogine & Rosenfeld, 1972).
It has often been remarked that Bohr's writings on the interpretation of quantum mechanics make scant reference to the mathematical formalism of quantum theory; and it has not infrequently been suggested that this is another symptom of the general vagueness, obscurity and perhaps even incoherence of Bohr's ideas. Recent years have seen a reappreciation of Bohr, however. In this article we broadly follow this "rehabilitation program". We offer what we think is a simple and coherent reading of Bohr's statements about the interpretation of quantum mechanics, basing ourselves on primary sources and making use of and filling lacunas in|recent secondary literature. We argue that Bohr's views on quantum mechanics are more firmly connected to the structure of the quantum formalism than usually acknowledged, even though Bohr's explicit use of this formalism remains on a rather global and qualitative level. In our reading, Bohr's pronouncements on the meaning of quantum mechanics should first of all be seen as responses to concrete physical problems, rather than as expressions of a preconceived philosophical doctrine. In our final section we attempt a more detailed comparison with the formalism and conclude that Bohr's interpretation is not far removed from present-day non-collapse interpretations of quantum mechanics.