Synthese (2011) 182:149–163 DOI 10.1007/s11229-009-9617-6

What are the phenomena of physics? Brigitte Falkenburg

Received: 20 May 2009 / Accepted: 3 June 2009 / Published online: 14 July 2009 © Springer Science+Business Media B.V. 2009

Abstract Depending on different positions in the debate on scientific realism, there are various accounts of the phenomena of physics. For scientific realists like Bogen and Woodward, phenomena are matters of fact in nature, i.e., the effects explained and predicted by physical theories. For empiricists like van Fraassen, the phenomena of physics are the appearances observed or perceived by sensory experience. Constructivists, however, regard the phenomena of physics as artificial structures generated by experimental and mathematical methods. My paper investigates the historical background of these different meanings of “phenomenon” in the traditions of physics and philosophy. In particular, I discuss Newton’s account of the phenomena and Bohr’s view of quantum phenomena, their relation to the philosophical discussion, and to data and evidence in current particle physics and quantum optics. Keywords Analytic-synthetic method · Bohr · Newton · Phenomena · Physics · Scientific realism 1 Introduction Let me begin with a commemorative remark about Daniela Bailer-Jones. I had invited her to give a talk on models at the first fall meeting of the Philosophy Working Group of the German Physical Society, at Heidelberg, in October 2004. She had accepted the invitation long before. But it turned out that she had to go to the hospital once more for an operation and would come out just a few days before the meeting. So she told me that she would not be able to come. I asked her to send me a piece of her work on

B. Falkenburg (B) Fakultaet 14, Institut fuer Philosophie und Politikwissenschaft, Technische Universitaet Dortmund, 44227 Dortmund, Germany e-mail: [email protected]

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models and let me present it at the meeting. The day of the meeting she might come for the discussion, or not, depending on her actual health state. She was very fond of this idea and sent me the chapter on Models, Theories and Phenomena1 from her book. Finally, she did not only show up, pale and tough, but gave us an impressive talk. The chapter on Models, Theories and Phenomena remained all of her work on phenomena. One of its virtues is that it relates separate discussions of recent philosophy of science which belong together. It conceives of the relation between models and phenomena in the following way. Like the Cartwright school, Bailer-Jones considered models to be mediators between theories and the phenomena. Like Bogen and Woodward, she considered the phenomena of science to be in the world. Like Hacking, she emphasized that scientific phenomena are stable, reproducible, and regular.2 In particular, she made the following claims about models and phenomena: Models are about phenomena in the world. Models are abstract, whereas phenomena are concrete and empirical. Nevertheless, the phenomena of a science are typical. They are prototypes of a certain kind.3 I take models to be about phenomena in the world […]. A phenomenon has empirical properties, whereas a model that is a structure does not. The phenomena that are explored by modelling are concrete in the sense that they are, or have to do with, real things, real things with many properties – things such as stars, genes, electrons, chemical substances, and so on. One tries to model, however, not any odd specimen of a phenomenon, but a typical one. […] The prototype has all the properties of the real phenomenon; it is merely that the properties are selected such that they do not deviate from a ‘typical’ case of the phenomenon. […] the prototype of a phenomenon still counts as concrete, because it […] could exist in just this manner. Hence, according to Bailer-Jones phenomena are concrete, empirical matters of fact in the world which exhibit certain typical features. This view of scientific phenomena is not uncontroversial. The philosophers of science do not agree about the question of what are the phenomena of science. Their answers depend on their respective positions in the debate on scientific realism. For scientific realists, phenomena are the matters of fact in nature which are explained and predicted by physical theories. According to this view defended by Bogen and Woodward, the phenomena are what the physicists call effects: the Einstein–de Haas effect, the Bohm–Aharanov effect, the quantum Hall effect, etc. But for empiricists like van Fraassen, the phenomena of physics are the appearances, that is, what can be observed or perceived by mere sensory experience. From an empiricist point of view, there are no unobservable phenomena of physics. For constructivists, in turn, the phenomena of physics are not in the world. From a 1 Bailer-Jones (2009, Chap. 6). 2 Cartwright (1983, 1999), Morgan and Morrison (1999), Bogen and Woodward (1988), Hacking (1983). 3 All quotations from Bailer-Jones (2009, Chap. 6).

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constructivist point of view, the phenomena of physics are artificial, they are nothing but the structures generated by experimental and mathematical methods. As I will show in the following, Bailer-Jones’ account of the phenomena agrees with the tradition of physics and philosophy. In particular, it comes close to Newton’s views about the phenomena of physics, to Kant’s concept of a phenomenon and to Bohr’s views about quantum phenomena. Only in recent physics, their ways to talk about phenomena is replaced by more specific ways of talking about data or evidence.

2 The debate on scientific realism For physics, the tradition of saving the phenomena was most important. In ancient astronomy, the attempt to save the phenomena was related to the description of the apparent motions of the celestial bodies. The phenomena were nothing but the observed positions of the moon and the planets at the night sky. The Copernican revolution, however, raised the question for the true motions of the celestial bodies and their causes. Galileo used the telescope in order to investigate the celestial motions and the experimental method in order to investigate the laws of mechanical motions. Kepler stated the laws of the elliptic motions of the planets around the sun. Newton unified Kepler’s and Galileo’s laws in his theory of universal gravitation. With Galileo’s experimental method, he investigated the optical phenomena of light propagation, diffraction, and dispersion, in particular, the decomposition of white light into colors by means of a prism. With the beginnings of modern physics, the debate about scientific realism arose. Ancient astronomy aimed at the description of the apparent motions of the celestial bodies. Ptolemy’s system of cycles, epicycles, and decentres made a most accurate description possible. It was open for corrections and made it possible to give very good approximations to the complicated observed planetary motions. With regard to the phenomena, Copernicus’ heliocentric system with circular planetary motions was much worse. But it claimed to explain the apparent planetary motions in terms of the true motions of the celestial bodies. The Aristotelian opponents of the Copernican system interpreted it from an instrumentalist point of view. They considered it to be a mere mathematical tool, as one hypothesis amongst others. The debate about the Copernican system was a debate about truth, about the claims of scientific realism. It was closely related to the question of what are the phenomena of physics. Galileo took his observations of the phases of Venus and the moons of Jupiter as phenomena that proved the truth of the Copernican system. His Aristotelian opponents refused to look through the telescope. They did not accept these observations as genuine phenomena. Hence, the opposition between empiricism and scientific realism is as old as modern physics. It deals with the question of whether the laws of mathematical physics are true and whether unobservable entities such as forces, fields, atoms, and subatomic particles exist. It also deals with the question of what are the phenomena of physics and which tools are employed to observe them. The successes of modern physics did not eliminate instrumentalism and empiricism. In view of subatomic physics, relativity, and quantum theory, the debate on scientific realism went on. The defenders of scientific realism follow the founders of modern

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physics. They admit that scientific phenomena are given by means of technological devices such as the telescope, the microscope, the electron microscope, or the particle detectors and computer devices of particle physics and quantum optics. They claim that these phenomena are physical effects (such as the photo effect, Zeemann effect, Bohm–Aharanov effect, Quantum Hall effect, etc.). The term “effect” indicates that these phenomena have causes, in particular, forces, fields, potentials, or interactions of subatomic particles. According to scientific realism, the phenomena of physics are stable facts of nature predicted and explained by physical theories. This is Bogen and Woodward’s view.4 Contrarily, modern defenders of empiricism and instrumentalism stand in the tradition of Aristotle and Galileo’s opponents. Empiricists claim that the appearances only are given by sensory experience. For Mach, Carnap, Suppes, or van Fraassen, the phenomena of physics consist in sense data, observable measurement outcomes, models of data, or empirical structures. More radically, constructivists consider the phenomena of physics to be artefacts, as opposed to the facts of nature, and the physical effects mentioned above to be technological and mathematical constructs. There were several constructivist schools, from the Neokantian Marburg school to current social constructivism.5 According to the latter, scientific phenomena stem from the fabric of the laboratory and scientific theories are socio-cultural products. In this way, the phenomena of physics and the unobservable agents behind physical effects are debated until today. 3 Newton: analysis and synthesis of the phenomena In physics, there is also no unambiguous concept of the phenomena. What the phenomena are depends on the theoretical and experimental context. This ambiguity dates back to Newton’s use of the term. In the Opticks, Newton identified the phenomena with the observable effects of his experiments. He investigated the phenomena of light propagation and refraction, the colours obtained from white light with a prism, the re-composition of white light from the spectra, etc.6 In contradistinction to these directly observable phenomena, the phenomena of the Principia are the planetary motions described by Kepler’s laws. Hence, they are phenomenological laws, or mathematical structures which empiricists may accept as an empirical structure or data model.7 Both kinds of Newton’s phenomena have common features. They are regular, predictable, law-like, and hence, typical. They are closely related to Newton’s methodology of analysis and synthesis. This methodology was familiar to scientists from Galileo to Bohr and to philosophers from Descartes to Mach.8 Since it is no longer 4 Bogen and Woodward (1988). 5 See Cohen (1883, 1986), Natorp (1910), Pickering (1984), Latour and Woolgar (1979), Knorr-Cetina

(1999). 6 Newton (1730). 7 Newton (1729, 401 ff., or 1999, 797 ff.). 8 The method dates back to Euklid’s Elements. It was opposed to Aristotle’s methods of deduction and induction; see Engfer (1982). The founders of modern physics applied it to natural phenomena, as Galileo‘s resolutive-compositive method (where “resolutive-compositive” are just the Latin terms for

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known in current philosophy of science, it should be sketched shortly. In his preface to the second edition of Newton’s Principia, Cotes explains it as the method of those whose natural philosophy is based on experiment. […] they proceed by a twofold method, analytic and synthetic. From certain selected phenomena they deduce by analysis the forces of nature and the simpler laws of those forces, from which they then give the constitution of the rest of the phenomena by synthesis. The method is identical with the famous “induction” of “propositions gathered from phenomena” pinned down in Newton’s methodological rules at the beginning of Book Three of the Principia, in Rule 4.9 It should not be confused with induction in a modern, empiricist sense. According to Rule 1 and Rule 2, it employs causal analysis. Rule 1 is a principle of causal parsimony, of admitting “no more causes” than are “sufficient to explain the phenomena”. Rule 2 requires assigning the same causes to “natural effects of the same kind”.10 Hence, the aim of causal analysis is to explain phenomena, where the latter are natural effects of a certain kind. In the Opticks, Newton describes the same method of analysis and synthesis as follows: As in mathematics, so in natural philosophy, the investigation of difficult things by the method of analysis ought ever to proceed the method of composition. This analysis consists in making experiments and observations, and in drawing general conclusions from them by induction […] By this way of analysis we may proceed from compounds to ingredients, and from motions to the forces producing them; and in general, from effects to their causes, and from particular causes to more general ones […] And the synthesis consists in assuming the causes discovered, and established principles, and by them explaining the phenomena proceeding from them, and proving the explanations.11 Here, too, the phenomena are subject to causal analysis. The quotation shows that for Newton the causal analysis of the phenomena has two aspects, namely the search of the ingredients or parts of a given compound and the search for the forces behind the motions. In the Principia, Newton explains the motions of the celestial bodies and other mechanical phenomena in terms of gravitation as a universal force. In the Footnote 8 continued “analytic-synthetic”); see, e.g. Losee (1993). Descartes gives a general version of it in the second and third rule of his Discours de la méthode. He and his rationalist followers (Leibniz, Spinoza, Wolff) carried the method over to philosophy. They attempted to model philosophical knowledge after mathematics and/or physics, in order to give undoubted foundations to metaphysics. Kant criticized the confusion to which the method led in seventeenth and eighteenth century metaphysics. In nineteenth century, the method of analysis and synthesis disappeared from philosophy, giving place to the modern empiricist view of induction and deduction. Nevertheless, residues of it are still present in Mill’s account of causal analysis [see Mill (1843), Book III: Of Induction, Chaps. V–VIII]. Mach (1905, 256 ff.), indeed only discusses the origins of the method in ancient mathematics. 9 See the rules at the beginning of Book Three of the Principia, Newton (1729, pp. 398–400; 1999, pp. 794–796). Quotation: Newton (1999, p. 796). Cf. also Falkenburg (2000, Chap. 1), Falkenburg and Ihmig (2004); Ihmig (2004a,b). 10 Ibid., pp. 794–795. 11 Newton (1730, pp. 404–405).

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Fig. 1 Spectral decomposition and re-composition of light. Opticks, Book One, Part II, Exp. 13 (Newton 1730, p. 147)

Opticks, Newton explains the constitution of light and other optical phenomena in terms of colours and hypothetical light atoms. In particular, he shows how white light is decomposed into the colours by a prism and recomposed by superposing the spectra from two parallel prisms (see Fig. 1). In both fields of physics, the phenomena are the starting point of causal analysis. The only difference between the Principia and the Opticks is that Newton is able to give a full-fledged mathematical theory of gravitation explaining the phenomena of mechanics, whereas he is not able to derive any force of the atomic constituents of light or matter, to say nothing of their mathematical description. Regarding the atoms, Newton’s analysis of the optical phenomena does not establish theoretical principles that explain the optical phenomena. Here, the second step of the analytic-synthetic method, the “synthesis”, is missing. Therefore, Newton presented his atomistic hypothesis only in Query 31 at the end of the Opticks. This hypothesis is based on a further methodological principle of physics, namely the assumption of the self-conformability or unity of nature: And thus nature will be very conformable to her self and very simple […].12 This principle is closely related to the belief that the phenomena are connected by laws. In the Principia, the law of gravitation unifies the terrestrial and celestial phenomena of mechanics, namely Galileo’s and Kepler’s motions. In Cajori’s appendix to Motte’s English translation of the Principia, this is demonstrated by the following thought experiment. Imagine a stone cast from a very high mountain The greater velocity the stone gets by the impact, the closer its parabolic trajectory approximates the elliptic orbit of a satellite (see Fig. 2). Later physicists followed Newton, by identifying the phenomena with natural effects of a certain kind, which are given at any stage of research. This account of the phenomena is closely related to the analytic-synthetic method of Newton and 12 Ibid., p. 397. Indeed, his hypothesis that matter consists of atoms is also supported by Rule 3 of the Principia, the rule of generalizing the extensive quantities and the impenetrability of mechanical bodies to all parts of matter including the atoms.

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Fig. 2 Connection between Galileo’s and Kepler’s motions. Principia, Appendix (Newton 1729, p. 551)

his followers. The phenomena are given in the sense that they are the starting point of causal analysis. Vice versa, they are the explananda of physical theories in the sense that they are the end point of the deduction from the principles found by causal analysis, i.e., the opposite synthetic step of the method. In this way, the phenomena of physics came to be any kinds of observable appearances, phenomenological laws, experimental results, measurement outcomes, and unexpected as well predicted physical effects. The phenomena of physics may be theory-laden in many regards, as far as they are given in this sense, waiting for causal analysis and theoretical explanation.13 From a Newtonian point of view, the empiricist claim that the phenomena of science are sense data is based on a deep misunderstanding of scientific methodology. The epistemic goals and the methods of physics are committed to scientific realism. They aim at finding phenomena that may be far from being obvious, at their causal analysis, and at their mathematical explanation. 4 Kant: phenomena and the laws of nature Kant’s philosophical account of the phenomena was based on Newton’s views, but it also took up the traditional philosophical distinction of phenomena and noumena. According to Descartes and his rationalist followers, our sensory perceptions may be deceptive and only intellectual knowledge is reliable. The rationalist credo was that the phenomena perceived by the senses do not give as access to reality proper. Their distrust in the phenomena traced back to Plato. Leibniz’ distinction is most typical. For Leibniz, the phenomena consist in the empirical matters of fact and their spatiotemporal order. The phenomena that we perceive are illusory though well-founded by the monads. His monads are conceived as unobservable, immaterial noumena or things-in-themselves which are the only real substances in the world. The empiricists, on the other hand, stuck to Aristotle’s tradition. They defended the doctrine that the phenomena are all that we know. In contradistinction to Descartes or Leibniz, 13 In my Particle Metaphysics (Falkenburg 2007, Chaps. 2–3), I show how the phenomena, or empirical basis, of particle physics became more and more theory-laden and complex in the course of scientific progress, from the discovery of the electron to the era of the big particle accelerators and highly sophisticated particle detectors.

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Locke criticized the notion of a substance or thing-in-itself as an unclear and confused concept. Later, Hume claimed that such metaphysical concepts are just based on psychological habit. Kant wanted to reconcile the tenable aspects of seventeenth/eighteenth century empiricism and rationalism. For him, both positions were in need of revision. Hume’s empiricism denied the necessity of the principle of causality and the laws of nature, including in particular Newton’s law of gravitation. But the opposing claims of rationalism about metaphysical substances as the only existing reality led to never-ending metaphysical struggles. In view of Newton’s physics, both positions generated problems. In order to resolve them, Kant developed his critical philosophy.14 From rationalism, he kept the ambitious project to model philosophy after the exact sciences. From empiricism, he kept the principle that we only can have objective knowledge of the phenomena. According to the Critique of Pure Reason, the phenomena are concrete, empirical matters of fact in the world, whereas the noumena are things-in-themselves of which no experience is possible, and hence no objective knowledge. Kant’s phenomena are neither identical with the empiricists’ sense data nor with the rationalists’ deceptive perceptions. They are structured sense data. Two kinds of structures are a priori imposed on them: the pure forms of intuition (space and time) and the pure of the understanding (the categories, e.g. substance and causality). On the one hand, the phenomena are given in pure intuition. As such, they are singular. That is, they are given as concrete empirical things and events that are spatio-temporally individuated. On the other hand, the phenomena underlie general laws of nature such as the principle of causality. As such, they behave law-like. In particular, all empirical things are mechanical bodies that obey Newton’s law of gravitation. The relation between the phenomena in general and the phenomena of Newton’s physics was investigated by Kant, too. According to the Metaphysical Foundations of Natural Science, the phenomena of physics (and of any science proper) have a mathematical structure. Due to this mathematical structure, Kant’s scientific phenomena are typical in the same sense as Newton’s phenomena, which were explained in the last section. In nineteenth century philosophy, Kant’s views about the empirical phenomena and their Newtonian structure did not become influential. Post-Kantian German idealism did followed Lambert’s rather than Kant’s views about the phenomena. Lambert was physicist and philosopher. As a physicist, he investigated phenomena of light and developed methods of photometry. As a philosopher, he referred to Plato’s view that the phenomena only seem to be real. His New Organon contains a part titled Phenomenology, or Doctrine of Appearance.15 According to this doctrine, the appearances are what seem to be. For light phenomena, both views of the phenomena perfectly agree. This is not only true for the colours, the paradigmatic non-primary qualities. It is also true for the phenomena of luminosity that Lambert investigated. Lambert’s Phenomenology became influential in the phenomenological tradition of philosophy from Hegel to Husserl and Heidegger.

14 See Falkenburg (2000). 15 See Lambert (1764).

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Nevertheless, Kant’s view of the phenomena became most influential in nineteenth and twentieth century physics. In particular, they underlie Bohr’s views about quantum phenomena.16 5 Bohr: complementary quantum phenomena In order to develop the atomic model of 1913, Bohr had substantially weakened the explanatory claims of Newtonian physics. Bohr’s quantization postulates did not only violate classical radiation theory but also Newton’s principle of the unity of nature. In the transition from the classical to the quantum domain, nature is neither “conformable to her self” nor simple.17 At the end of his Nobel lecture of 1922, Bohr gave his own account of scientific explanation: By a theoretical explanation of natural phenomena we understand in general a classification of the observations of a certain domain with the help of analogies pertaining to other domains of observation, where one has presumably to do with simpler phenomena. The most one can demand of a theory is that this classification can be pushed so far that it can be contribute to the development of the field of observation by the prediction of new phenomena.18 For Bohr, too, the phenomena of physics are explananda of theories. But for him, an explanation is nothing but the classification of observations in terms of analogies. In this point, he follows Mach’s empiricist epistemology rather than Newton. Theories do not aim at the description of the “true causes” of the phenomena19 but only at the prediction of phenomena. This is an instrumentalist view. Physical reality is in the phenomena, but not in a literal understanding of the analogies that help to classify them in order to find out their structure. Nevertheless, Bohr was familiar with the traditional method of analysis and synthesis. When this method disappeared from the philosophy of science in the course of the triumph of empiricism,20 the expression “analysis and synthesis” was still omnipresent in the philosophical contexts that influenced Bohr. His philosophical teacher Høffding used “analysis and synthesis” as a standing expression in his books, and so did Bohr in his later philosophical writings without ever defining them.21 We may also assume that Bohr was familiar Newton’s Principia and Opticks. (Was there any physicist of 16 In nineteenth century science, Kant mainly was present due to Neokantian influences. Helmholtz investigated the foundations of Kant’s epistemology in physical geometry as well as in physiology. Bohr’s views about Kant were influenced by his philosophical teacher Høffding; see Faye (1991) and Pringe (2007). 17 See Newton (1730, p. 397). 18 Bohr (1922), last page (BCW, Vol. 4, p. 482). Bohr’s account of theoretical explanation traces back to

Høffding’s account of analogies and explanation, where the latter was clearly influenced by Mach’s views. 19 See Newton’s Rule 1, Newton (1729, p. 398; 1999, p.794). 20 See note 8. 21 See in particular Bohr (1958) and the article “Analyse et synthèse” in Chevalley (1991, pp. 373–378). Chevalley emphasizes that the term “analysis and synthesis” had also a canonical sense in the Neo-Humboldtian philosophy of language, in Göttingen, the German stronghold of quantum mechanics, in the late twenties; ibid., p. 377.

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Bohr’s generation who did not look into Newton’s Principia and Opticks?) Due to his philosophical education, he could not miss to understand correctly the method of analysis and synthesis in the Queries of the Opticks and the four methodological rules in the Principia. In Bohr’s philosophical writings, the Newtonian sense of the expression “analysis and synthesis” indeed repeatedly occurs.22 After the birth of quantum mechanics, he emphasized that Planck’s quantum of action indicates the limitations of experimental analysis.23 Later, he claimed more generally that biology shows the limitations of mechanical analysis.24 Bohr’s report of his discussions with Einstein, first published in 1949, mentions the problem of “obtaining the proper harmony between analysis and synthesis of physical phenomena” and the impossibility of “attempting a causal analysis of radiative phenomena”.25 On the basis of such text passages, I take it for granted that in the context of quantum mechanics Bohr used the concepts of analysis and synthesis roughly in Newton’s sense. For Bohr, the limitations of experimental and causal analysis in the quantum domain indicate that the concept of a subatomic physical object has to be rejected. There are no quantum objects, there are only quantum phenomena. The quantum mechanical wave function? does not have concrete physical meaning, it only is abstract and symbolic. The only objects of subatomic physics are complementary quantum phenomena. In accordance with Bohr’s account of theoretical explanation, they are analogues of the corresponding phenomena of classical physics, namely particle tracks or wave interference. He introduced these ideas in his famous Como lecture of 1927. There, he emphasized that the “essence” of quantum theory may be expressed in the so-called quantum postulate, which attributes to any atomic process an essential discontinuity, or rather individuality, completely foreign to the classical theories and symbolised by Planck’s quantum of action.26 Here, “individuality” has to be understood in the sense of being indivisible.27 Planck’s quantum of action indicates limitations of experimental analysis, giving rise to the discontinuity “or rather individuality” of the quantum jumps. In addition, Planck’s quantum of action plays a crucial role in Heisenberg’s uncertainty relations. The latter, in turn, indicate the limitations of causal analysis in the quantum domain: The very nature of the quantum theory thus forces us to regard the space-time coordination and the claim of causality, the union of which characterises the classical theories, as complementary but exclusive features of the description […].28 22 See the examples given in Chevalley (1991, pp. 374–375). 23 See below. 24 See Chevalley (1991, p. 374), pointing to Bohr (1961, p. 159); Light and Life (Bohr 1933). 25 Bohr (1949, pp. 10–11). 26 Bohr (1928, p. 88). 27 This was already noted by Meyer-Abich (1965). 28 Bohr (1928, p. 90).

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Fig. 3 Particle tracks observed in nuclear emulsions (Powell et al. 1959, p. 31)

Fig. 4 Compton scattering (below) and event reconstruction (above) (Compton 1927)

According to the Como lecture, the classical theories of particle or wave are complementary descriptions that only together give a full account of the quantum domain. Bohr emphasizes that we are not dealing with contradictory but with complementary pictures of the phenomena, which only together offer a natural generalisation of the classical mode of description.29 The most typical particle-like phenomena of quantum physics are: the particle tracks observed in the bubble chamber (Fig. 3); and the Compton effect in which a single photon gives an observable kick or momentum transfer to an electron, by losing energy and lowering its frequency (Fig. 4). The most typical wave-like phenomena of quantum physics are: the interference fringes observed when (a) electrons or (b) gamma rays are sent through a crystal (Fig. 5). 29 Ibid., p. 91.—For Bohr’s complementarity view of quantum mechanics, see also Falkenburg (2007,

pp. 272–277).

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Fig. 5 Electron diffraction (a) and gamma ray diffraction (b) (Raether 1957, p. 443)

These quantum phenomena are complementary in the following sense. Electrons may either cause particle tracks or interference fringes, depending on the experimental conditions. Similarly, photons may either cause a momentum kick or interference fringes, again depending on the experimental conditions. But there are non experimental conditions under which the tracks (or the momentum kick, respectively) can be observed at the same time. Why are there no quantum objects, for Bohr? According to the Como lecture, classical physical objects are defined in terms of spatio-temporal and causal properties, where these properties can be observed simultaneously. But for quantum phenomena, the spatio-temporal and causal properties cannot be observed at once. The spatio-temporal properties of a physical object are its space–time coordinates. The causal properties, for Bohr, are expressed in terms of momentum–energy conservation. For the alleged quantum ‘objects’ the conditions of observation are at odds with the possibilities of defining a full-fledged physical object in terms of space–time coordinates and momentum–energy conservation. Bohr’s example is the Compton effect, in which the photon shows particle-like nature by giving a momentum kick to an electron, whereas the interference fringes of light sent through a double slit are evidence of a wave-like space–time coordination of light propagation. The particle-like momentum kick is described in terms of classical mechanics, the wave-like light propagation in terms of classical electrodynamics. Hence, there are no quantum objects for Bohr but only complementary quantum phenomena. Only the latter are concrete phenomena. Only the complementary quantum phenomena have either a spatio-temporal or a causal representation. For Bohr, they are intuitive in Kant’s sense, in contradistinction to the abstract and symbolic formalism of quantum mechanics. They are observations obtained under specified circumstances including the experimental arrangement. As such, they are individual (i.e., indivisible), complementary (i.e., mutually exclusive), and in correspondence to the classical models of wave or particle. This account of the quantum phenomena turned out to be closely related to Newton’s methodology of analysis and synthesis, given that the “individuality” or indivisibility of quantum phenomena means that for Bohr,

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the Planck’s quantum of action indicates the limitations of experimental and causal analysis. 6 Conclusions According to the approaches discussed in the preceding sections, the phenomena of physics have the following features. They are (i) spatio-temporally individuated objects and events in the world, i.e., concrete; (ii) given by observation or measurement, i.e., empirical; and (iii) explained in terms of laws and causal stories, i.e., typical, regular, or law-like. The phenomena of physics in the broadest sense cover a wide range of different levels of observation measurement, and experimentation. Only about this last point, scientific realism and its opponents empiricism or constructivism disagree. Nevertheless, the latter positions rely on basically correct insights. The phenomena of physics are empirical structures constructed by experimental and mathematical methods. But strict empiricism and radical constructivism exaggerate as regards the claim that the phenomena should be directly observable. They miss the complicated architectonics of modern physics, according to which the causal explanations of one stage of research may bring out the phenomena of the next stage of research, which again are subject to causal analysis, the results of which will bring out the next deeper level of phenomena, or physical effects, or explananda of physical theories. At all levels of causal analysis, however, the phenomena of physics have the following common features. They are concrete matters of fact in nature. They are empirical insofar they are given by means of observation, measurement, or experiment. And they are typical insofar as they are identified by classification and in terms of mathematical structures. Indeed, the approaches of Bailer-Jones, Bogen and Woodward, Newton, Kant, and Bohr, notwithstanding their differences basically agree about these features of the phenomena. However, finally the following point has to be noted. The concept of a phenomenon is a pre-theoretical, informal notion of physics. In several areas of current physics, it has been eliminated in favor of more precise concepts. In current particle physics, it is replaced by talk about data, events, and evidence. Only in current quantum optics, Bohr’s use of the notion of a quantum phenomenon survived. After all, the concept of a phenomenon has a philosophical origin and it remains to be a philosophical term. References Bailer-Jones, D. M. (2009). Scientific models in philosophy of science. University of Pittsburgh Press (to appear). Bogen, J., & Woodward, J. (1988). Saving the phenomena. The Philosophical Review, 97, 303–352. Bohr, N. (1922). On atomerne bygning. Nobel lecture, December 11, 1922. German translation by Pauli, W. (1923). Über den Bau der Atome. Naturwissenschaften, 11, 606–624. English translation by Hoyt, F. C. (1923). The structure of the atom. Nature, 112, 29–44. Danish and English text in BCW (Bohr’s Collected Works), Vol. 4, pp. 425–482. Bohr, N. (1927). The quantum postulate and the recent development of atomic theory. Como lecture. Modified version: Bohr, 1928. Both versions in BCW, Vol. 6, pp. 109–158.

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