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.
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.
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.
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.
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.
The ideas in Heisenberg's paper on quantum mechanics in 1925 mark the beginning not only of a new phase in modern physics, but also of Heisenberg's own philosophical journey. This thesis examines that journey between 1925 and the late 1950s by situating Heisenberg's philosophy of quantum mechanics in the context of his encounters with his contemporaries as well as the context of various strands of thought in the German-speaking world at the time. Heisenberg's early philosophical critique of the 'classical' viewpoint between 1925 and 1927 bears the decisive influence of Einstein's theory of relativity, more specifically, the positivism he saw as underpinning Einstein's emancipation from Newtonian physics. The positivist influence on Heisenberg's early attitude to quantum mechanics is evident in three ways: (a) his invocation of an observability principle in 1925 to justify the renunciation of the concept of the electron orbit, (b) an instrumentalist conception of understanding, which characterised Heisenberg's response to Schrodinger's demand for classical visualisation in space and time in 1926-7, and (c) the introduction of an operational definition of concepts such as position and velocity in 1927, in an attempt to replace the concepts of classical physics. But after discussions with Bohr and Einstein in 1926-7, Heisenberg soon recognised what we might term his `empiricist' viewpoint was problematic. In 1927 Heisenberg's thought undergoes a shift away from the `empiricist' viewpoint that had underpinned his early philosophy of quantum mechanics. The nature and scope of this transformation, which forms the central theme in this thesis, has, up until now, been poorly understood and often completely neglected. Through his discussions with Bohr, Heisenberg came to the realisation that despite their limitations, classical concepts were conditions for the possibility of the description of all experience. This marked the abandonment of his earlier attempt to replace classical concepts with quantum concepts. The recognition of the primacy of classical language forms the point of departure for much of Heisenberg's later thought, which brought him into contact with the attempts in the German-speaking world in the 1920s to reconstruct Kantian epistemology. By the mid-1930s, Heisenberg advocated a 'pragmatic transformation' of Kantian philosophy, in which classical concepts were held to be a priori in the sense that they remained the conditions for the possibility of experience, but were no longer held to be necessary or universal in a strict Kantian sense. After 1940 Heisenberg saw the paradoxes of quantum mechanics under the aegis of what can be termed a 'transcendental conception of language', according to which language is not a mere tool, but actively shapes, gives form, and objectifies, our 'reality'. The limits of a classical 'description' in quantum mechanics therefore came to signify for Heisenberg, the limits of 'objective reality'. While Bohr exerted perhaps the most important philosophical influence on Heisenberg, their intellectual relationship was characterised by disagreement and misunderstanding. This is most strikingly displayed in their respective views on wave-particle duality and complementarily. While after 1927 Heisenberg accepted Bohr's basic insight that our knowledge of the quantum world is mediated through classical language, he did not share Rohr's interpretation of complementarity. While Heisenberg certainly used terms such as 'complementarity' and wave-particle duality' in his writings, a close reading reveals that these terms had very different meanings for the two physicists. This is particularly evident in the contrast between Heisenberg's notion of wave-particle equivalence and Bohr's idea of complementarity. In bringing to light these divergences between Bohr and Heisenberg, this thesis lends further weight to the view - already advocated by scholars such as John Hendry and Mara Beller - that the so-called Copenhagen interpretation of quantum mechanics should not be thought of as a unified philosophical position, but actually comprises a number of different strands.
The complex relation between quantum mechanics and classical mechanics is crucial in the philosophy of modern physics, and it cuts across current quantum physics. This paper is divided in two parts. In the first part I will offer a critical analysis of the role that decoherence plays in the account of the quantum-classical limit. In the second part I will mention three ways in which philosophers are engaging with the realist interpretation of quantum mechanics in light of the assessment that the problem of the quantum-classical limit is still open to debate. My main claim is that the problem of the quantum-classical limit is overrated and it receives too much attention for the realist who looks at quantum mechanics. The question that the realist wants to focus on is the crucial interpretation question: what is a quantum system?
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.