Was ist und was sollen die Kopenhagener Deutung?

to find out what happens... when properties stop being virtual ... and start getting real

“Tiden gaaer, Livet er en Strøm, sige Menneskene, o. s. v. Jeg kan ikke mærke det, Tiden staaer stille og jeg med. Alle de Planer, jeg udkaster, flyve lige lukt tilbage paa mig selv; naar jeg vil spytte, spytter jeg mig selv i Ansigtet.”

• Søren Kierkegaard, Enten — Eller.

Contents

1. Introduction

2. Varieties of Naturalist Realism

   1. Sellars’ Concept of Scientific Realism

   2. Hacking’s Entity Realism

   3. Bohr’s Place Among the Realists

PAYWALL

3. Experimental Illustrations of Quantum Mechanics

   1. Structure and interpretation of a scattering event

   2. Kibble balance and the kilogram

4. Niels Bohr’s Fundamental Commitments

   1. The Analysis of Classical Concepts

   2. Complementarity as a Relation

   3. Correspondence Arguments

   4. Kinematic/Dynamic Duality

5. Conclusions

Introduction

Do you believe in the Copenhagen Interpretation of Quantum Mechanics? Whether you do or don’t - can you even say what it is? In the linked Wikipedia page, I count at least 13 distinct citations to the effect that nobody actually agrees what the (or a?) Copenhagen Interpretation of Quantum Mechanics is. And yet, this interpretation - or small cloud of interpretations - is predominant. Most quantum mechanics classes implicitly adopt a Copenhagen-style stance. If you include the bits of introductory physical chemistry which includes quantum mechanics, this approach to the interpretation of experience and experiment where the quantum of action is relevant is by a wide margin the vast majority of experience that humanity has with quantum theory.

This post is a journey into the Copenhagen Interpretation of Quantum Mechanics (CIQM) from the point of view of Niels Bohr. Except insofar as I present Bohr’s approach to the interpretation of experience and experiment in atomic physics as reasonable and coherent, I will not be arguing for or against any interpretation of quantum mechanics. All interpretations of experience and experiment in atomic physics are inconsistent with the intuitive linkage between kinematics and dynamics that we call classical physics. Thus all interpretations face trade-offs in what to keep and what to jettison in building what Bohr called the “rational generalization of classical physics”. And all interpretations, as a matter of historical fact, originated in deep dialog with Niels Bohr’s approach - whether considered inside the cloud of CIQM (such as Heisenberg & von Neumann’s approach) or outside (such as Bohm & Everett’s approach). Thus understanding Bohr’s motivations can help one clarify why one’s own preferred approach had to start the way it did.

Put briefly, Bohr’s motivation for the design of CIQM is to start with uncontroversially real entities - microscopic particles detected asymptotically both upstream and downstream of an interaction event - then work inwards towards the interaction event, developing a continuous description of nature in the light of quantum complementarity trade-offs. This is not necessarily the continuity of a trajectory in the sense of classical mechanics, but the continuity of a disciplined physical description across contexts in which classical concepts can acquire and lose precision in interacting ways. Like Kierkegaard’s author A quoted above, in the continuous stream of quantum description we find the notions that everyone once was so confident in - time, energy, space and momentum - separate from experience and become abstract rather than descriptive.

In a previous CVAR article, we discussed the influence of quantum mechanics on philosophy not in terms of its technical content, but what it opened. In the heyday of classical mechanics, especially the 19th century, one could argue that there was one notion of determinism - the one implicit within the practice of mechanics. People who had alternative ideas about the flow of nature, even practicing scientists like William James, often had to phrase their ideas as an abandonment of strict science - a plea for freedom. Today it would be perverse to claim that there is only one “scientific” notion of determinism relevant for philosophy. Quantum mechanics has its own natural notions of determinism, distinct from that of classical mechanics and no less fundamental for it. The mechanical determinism of everyday experience is founded on (perhaps even “emergent from”) quantum underpinnings.

Niels Bohr was the first to build a language for quantum mechanics - an “interpretation” as it were. I would argue that refined forms of this language (CIQMs) continue to structure the connection of quantum mechanical ideas to phenomenology. Bohr’s language, at least in the first form refined to modern standards, is the CIQM. Some of the surface features of Bohr’s CIQM language are a little foreign: stationary states rather than eigenstates, correspondence as arguments within quantum mechanics rather than between quantum and classical mechanics, complementarity instead of non-commutativity, classical concept rather than observable etc.. If we dig a little further, we will find a complete and consistent language for the rational interpretation of atomic physics. Not the only such language, but one with strong motivations and goals. Linguistic evolution has even occasionally favored Bohr - “classical concept” is clearly better nomenclature than “observable” (since observables can only be observed in certain contexts).

Further, as a matter of historical fact, all the approaches to that task originated in engagement with Bohr’s interpretation. Thus for both historical and technical reasons, not only Bohr’s results, but also his methods, his approach and his philosophy are of deep interest to anyone who is facing the interpretation of experience in the face of uncertainty - which is to say everyone.

One of the philosophically innovative aspects of Bohr’s group of ideas was that he treated uncertainty as both real and relational. This enters quantum mechanics through strict “complementary pairs”, but complementarity can also be understood as a general form of relation. The relational tension involved in complementarity is different from treating uncertainty as merely virtual or predicative. The predicate “... are circular” in the sentence “Tires are circular” is an example of a virtual predicate - the circularity of tires is only an approximation to the real state of the tire but it is an approximate truth that depends only on the state of the tire. In order to clear up these confusing philosophical issues, we will start with a section on what realism even means in order to see whether “Bohr is a realist” is virtual or real.

Next, we can go over the analysis of quantum mechanical experiments to see how Bohr’s language broadly structures quantum experience. I start with Bohr’s initial motivation in large angle scattering and spectroscopy. What is interesting here is the CIQM emphasis on relativity even in low energy interactions. Bohr needed relativity to connect the real objects of quantum experiments (electrons, protons, etc.) through meaningful language, even before Bell and other inequalities foregrounded the role of relativity quantitatively. Then we quickly go over some of the fundamentals of Kibble balance physics. We will find that some of Bohr’s controversial concepts - such as complementarity - play an interpretive role in the analysis of one of the processes by which kilogram standards are realized, even though quantitative uncertainty relations are not the practical limitation.

From this analysis, I will motivate what I believe to be Niels Bohr’s fundamental commitments. Broadly

1. Classical concepts structure operational meaning;

2. Complementarity tells us which descriptions cannot be unified;

3. correspondence tells us where and how operational meaning enters;

4. kinematic/dynamic duality tells us why this tension is unavoidable.

I believe that Niels Bohr, when interpreted in the context of experimental physics, was not only a great physicist but also one of the most original and important philosophers of uncertainty in the 20th century. Even if you disagree with him, as John Bell did, his insights are usable to structure alternative ideas, as John Bell did. Thus, I will end this article trying to justify this opinion by describing the appeal of Bohr from the general point of view of CVAR.

Varieties of Naturalist Realism

“”Real” is a word invented in the thirteenth century to signify having Properties, i.e. characters sufficing to identify their subject, and possessing these whether they be anywise attributed to it by any single man or group of men, or not.”

• C S Peirce, “A Neglected Argument for the Reality of God”

“Virtual [Lat. virtus, strength, from vir, a man]: Ger. virtuell; Fr. (1) virtuel; Ital. (1) virtuale. (1) A virtual X (where X is a common noun) is something, not an X, which has the efficiency (virtus) of an X.”

• C S Peirce, Baldwin’s Philosophical Dictionary

A tire is real plastic but a virtual circle - the edges of a tire are only approximately the same distance from any possible center of the tire. On a Peircean reading, one useful way to mark the distinction is that real predicates admit no contradiction, whereas virtual predicates may tolerate small internal tensions. A really plastic tire cannot be made of glass, but a virtually circular tire can be a little elongated.

The demarcation between the real and the virtual is a long running contest in the history of philosophy. Heraclitus denied the reality of many cherished fictions. The “way up” and the “way down” are the same way: distinctions that seem ontologically deep dissolve when traced through the dynamics that generate them. They are not real as independent ontological primitives in the Peirce sense.

Democritus was even stricter in his reductionism. The only predicates which qualify for reality are descriptions of microscopic bits of matter and their configurations. Consider a familiar atomist thought experiment: tracking the history of a bit of matter. Once it lived in stone, but over time it was ground into earth. From earth the bit was absorbed and became plant. From plant the bit was eaten by a woman and became nutrient. From nutrient it passed into a fetus and became boy. Can the same bit be stone, earth, plant, woman and boy? No, thus these denotations are but virtual. Only the microscopic configurational structure of the atom is real.

In the generation before Bohr, physics was locked into a struggle between different claims to the crown of reality. The main battle line was the question of the reality of atoms. Physicists like Ernst Mach gave philosophical reasons to deny the reality of atoms as ontological necessities. A fluid, Mach might say, really appears to be a continuum. It only virtually appears to be a swarm of particles. Statistical mechanical methods bring in concepts like “the finite number of molecules in a patch of fluid” only as virtual predicates - façon de parler, metaphors for the real appearance. Other physicists like Boltzmann and Clerk Maxwell kept up a realist view - the predicate “the finite number of molecules in a patch of fluid” really applies to the patch.

Einstein’s 1905 contributions effectively decided the question in physical terms in favor of the atom realists. Mach’s philosophical ideas are certainly still of interest. His Science of Mechanics can be read with pleasure and profit even today. But the statistical mechanical predicates apply to patches of fluid in the same sense that mass predicates do. They are as “real” as any physical predicate.

Bohr was only nineteen years old when Einstein’s Annus Mirabilis papers were published in 1905, still an undergraduate studying physics. But he was not silent or neutral about this debate. Bohr was a forthright Einsteinian realist about atoms and statistical mechanics. Even as late as 1962, Bohr’s lecture “Light & Life Revisited” contains forthright statements about the reality of atoms and confirmation of statistical mechanics as major scientific breakthroughs. In fact, virtually all of Bohr’s synoptic or historical articles contain statements about the reality of atoms and molecules as actual “stuff” and by implication not façon de parler.

However, Bohr’s work in interpretation of quantum mechanics forces a wrinkle into this easy conception of the real and the virtual. If a pair of predicates are “complementary” (whatever that means), then it seems that they tolerate small tensions and thus become effectively virtual. What is needed, then, is a development of the notion of reality to be compatible with complementarity. This might seem like a departure, but the concept of naturalistic realism has evolved greatly through the years. With this motivation, in this section, we will look at how modern philosophers have interpreted naturalistic realism and come to terms with where Bohr sits in these traditions.

1. Sellars’ Concept of Scientific Realism

Wilfrid Sellars’ “Philosophy and the Scientific Image of Man” is an excellent guide to the evolution of how what one might call (for the present purposes) ‘realism assignment’ evolves through philosophical discourse. His analytic and dialectical style can make for dense reading, so I will start with some interpretation with an eye towards bringing Sellars’ work in general method into contact with philosophy of physics particularly.

To begin with, Sellars emphasizes that what we’re doing when philosophizing in general - and thus assigning reality/virtuality to predicates in particular - is a practical and inferential “knowing how” rather than a “knowing that”. The point of comparing wheels to circles is to bring in the system of geometry for the solution of wheel problems. This “knowing how” includes the solution of ‘higher’ more abstract problems such as teaching, communication and leadership. Socrates asks Euthyphro how to apply the notion of the holy to Socrates’ case by asking for not an example but a definition.

Sellars calls the kind of systems of problem solving that he is examining “images”. An “image” is not the specific premises and analyses of a particular proposal, but the methodological and meta-problem solving ideals that are shared by a philosophical community. One kind of image in Sellars’ sense is the concept of the “role model”. In Descartes’ time, many lesser lights proved important geometrical theorems by adopting Descartes’ problem solving techniques, such as coordinate geometry. Descartes was their role model, not in the sense of his particular (and rather difficult) personality or even Descartes own theorems were reproduced, but in the sense that he set an image of a productive method in geometry.

Centuries later, Poncelet would provide a new image for geometric progress, moving away from the heavy calculations of coordinate geometry for the qualitative approach of projective geometry. His methods - ideal chords, complex points, the so-called “principle of continuity” - were productively criticized. The images of coordinate and projective geometry would grow in tandem with progress in knowledge, technology and critique. This demonstrates how Sellars’ notion of image is not static or abstract, but evolves with and guides scientific and philosophical practice.

Sellars first discusses what he calls the “manifest image” in philosophy and science. The motivating nerve of this image of scientific and philosophical practice is, as Peirce phrases it, that “the substance of a dream is not Real, since it was such as it was, merely in that a dreamer so dreamed it; but the fact of the dream is Real, if it was dreamed…” (A Neglected Argument for the Reality of God). That is to say, we can assign any experience to the real as an experience. One can see how the Cartesian image of science can be motivated by this: the clear and distinct predicate applies to the dream algebraic curve as much as to any algebraic curve. Peirce, like many psychologists of creativity, was fascinated by examples of scientists drawing on dream imagery for scientific practice.

This centering of the reality of experience as experience Sellars calls the “manifest image”, as it is based on what is manifest and direct to the experiencer. This basic vision guided problem solving by various schools long after Cartesian science faded into the background, the unity of its image lost to a century of innovation and critique. Idealist and positivist approaches to science also inherited the reality of experience as experience as a key source of certainty and reality in their theories.

Let’s see how this worked in a particular case, relevant to physics in general and Bohr in particular: John Stuart Mill’s famous analysis of matter.

“Matter, then, may be defined, a Permanent Possibility of Sensation. If I am asked, whether I believe in matter, I ask whether the questioner accepts this definition of it. If he does, I believe in matter: and so do all Berkeleians. In any other sense than this, I do not. But I affirm with confidence, that this conception of Matter includes the whole meaning attached to it by the common world, apart from philosophical, and sometimes from theological, theories. The reliance of mankind on the real existence of visible and tangible objects, means reliance on the reality and permanence of Possibilities of visual and tactual sensations, when no such sensations are actually experienced. We are warranted in believing that this is the meaning of Matter in the minds of many of its most esteemed metaphysical champions, though they themselves would not admit as much.”

• John Stuart Mill, An Examination of Sir William Hamilton’s Philosophy

Mill’s analysis demonstrates well Sellars’ point that ‘realism assignment’ is a ‘knowing how’, as Mill’s concluding point is that once we learn how to make do with a “Permanent Possibility of Sensation” there is nothing left over for matter to do. Still, there are many objections one can make to Mill’s analysis, as brilliant as it is. From a logical point of view, what about the gossamer spider’s web, the feeling of which destroys the web as such? This is now called the problem of so-called “finkish” dispositions. From a philosophy of language point of view, one can object that Mill has not eliminated metaphysics but rather displaced it. Mill has traded Hamilton’s “Unseen Universe” of matter for metaphysics with “permanent possibilities”, a modal conception. Quine could object that it’s clear what Hamilton’s quantifiers run over (spacetime events and what occupies them) but Mill’s quantifiers are mysterious. Is Berkeley such a wonderful role model that trading off clarity about the quantifier for him?

Sellars’ objection has to do with how Mill treats the notion of experience. The experiencer is metaphysically special, outside of the causal flow. Causality applies to his experiences, not the experiencer as an experiencer. He is a pointlike object, the stuff explanations are built off rather than something to be explained. But the new psychologies of James, Freud, Watson and Skinner - so different in method and tone - see the experiencing organism as an explanandum rather than an explanans. Sidney Morgenbesser put the matter to Skinner like this: “Let me see if I understand your thesis. You think we shouldn’t anthropomorphize people?”.

Sellars called this the Scientific Image of Man. As the scientific image increasingly displaces the manifest image in explanatory authority, experience becomes mere virtual on top of the real phenomena such as nervous states (James), competing neural circuits (Freud) or neurally integrated stimuli (Watson and Skinner). Boris Groys discusses a different, but structurally analogous, version of the scientific image in an unpublished manuscript of Kojeve. The alcoholic before the revolution says he drinks because he’s thirsty - his language centers on the manifest image as a method for solving the problem of his temporary sobriety. The alcoholic after the revolution says he drinks because of the social conditioning of his neural state and market clearing work to make a drink the cheapest path to physical equilibrium. He speaks of his experiences as something to be explained. Though both men act the same, truth and reality have been moved from first-person experience to third-person explanatory structure.

For Bohr, we need to discuss a different objection to Mill’s analysis. Here we leave behind Sellars’ specific text. Mill defines matter as a permanent possibility. This permanence gives matter a derivative sort of reality in Mill’s system. Mill, by doing so, accepts the Newtonian principle of the non-creation of matter as a stipulative differentiator between kinds of experience. But Bohr’s analysis of nature developed the thesis that the electrons observed in beta decay are created in the sense that they cannot be regarded as possessing determinate existence prior to the decay process. Landau and Heisenberg both considered this to be one of Bohr’s signature contributions to physics and the philosophy of physics.

This already shows that quantum physics - or at least Bohr’s CIQM - cuts against certain philosophies guided by the manifest image. But further analysis makes the grounds for the manifest image even more treacherous. The particle creation process is a process whose reality is guaranteed by correspondence arguments (more on this in a later section): the mother atom coming in from the asymptotic past and the daughter particles entering the asymptotic future are perfectly describable by classical concepts. But the process of particle creation is not describable by classical concepts. Particle creation mixes kinematic and dynamic concepts in a manner inconsistent with their complementarity. That is, particle creation describes a new particle in a tightly delimited region of spacetime with tightly delimited energy in a way that cannot be consistently connected to operationally defined classical concepts. In formal quantum theory, particle creation is represented by operators that are not observables and do not correspond to any jointly well-defined classical kinematic and dynamic quantities. Bohr says such concepts have a so-called “symbolic”, rather than operational/classical, meaning. If the particle creation process of beta decay is ascribed reality (as Bohr believes it should be), then we have a situation which appears to disagree with the core intuition of the manifest image. Potential experienceability would not be a necessary part of the scientific description of nature.

Summing up, analysis of beta decay finds broadly two regions who must be ascribed reality in different ways. The asymptotic regions Bohr can grant reality by correspondence arguments. But the spacetime region near particle creation cannot be ascribed reality just by indicating classical concepts. What Bohr needs at this juncture is a disciplined way of ascribing reality to the process of beta decay near the moment of decay for the continuity and “completeness” of interpretation. In the next subsection, I will use the work of Ian Hacking to highlight and illustrate how this can be done.

2. Hacking’s Entity Realism

“Philosophers long made a mummy of science. When they finally unwrapped the cadaver and saw the remnants of an historical process of becoming and discovering, they created for themselves a crisis of rationality. That happened around 1960.”

• Ian Hacking, Representing and Intervening

Hacking gives an excellent statement of the dialectic that brought him to entity realism in his book Representing and Intervening. Hacking began his book by locating his problem within the post-Kuhn and Sellars dialectic (“Philosophy and the Scientific Image of Man” came out in 1960). Before Sellars and others in that space, scientist behavior was treated as a simple out of which theories were constructed. But when scientist behavior was considered to be part of the scientific image of man, the explanations for said behavior included all manner of psychological activity, not just those which had been crowned ‘rational’. But Hacking is repelled by the approaches which easily move from the element of ‘irrationality’ (really, ‘explainability’) of scientist behavior to the virtuality of scientific concepts. “Long chains of amino acids,” he says “are really there to be spliced.”.

Hacking gives the following explanation of the dialectic that led to his entity realism. I will mix quotes and paraphrases to speed us along.

“For my part I never thought twice about scientific realism until a friend told me about an ongoing experiment to detect the existence of fractional electric charges. These are called quarks. Now it is not the quarks that made me a realist, but rather electrons. “

Hacking then describes the Millikan experiment in the usual way. A tiny charged oil droplet is suspended in a fluid (typically air) between two plates via electrostatic force. The mass of the droplet is worked out (being the product of density of the oil and the droplet volume, which can be measured optically), thus the force due to gravity is known. The charges on the plates are then switched and the drop falls. From the terminal velocity (measured optically), the total force on the droplet can be figured out from Stokes’ Law. Then the electrostatic force can be found simply by subtracting the force due to gravity. Is this necessary, ‘rational’? Not in the sense demanded by philosophical reconstruction, for it involves reasoning which is merely convenient rather than proven: the sphericity of the drops, the irrelevance of Brownian motion. “But the idea of the experiment is definitive.”, Hacking says.

What he means by “definitive” becomes clear as he continues to describe the attempt to measure the charge of the quark (then new physics). He says

“Small particle physics, however, increasingly suggests an entity, called a quark, that has a charge of e/3. Nothing in theory suggests that quarks have independent existence; if they do come into being, theory implies, then they react immediately and are gobbled up at once. This has not deterred an ingenious experiment started by LaRue, Fairbank and Hebard at Stanford. They are hunting for ‘free’ quarks using Millikan’s basic idea.”

The idea of the experiment is “definitive” in the sense that the image of Millikan’s practices continues to guide and inspire progress in the investigation of particles with small charges. He goes on to describe the quark hunting experiment. A niobium droplet (far smaller than the oil droplet used before) is suspended in a fluctuating magnetic field. Magnetometer measurement can fix the position and velocity of the niobium droplet far more accurately than the optical methods used by Millikan experiment. The charge on the droplet is altered and the data is processed to find if the charge change has an excess or defect on the scale of e/3. Hacking asked a physicist friend how they change the charge on the droplet.

“ ‘Well, at that stage,’ said my friend, ‘we spray it with positrons to increase the charge or with electrons to decrease the charge.’ From that day forth I’ve been a scientific realist. So far as I’m concerned, if you can spray them then they are real.”

• Ian Hacking, Representing and Intervening (emphasis in original)

What impressed Hacking about this is what he calls “the stability of the laboratory sciences”. From a philosophical theory of meaning point of view, the change from Millikan to quark language is a catastrophic change in meaning. The entity with the smallest negative charge at the level of fundamental constituents went from being the electron to the down, strange and bottom quarks. All the sentences that used the predicate “the entity with the smallest negative charge”, understood to be -e, are strictly false. But to practicing physicists, the result of the quark language revolution was not catastrophic, but conservative. Kendall, Friedman and Taylor who developed the deep inelastic scattering methods to investigate quarks spoke happily about their inspiration from Rutherford. LaRue, Fairbank and Hebard - as noted - were guided by the example of Millikan. Through this so-called “scientific revolution”, the images which guided experimental progress had not changed. As Hacking put it in his Historical Ontology:

“What we accumulate are experimental techniques and styles of reasoning. Anglophone philosophy of science has too much debated the question of whether theoretical knowledge accumulates. Maybe it does not. So what? Phenomena and reasons accumulate”

What have we learned from Hacking’s example? We learn that although entity meaning on the theory side may change catastrophically, experimental and observational roles need not. We thus have license to call “real” those entities which play a persistent part in our descriptions of nature as phenomenon and experience. The fundamental concept of Copernicus, that the System of the World might be deduced from patterns of the burning points in the sky, remains even as our observational culture moves on from the visual to the whole range of electromagnetic radiation and from light to the whole variety of cosmic rays. What remains now is to show how Bohr incorporates this notion of realism avant la lettre.

3. Bohr’s Place Among the Realists

Niels Bohr’s method of analysis can be applied directly to Hacking’s epiphanic moment. For now, we will only indicate Bohr’s CIQM concepts, with an analysis of the fundamental principles in a later section. Further, Bohr died in 1962 so he does not use the language of early 80s analytic philosophy of science or even of quarks (which were proposed after his death). But we can narrow down to the basic scattering language to see how this happens.

We will describe general features of a deep inelastic scattering experiment in Bohr’s style. The reason for this switch is that interpretation of the experiment Hacking discussed is less clear than deep inelastic scattering, a widely performed type of experiment. Besides, Hacking’s slogan was motivated by the laboratory manipulability of electrons generally not by the specifics of LaRue, Fairbank and Hebard. We can see the same structure, more abstractly, in later scattering practices that treat “sprays” of particles as stable experimental signatures. Further, deep inelastic scattering is a particularly strong test case for Bohr’s realism because it cleanly separates what we can describe classically from what we cannot. The very entities predicted by high-energy theory (quarks) are not independently observable as isolated entities, so if Bohr style analysis can assign reality to them then the notion of realism can be said to be sufficiently flexible to be truly interesting.

In a typical deep inelastic scattering experiment, a high-energy lepton (often an electron or a positron) and a hadron (such as a proton bound in a nucleus) enter into the interaction from the asymptotic past. Usually, the hadron is at rest with respect to the laboratory frame, making it a “target” for the lepton beam. Whatever frame we choose, the reality of these particles are anchored by the classical description of tracks, counts, and macroscopic records - i.e. reality via correspondence arguments. After the interaction, the experimenter reads hadronic final-state records. At high energies, this can include a literal “spray” of hadrons in a pattern called a jet.

Complementarity enters the scene through the process of interaction. In a way, Bohr already has dealt with a less complex and lower energy but still philosophically analogous process. Beta decay, which also involves particle creation, provides a simple example of the same interpretive structure: well-defined asymptotics connected by a non-classical interaction region. The analogy between beta decay/hadronic final-state formation means that, despite being physically very different, deep inelastic scattering has a similar interpretive structure. They both have Bohr-style correspondence arguments with well-defined asymptotic regions extended continuously via complementarity. During the tiny spacetime interval of the interaction process, complementarity delimits the definition of operational classical predicates such as energy and momentum. Those predicates take on a more virtual or symbolic meaning rather than strict operational definition.

Bohr and Hacking are in agreement about what is real. For instance, the “spray” of hadrons in a jet is very similar to the language used by Hacking. Correspondence arguments start the assignment of reality at this uncontested level, the level of third person explanatory structure. What Bohr’s notion of complementarity adds is what happens in between. This philosophical principle helps explain how the rigorous discussion of the deep inelastic scattering process as a lepton-quark interaction can be meaningful even when the interaction region cannot be described by classical pictures. The measurements provide information about the process of the system. And exactly insofar as that process has no operational meaning, its description is allowed to become symbolic. I would suggest that Bohr’s interpretation of quantum mechanics (or at least a form of the CIQM consistent with modern standards) is trying to be, in the sense of entity realism (which is just one variety of realism), a maximally realistic approach to atomic and nuclear physics.