From Data to Quanta

Niels Bohr's Vision of Physics

Slobodan Perovic

• Publisher: University of Chicago Press
• Year: 2021
• ISBN:
  9780226798479
• Topic: Biowissenschaften
• Subtopic: Wissenschaft Allgemein

The first comprehensive philosophical and historical account of the experimental foundations of Niels Bohr's practice of physics. Niels Bohr was a central figure in quantum physics, well known for his work on atomic structure and his contributions to the Copenhagen interpretation of quantum mechanics. In this book, philosopher of science Slobodan Perovi? explores the way Bohr practiced and understood physics, and analyzes its implications for our understanding of modern science. Perovi? develops a novel approach to Bohr's understanding of physics and his method of inquiry, presenting an exploratory symbiosis of historical and philosophical analysis that uncovers the key aspects of Bohr's philosophical vision of physics within a given historical context.To better understand the methods that produced Bohr's breakthrough results in quantum phenomena, Perovi? clarifies the nature of Bohr's engagement with the experimental side of physics and lays out the basic distinctions and concepts that characterize his approach. Rich and insightful, Perovi?'s take on the early history of quantum mechanics and its methodological ramifications sheds vital new light on one of the key figures of modern physics.

1: Introduction

Controversy

It is puzzling that, despite Niels Bohr’s exceptional prominence in the history of quantum physics and modern physics more generally, his physical concepts have often seemed unpolished, even misleading, to many physicists, historians, and philosophers of physics. As we will see shortly in more detail, this sort of attitude and Bohr’s controversial reputation among these specialists is still surprisingly widespread. Resolving this puzzle motivated me in part to write this book, and understanding it helped me focus on some key aspects of Bohr’s work. Although the comprehensive understanding of Bohr’s work offered in the following pages should resolve the puzzle, the book far exceeds this initial motivation and sheds light on the work of one of the key figures of modern physics.

The book develops a novel approach to Bohr’s understanding of physics and method of inquiry. My aim is an exploratory symbiosis of historical and philosophical analysis that uncovers the key aspects of Bohr’s philosophical vision of physics within a given historical context. I argue that his vision was largely driven by his endeavor to develop a comprehensive perspective on novel experimental work, and his gradually developing accounts of the main features of experimentation. I will show that his distinctive research contributions were characterized by a multilayered or phased approach of building on basic experimental insights inductively in order to develop intermediary and then overarching (master) hypotheses. The strengths and limitations of this approach made him a thoroughly distinctive kind of physicist who ought to be investigated in a cross-disciplinary manner. I offer one such endeavor.

In my historical analysis, I focus mostly on Bohr’s philosophical grasp of physics as it was driven by his practice during the early period of his work (roughly until the mid-1930s) when he developed his milestone contributions, while touching on a later period, substantially more removed from his actual “lab practice” and often addressed to a wider nonspecialist audience. The analysis highlights the crucial importance of experiment in his work, often in the form of “principles” based on experiment. It recognizes an important methodological coherence underlying Bohr’s approach. This is ultimately more important in understanding his vision of physics than any definite philosophical stance we may try to identify in his work; Bohr’s methodological concerns were prioritized throughout his career over more abstract epistemological and metaphysical concerns, as his famous debate with Albert Einstein demonstrates. When he explicitly addressed these further concerns, he did so by adding a final and, given the historical context, rather uncontroversial layer to his theoretical accounts. Finally, this methodological coherence is particularly apparent if viewed within the context of the community of quantum physicists who harbored distinctively different methodological approaches to quantum phenomena.

Philosophically, I develop an account of the relations between theory and experiment that prioritizes a semi-inductive (inductive-hypothetical) approach that shaped Bohr’s practice. In such an approach we recognize stages or layers of hypotheses of different levels of generality, starting from the basic experimental ones. The basic experimental hypotheses defined by everyday language and notions of classical physics remain foundational; but what counts as a general master-level hypothesis harboring novel potentially nonclassical concepts is subject to reevaluation with new incoming experimental knowledge, which often ends up reducing it to a supporting role. In fact, throughout the course of his work, Bohr reassessed various features of both forms of hypotheses through the connection and increasing distinction asserted by the correspondence principle as an intermediary hypothesis during the formation of early quantum theory—and then, later on, during the formation of quantum mechanics, by the uncertainty principle.

When it comes to his main contributions, in short, Bohr’s model of the atom introduced an ambitiously general theoretical approach synthesizing diverse research endeavors, while the correspondence principle was more speculative, and was intended as a forward-facing methodological device attempting to link rational theory and experimental work. During the development of quantum mechanics about a decade later, complementarity was developed as a synthetic perspective embedded in the experiments, primarily to explicate the limits and relationship between novel and diverse formal approaches and methods.

Yet before we fully unpack this account, let us return to the puzzle I sketched above, as an intriguing but a useful entry point to an understanding of Bohr’s vision and practice of physics.

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Bohr’s peers considered him, “by tacit consent . . . the leader towards whom all turned for guidance and inspiration” (Rosenfeld in Bohr 1972–2008, vol. 1, xxxi). Yet some aspects of his work were vigorously criticized by some of the most prominent physicists of the era, such as Erwin Schrödinger and Einstein. Although the view of the microphysical world Bohr advocated in the mid- and late 1920s under the auspices of his principle of complementarity appears to have been closely tied to the experimental advances of his day, Schrödinger and Einstein were very reluctant to embrace it, seeing it as an obscure attempt to reconcile mutually contradictory concepts of particles and waves. And Bohr’s precomplementarity correspondence principle concerning classical and quantum states (Bohr 1913a, 1922a), though central to the so-called old quantum theory, initially created a similar controversy, and was seen by some as embracing the essentially conflicting features of quantum and classical mechanics. Moreover, his breakthrough model of the atom (Bohr 1913a, 1913c, 1913d) was criticized throughout the period and, in fact, has been criticized ever since for discarding the spatial continuity of physical processes by introducing the “quantum jumps” of electrons from one discrete energy state (i.e., an orbit around the atomic nucleus) to another.

Yet these early criticisms pale in comparison to the more comprehensive ones developed by philosophers, historians, and physicists over the last several decades. For instance, Bohr’s complementarity principle has been deemed an obscurantist account inherently open to diverse and mutually exclusive interpretations, and his approach to quantum phenomena has been judged an unprincipled imposition of his dogmatic metaphysical preferences on dissenters (e.g., Beller 1999, 1997, 1992; Bitbol 1996, 1995; Bub 1974).

Some authors have made determined efforts to debunk the principle of complementarity. James T. Cushing (1994) regards it as mostly empty rhetoric that operates through intellectual intimidation, while Jeffrey Bub (1974) argues that the complementarity principle is akin to Immanuel Kant’s or Ludwig Wittgenstein’s ultimately impenetrable philosophy. While Bohr’s complementarity “endows an unacceptable theory of measurement with mystery and apparent profundity, where clarity would reveal an unsolved problem” (ibid., 46), “Bohr’s contribution to the Copenhagen Interpretation” is “that of a remarkably successful propagandist” (ibid., 45). This harsh ideologue in the quantum physics community saw “the statistical relations of quantum mechanics as the confirmation of an approach to the problem of knowledge that had fascinated him since youth” (ibid.), and imposed it on others.1 For his part, Imre Lakatos (1970) saw the continuous development of Bohr’s model of the atom as a prime example of a degenerating research program; others followed suit, deeming the model inherently inconsistent (Jammer 1966).2

The physicist John Bell (2001, 197), well known for his foundational theorem in quantum mechanics, says:

Rather than being disturbed by ambiguity in principle . . . Bohr seemed to take satisfaction in it. He seemed to revel in contradictions, for example between “wave” and “particle,” that seem to appear in any attempt to go beyond the pragmatic level. Not to resolve these contradictions and ambiguities, but rather to reconcile us to them, he put forward a philosophy, which he called “complementarity.”

Harsh words indeed, from arguably the most important figure in the post–World War II physics of the foundations of quantum mechanics.

The litany of complaints continues to this day. More recently, the harsh criticism reached more popular venues like Forbes magazine, where a prominent science journalist, Chad Orzel, (2015) stated, “Bohr is a pretty bad example of philosophy in physics, as he was maddeningly vague and a horribly unclear writer.” His harsh assessment of Bohr’s work and his impact on physics is aligned with that of Tim Maudlin (2018): “The obscurantism of Bohr and Heisenberg, which became known first as the ‘Copenhagen Interpretation’ and in its later incarnations as ‘Shut Up and Calculate,’ is a self-conscious abdication of the aim of physics, namely, to understand the nature of the physical world.” In his latest, introductory book on quantum mechanics (Maudlin 2019), Maudlin deliberately omits Bohr’s work and the Copenhagen Interpretation, as it does not meet the standards of a physical theory that “clearly and forthrightly address[es] two fundamental questions: what there is, and what it does” with “sharp mathematical description” and “dynamics . . . by precise equations describing how ontology will evolve” (ibid., xi). Instead of informing the reader of the details of Bohr’s work, Maudlin refers his reader to previous unforgiving criticisms of it, as “our time is better spent presenting what is clear than decrying what is obscure” (ibid.).

Similar sentiments are echoed in a historical analysis aimed at a wider audience; Adam Becker’s 2018 book provides an intriguing counterfactual history of quantum mechanics based in part on these sentiments.3 And we should mention a prominent philosopher of science, namely David Albert, who recently offered an acerbic, albeit entertaining, version of an attitude found in the philosophy of the physics community, in a podcast series run by the outspoken theoretical physicist Sean Carroll.4

This sort of criticism and the impression it generates must be set against Bohr’s stature among his contemporaries, if we agree that it is “a distortion to see Bohr’s views as basically stemming from an a priori philosophical background” (Dieks 2017, 307). After his first big breakthrough on the model of the atom in 1913, Bohr quickly assumed a central role in the quantum physics community. Paul Ehrenfest’s comment to a young physicist in 1929 convincingly attests to his status: “Now you are going to get to know Niels Bohr and that is the most important thing to happen in the life of a young physicist” (Casimir 1968, 109).

What happened to Bohr’s reputation? Why are we now getting such a different reaction to his work? And how is this relevant to our understanding of his physics and its methods?

The skeptical attitudes to Bohr’s work are diverse—voiced by specialists on the history and philosophy of quantum mechanics, various philosophers of physics, philosophers of science, physicists, science journalists, and so on—but many are an unfortunate consequence of the fact that both critics and defenders (e.g., Howard 2007; Landsman 2006; Chevalley 1994; Faye 1991; Murdoch 1987) too often focus on the search for an exact metaphysical or epistemological account to which they think Bohr may have subscribed and which, in turn, may have shaped his major contributions to physics. I should note that, in general, just as harsh criticism often deflects the analysis from what I consider the central aspect of Bohr’s work—his methodology—so too does the enthusiasm of certain philosophers who rush to ascribe to Bohr certain metaphysical or epistemological views. Bohr’s vision of physics has been identified as Kantian (Bitbol 2017; Cuffaro 2010; Chevalley 1994; Kaiser 1992), a transcendental research program (Bitbol 2017), an account of relational holism (Dorato 2017), or an account ripe with semantic and metasemantic implications (Osnaghi 2017). Parallels have recently been drawn between his work and Pragmatism in philosophy (Faye 2017; Heilbron 2013, 33), and even between his ontological views and the religious views of Søren Kierkegaard and those in the Talmud (Clark 2014). Also, over the last few decades many authors have concentrated on side issues in Bohr’s work, focusing, for example, on his free-thinking mature phase, which often veered from a concrete experimental context but is philosophically intriguing. Moreover, leading physicists both sympathetic to and skeptical of Bohr’s work (typically his later work) have insisted on Bohr’s wholly spontaneous, as it were, and purely intuitive process of generating ideas, sometimes calling it a divine glance (Kramers 1935, 90).

Analyses of this sort are often valuable for understanding Bohr’s overall vision of the physical world and related philosophical issues if we approach them cautiously. David Favrholdt (1992), by contrast, has articulated an opposing and perhaps exaggerated view, saying that since Bohr never studied philosophy systematically, his contributions stem from his physics alone (Heilbron 2013, 33). As we will see in due course, however, it is not always easy or desirable to try and disentangle physics from philosophical concerns, particularly epistemological and methodological concerns, especially when studying the emergence of a completely new theory. Yet there is something more fundamental at the core of Bohr’s approach to physics that many philosophical interpreters and critics have missed—something other than what his occasional intellectual flourishes, musings, and wide-ranging elaborations of microphysical phenomena suggest if they are read apart from his practice of physics. Philosophers have often focused on these, but they are often a sideshow, especially when isolated from the rest of his work. The story of Bohr’s method and the vision behind it may be disappointingly conventional in comparison.

In that sense, my account of Bohr’s work is deflationary. A number of important philosophical influences on various aspects of, and stages, in Bohr’s work can certainly be identified,5 but the core of what Bohr did as a physicist is, generally speaking, a pretty standard experiment-driven inductive approach, a particular strand that he perfected and improved in the context of founding quantum theory and quantum mechanics. And as Dennis Dieks (2017, 303) recently emphasized, Bohr’s works read very differently from “the tradition of foundational work that started in the early 1950s.” One remarkable aspect of his approach is that he consciously and uncompromisingly stuck to it wherever it led him, including renouncing some of the key aspects of what came to be recognized as classical physics, as well as the principles other prominent physicists deemed inescapable, even though he was fully aware of potential losses entailed by such moves.

Instead of taking the metaphysical and epistemological background as the starting point for an analysis of Bohr’s work, I undertake a historically sensitive philosophical analysis of the method that produced his breakthrough results. We primarily need to understand Bohr’s method, the exact role Bohr played in the physics community, and the reasons for both. Only then can we properly judge his accomplishments and end the controversy. I also expect the story about Bohr’s vision of physics to clarify for philosophers of science interested in methodological questions how exactly the repairs on the sailing ship of science were made, to use a well-worn phrase, during this major episode in the history of science, and thus to enrich their current inquires (e.g. what constitutes the basic experimental level of the inductive process, how the much discussed notion of “bridge laws” relates to the intermediary hypotheses crucial for the inductive process pursued by Bohr and others, the nature of modelling during the crafting of the quantum theory, and so on).

As the above brief summary suggests, there are two ways of approaching Bohr’s work. First, we can attempt to understand his accounts of the physical world with the help of known and well-developed philosophical terms and doctrines. This doctrinal way is often favored by philosophers’ attention. Second, we can try to understand his underlying approach to physical states and processes, and establish whether certain unifying principles or heuristics underlie it. The latter methodological kind of analysis can be pursued fairly independently of the former,6 yet in my view it should ground the former, not the other way around.

The main danger of the first way is in reading preferred philosophical terms and doctrines into Bohr’s account while distorting or even ignoring the methodological understanding. Moreover, the debates among physicists and their underlying epistemological and ontological views were in flux when Bohr produced his most important results. The physicists understandably kept crafting their positions, giving them up, changing them. Typically, only rough and barely adequate distinctions can be identified within such evolving understandings when assessed by the usual conceptual machinery of contemporary philosophy, despite our best efforts to “nail them down.” In Bohr’s case, however, the key general methodological understanding—the understanding of how exactly one ought to craft physical theory—remained stable, in contrast to, for example, Arnold Sommerfeld’s methodology, which substantially changed between different periods of his work (Seth 2010). It is possible, of course, that there are no underlying unifying principles or heuristics and no coherent approach to physical states and process throughout Bohr’s work, and that the doctrinal understanding may be the only way to understand him. But my analysis suggests otherwise.

Bohr’s later elaborate metaphysical and epistemological considerations were typically derivative of his scientific practice and his reflections on the immediate method that shaped it. Yet sometimes, especially in his mature phase, they were fairly removed from it and targeted a wider nonspecialist audience. Therefore, it is crucial to understand properly the connection between Bohr’s development of his epistemological attitude to scientific research, the metaphysical ideas he occasionally commented on, and the actual method of finding out about microphysical processes he gradually developed and excelled in. In fact, for most of his active career the methodological concerns and their tangible epistemic ramifications that were close to the experimental context were much more prominent and fundamental than the abstract epistemological and ontological considerations in his reflections. Bohr’s work is certainly more “indicative of a physicist’s rather than a philosopher’s attitude” (Dieks 2017, 307) if we are thinking of a professional “philosopher” of the last hundred years or so. Yet Bohr’s attitude was shared by only some members of the physics community at the time; as we will see in due course, others pursued physics in a much more philosophical (in the above sense) or mathematically driven manner.

Bohr’s Vision and Practice of Physics

The primary goal of this book is to trace the structure of Bohr’s approach to physics by drawing on numerous historical studies. Its secondary goal is to assess critically a broad array of views of Bohr, which are often critical and often, as we have seen, portray him as a physicist who bullishly pursued his prior, quite contentious metaphysical and epistemological views.

The main goal of this book, that of advancing a methodological understanding of Bohr’s work, is pursued through a historically motivated philosophical assessment of the scientific method as a constant and vigorous interplay between theory and experiment in the early development of quantum physics. Generally speaking, it is a case study that identifies the crucial traits of the experimentalist strand in modern physics prior to its transition to the industrial-scale science of the second half of the twentieth century, on the one hand, and to mathematically driven abstract th...