High-Energy Physics and Reality – Some Philosophical Aspects of a Science Ph.D. Thesis by Henrik Zinkernagel February 1998 Supervisor Benny Lautrup Acknowledgements This project was made possible by the Center for the Philosophy of Nature and Sci- ence Studies at the Niels Bohr Institute, University of Copenhagen. It is a pleasure to thank Jens Bang, Tian Y. Cao, Robert S. Cohen, Finn Collin, Peter Galison, Jørgen Beck Hansen, Peter Hansen, Holger Bech Nielsen, Silvan S. Schweber, Chris LLewellyn Smith, Thomas S¨oderqvistand Peter Zinkernagel for stimulating discus- sions on physics and philosophy. Moreover, I would like to thank Alfred I. Tauber for hospitality during my stay at the Center for Philosophy of Science, Boston Uni- versity. Finally, I thank Claus Emmeche, Benny Lautrup, and Svend Erik Rugh for discussions, encouragement, and support all along the way. Members of the examination committee: Prof. James T. Cushing (University of Notre Dame) Prof. Poul Olesen (University of Copenhagen) Prof. Stig A. Pedersen (Roskilde University) 1 Contents 1 Introduction 4 1.1 The method . 5 1.2 Structure of the thesis . 5 2 Reflections on Science 7 2.1 A spectrum view on realism . 7 2.1.1 In between . 8 2.2 Theory-ladenness of data . 9 2.3 The social constructivist turn . 11 2.4 Underdetermination of theories by observations . 14 2.5 Other positions on the realism issue . 16 2.5.1 A break for the scientist . 17 2.6 A historicist view on science . 18 2.7 Objectivity . 20 2.7.1 Conditions for description and objectivity . 22 2.7.2 Classical physics and objectivity . 23 3 The Structure of High-Energy Physics 26 3.1 Theories leading to HEP . 26 3.2 The Standard Model . 27 4 Experimental High-Energy Physics 31 4.1 Experimental evidence . 31 4.1.1 Studying the properties of quarks . 33 4.1.2 The reality of quarks . 35 4.2 Weak neutral currents — facts or artifacts? . 36 4.2.1 The discovery of the neutral currents . 36 4.2.2 Pickering and social constructivism . 39 4.2.3 Galison and historicism . 40 4.2.4 Miller and Bullock, and realism . 41 4.2.5 The trust in neutral currents . 42 4.3 The electron g-factor . 43 4.3.1 g 2 experiments . 45 4.4 The trust− in, and interpretation of, experimental results . 46 5 Alternative Theories of High-Energy Physics? 48 5.1 A brief history of the vacuum . 48 5.2 QFT and the vacuum . 49 5.3 The Casimir effect and the interpretation of the vacuum . 51 5.3.1 The QED vacuum as a consequence of field quantization . 52 5.3.2 Source theory approach by Schwinger . 54 5.4 QFT vs. source theory . 56 5.4.1 Ontology of QFT . 57 5.5 Other alternatives to QFT? . 59 5.6 Theory selection and reality . 60 2 6 Conclusions 62 7 List of Works 64 A Sociology of Science – Should Scientists Care? 66 B Conditions for Objectivity 78 C An interview with C. LLewellyn Smith 94 D Referential Realism and Appropriate Technology 100 E g 2 and the trust in experimental results 104 − F The Casimir effect and the interpretation of the vacuum 123 3 1 Introduction What I want to address in this thesis is the relationship between High-Energy Physics (HEP) and reality. First, however, I will have to discuss what can pos- sibly be meant by these terms. The name HEP refers to the fact that studies of the microscopic structures of matter often consist in analyzing the reaction prod- ucts of high energy particle collisons1. Questions about reality (e.g. what reality consists of), and their relations to the notions of truth and objectivity, have been discussed throughout intellectual history, and not least in connection with science. Consider for example the statement: ”The goal of physics is to understand nature at the most fundamental level.” In first approximation this amounts to saying that the goal of physics is to understand reality — simply by equating ‘nature’ with ‘reality’. An impressive arsenal of philosophical arguments is raised against such an interpretation immediately. For instance: ”Are you making claims of reality in itself or as it appears to you”, ”Is this distinction possible in the first place?”, ”Does ‘understand’ mean that the theories of physics are true, and if so, in what sense?”, ”How will physicists ever know if the goal has been obtained?”, and ”In what sense does physics deal with the most fundamental level?”. These questions involve issues such as the conditions for obtaining knowledge, the possibility of reducing different branches of science to others and the meaning of concepts. Traditionally, philosophy distinguishes between epistemological and ontological, or metaphysical, questions. Epistemology addresses the nature of acquisition and justification of knowledge whereas ontology is about what lies behind our experiences of the world — what the world is really like. Sometimes ontology, or metaphysics, has been ridiculed as unnecessary and speculative, but the ontological aspects of questions such as ”is it possible to reduce biology to physics or is there a genuine division between the physical and the mental?”, or ”what are the causes or expla- nations for the phenomena we see” continue to attract interest from philosophers and scientists alike. Though the distinction between epistemology and ontology is useful, it is not always sharp. For instance, if one embraces the epistemological idea that scientific knowledge ultimately corresponds one-to-one with reality, then some ontological questions about what really exists are naturally answered. An investigation of the philosophical aspects of HEP must necessarily, within the scope of this work, be selective regarding aspects of the HEP-reality relation- ship. A hint of such selectiveness, or focus, is provided by the philosophy of science. Specifically, the two topics ‘theory-ladenness of experiments’ and ‘underdetermina- tion of theories by observations’ serve well to illuminate some epistemic aspects of HEP with ontological consequences. Very crudely, the content of the first is that experiments are always seen in a certain theoretical light, so that data are never pure. Underdetermination means that there is, in principle, always more than one theory capable of explaining the same set of observations. The ideas of theory-ladenness and underdetermination will provide the starting point for the philosophical discussions of HEP to follow. But first I will review and comment upon some of the recent approaches to the philosophical understanding 1Sometimes this branch of science is referred to as elementary particle physics or simply particle physics. In a chapter 3, I will discuss the structure of HEP in more detail. 4 of science in general. In addition, I will address a more semantic aspect of science which suggests certain constraints on the epistemology of science. Indeed, Niels Bohr is often quoted as saying that science aims not to understand nature but what we can meaningfully say about it. This has lead others to analyze the language we use for a scientific description and the conditions which govern this activity. Such a project has been pursued by the philosopher Peter Zinkernagel with whom I have been discussing physics, epistemology and his theories on these matters for many years. Despite my efforts to narrow the scope of study, I initially cut across a large number of issues related to the philosophy of science tradition. While this necessarily leads to some loss of details, it is my hope that the broad range of philosophical issues mentioned will put specific discussions of HEP in a proper context. 1.1 The method Coming from a physics background, the method in this work has primarily been learning and applying. More specifically, I have used my background to analyze parts of HEP in light of the philosophical discussions and positions which I have continually studied. At the same time, I have attempted to keep a critical eye on the philosophical underpinnings of the conclusions about the relationship between science and reality which have been given in the literature. The emphasis in this project is both on original case-studies of HEP and case- studies done by other scholars. It is well known that strict generalizations from single events, or even classes of events, are impossible. This is the philosophical problem of induction. On the other hand, if one is to understand science, or at least obtain some kind of overview, it is necessary to draw lessons from case-studies. By pointing to some of the problems raised by case-studies, one can illuminate the HEP-reality relationship from different perspectives. Except for many stimulating discussions with physicists and philosophers alike, the main input to the original case-studies presented in this work has come from the published literature. 1.2 Structure of the thesis Although the discussion at times will be technical, I have tried to keep this thesis readable for all who have a general knowledge of science and philosophy. While the following can be read on its own, I will make explicit reference to the appendices which contain a substantial part of the actual research I was involved in during the course of my studies (see chapter 7 for an overview). I begin in chapter 2 with a discussion of various philosophical positions on the relations between science and reality. Short introductions will be given to the ideas of incommensurable paradigms, theory-ladenness, and the underdetermination of theories by observations. Recently, these philosophical ideas have provided fuel for the social constructivists who have argued for a strong context-ladenness of science. I give a brief introduction to this branch of science studies and discuss how it is related to epistemology and ontology.