Representing the Unobservable

Inhaltsverzeichnis

1. Frontmatter

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2. 1. Introduction

   • Open Access

   Markus Ehberger

   Abstract

   In this introductory chapter I will first describe the modern concept of the virtual particle in a non-technical manner and secondly emphasise the role of analytical categories and modern-day conceptions in a historical study of conceptual development. I will synthesize a preliminary history of the virtual particle concept from the available literature in the third subsection. Inconsistencies in the secondary literature are revealed and guiding questions will be formulated. In the last section, an overview of the contents of this book is given and the chapters most interesting for specific groups of readers are indicated.

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3. 2. How to Conceive of the Concept of Virtual Particles in a Historical Study of Its Development

   • Open Access

   Markus Ehberger

   Abstract

   This chapter serves three purposes: (1) I will discuss the modern concept of the virtual particle, its role in theoretical physics and in the philosophy of science in order to make the concept more accessible to the reader. (2) With this deeper understanding of the concept I will describe a pragmatically informed way of accessing the history of the virtual particle concept. I will introduce my main analytical categories: concepts as tools, concepts as representations, the narrative and the false prediction. (3) I will exemplify the use of my historical and analytical tool kit through one central example: The concept of the virtual particle in Richard Feynman’s Space-Time Approach to Quantum Electrodynamics (1949).

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4. 3. The Community of Practitioners

   • Open Access

   Markus Ehberger

   Abstract

   According to the approach described in Chap. 2, studying the development of a concept entails the study of scientific practices as well as epistemic and social constellations. For the reader not familiar with the development of the community of theoretical physicists in the years from 1923 to 1949, this chapter will give an outline of this network. It will also serve as a backdrop when the overall development fades into the background in the in-depth discussion in the chapters to come. I will describe the development of the community of practitioners starting shortly before the development of quantum mechanics and reaching out to the shift of research centres to the US in the post-WWII period.

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5. From Virtual Oscillators to Virtual Transitions (1923–1929)

   1. Frontmatter

   2. 4. The BKS Theory and the Light-Quantum Hypothesis: Virtual Entities and Transitions to Intermediate States, But in Different Conceptual Frameworks (1923–1925)

      • Open Access

      Markus Ehberger

      Abstract

      Prior to the development of matrix and wave mechanics, virtual entities figured prominently in the so-called BKS theory. According to some historians, the virtual oscillators of the BKS theory and their mathematical representations are directly connected to the concepts of virtual transitions/states. I will analyse the meaning of the term “virtual” in BKS, discuss the theory’s most important conceptual building blocks and its impact on dispersion theoretic accounts, most importantly the Kramers-Heisenberg paper of 1925. Then I will turn to a development that has nearly faded from sight: Shortly before the development of quantum mechanics, but from a light quantum point of view, physicists developed the idea that scattering and dispersion is composed of successive emission and absorption processes through “intermediate states”. The discussion of both of these strands, BKS and the light-quantum viewpoint, will serve as basis for the further development and for an important question to be answered in the following: Was there a generative connection between virtual oscillators and what came to be known as virtual transitions?

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   3. 5. Dirac’s Verbal Model: Making Transitions a Quantum Concept (1927)

      • Open Access

      Markus Ehberger

      Abstract

      This chapter focuses on Paul Dirac’s introduction of a verbally explicit description of scattering in terms of two successive emission and absorption processes, each coupled to a transition of the atom, not obeying energy conservation and located on the level of probability amplitudes. Historical actors called these energy non-conserving transitions “virtual transition” later on. I will contextualize Dirac’s introduction of this verbal model in reference to his research at the time, his interpretation of quantum theory and his rather pragmatic to flexible approach to mostly philosophical aspects of the interpretation of quantum theory. According to my argument, one of the most important reasons for Dirac to come up with this description was its usefulness in the practice of theory. A comparison between Dirac’s concepts and the concepts of the old quantum theory shows that neither the “intermediate states” of the light quantum point of view nor the “virtual” entities of the BKS theory had an important influence on Dirac.

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   4. 6. The Raman Effect: How Virtual Transitions Became “Virtual” (for the First Time) and Real Transitions Were Excluded from the Conception of Scattering (1928–1929)

      • Open Access

      Markus Ehberger

      Abstract

      The so-called Raman effect was the first empirical verification of a formula, which made prominent use of the concepts of energy non-conserving transitions. In the discussion surrounding its interpretation, virtual transitions were called “virtual” for the first time. I will contextualize this first usage of the terminology by C. V. Raman in relation to his own first simplistic interpretation of the effect named after him, describe the empirical material which made a reassessment necessary and discuss the terminological shift which came with Raman’s invocation. Contrasting this development, I will turn to the nearly simultaneous realistic reading of virtual transitions by Yakov Il’ich Frenkel. But neither Frenkel’s nor Raman’s interpretation could be upheld when confronted with the empirical material. While both Raman and Frenkel could uphold specific aspects of their conceptualization of scattering, it was clear that virtual transitions were neither ineffective nor actual.

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6. Theoretical Practice with Virtual Transitions (1928–1942)

   1. Frontmatter

   2. 7. Scattering and the Sea: Antiparticles and Intermediate States (1928–1931)

      • Open Access

      Markus Ehberger

      Abstract

      The so-called Dirac sea, an infinity of electrons filling up all negative energy states in the world, was the most common conception of the vacuum and anti-matter during the 1930s. In this chapter, I will review the development of this conception with specific focus on the role of virtual transitions (modern terminology). According to the argument developed in the first four sections, it was the occurrence of negative energy states as virtual ones that led Paul Dirac to a reconsideration of the negative energy states in general terms. In the further development and as my argument will go, the description of the scattering mechanism pointed to conceptual inadequacies of Dirac’s initial description of a hole in the sea as a proton. This chapter will thus discuss the impact of virtual entities on the interpretation and development of the conceptual framework of quantum electrodynamics. Furthermore, I will describe one instance of what I called a “false prediction” and discuss the relation of Dirac’s reasoning to arguments brought forward in the realm of Beyond Standard Model Physics today.

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   3. 8. The Practice of Time-Dependent Perturbation Theory (Part I): Formal and Conceptual Extensions (1929–1936)

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      Markus Ehberger

      Abstract

      This and the following chapter will provide for an in-depth discussion of the theoretical practice of QED during the 1930s. In this chapter, I will start out with a discussion of two dissertations (and connected papers) on quantum electrodynamics finished in Göttingen where Dirac had developed his verbal model. Both Maria Göppert-Mayer and Victor Weisskopf extended the realm of processes discussed with Dirac’s verbal model and the set of representations that were used. One of the outcomes was the first extension of perturbation theory to an arbitrary order by Victor Weisskopf (1933), later on at times referred to as “Weisskopf’s formula”. In the second part of this chapter I will turn to the most popular didactic works on quantum electrodynamics during the 1930s. I will discuss the role of the intermediate states and energy non-conserving transitions for these works (and papers related to them) and synthesize a general algorithm for calculations on QED during the 1930s.

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   4. 9. The Practice of Time-Dependent Perturbation Theory (Part II): Virtual Possibilities, Modes of Representation, and the Reprise of the Schüttelwirkung (1934–1942)

      • Open Access

      Markus Ehberger

      Abstract

      This chapter, as the prior one, will provide for detailed studies of the practice of QED during the 1930s. It will focus on the scattering of light by light, higher order calculations in radiation theory, the Delbrück effect and the theory of radiation damping. The historical study of light-by-light scattering will reveal another instance of what I called a “false prediction”, it will allow for a discussion of the rather curious terminology of “virtual possibilities”, which provides for a connection to the BKS theory, and it will exemplify the general algorithm of QED discussed in the previous chapter. Bernhard Kockel’s dissertation came with another instance of a “false prediction”, provided for a discussion of the exclusion principle for virtual entities and allows to compare the usefulness of the representational formats of tables and term scheme diagrams. The approaches to the Delbrück effect exemplify how specific ways of accessing a theory were upheld and how a “false prediction” can lead to the further evaluation of the formalism (in this case resulting in Furry’s theorem). Finally, the theory of radiation damping nicely demonstrates how the intermediate or “virtual” states could be manipulated by the historical actors. Its failure shows, that this could not be done at will. I will conclude this chapter with an analysis of the conceptual strands opened up in this and the prior chapter. It will bring together the most important conceptual results of the last two chapters.

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7. From Virtual Transitions to Virtual Particles (1930–1949)

   1. Frontmatter

   2. 10. In Between: Traces of the Virtual Particle During the 1930s

      • Open Access

      Markus Ehberger

      Abstract

      During the 1930s, historical actors discussed phenomena that made the concept of a virtual particle, contrasted to the concept of a virtual state, rather suggestive. The divergences of QED and phenomena of nuclear physics rested – when put into the framing of Dirac’s verbal model – on specific particles being present only in intermediate or “virtual” states. Studying these phenomena in the necessary depth will reveal how a verbal description of mathematical structures was only slowly established when physicist considered the phenomena to be essentially static, how specific mathematical formulations made conceptualizations more or less suggestive, and how basic pillars of the conceptual framework, like the concept of a particle or the vacuum, were slowly altered in connection to the concepts under study. In the epistemically uncertain terrain of quantum field theory in the late 1930s, the verbal structures which I called a “narrative” gathered prominence for the qualitative evaluation of theory and diagrams, Feynman-like but without calculational purpose, were proposed. Pulling the strings of conceptual development together in the concluding analysis, we can see that by the late 1930s a conceptual overlap, potentially tension between virtual particles and virtual processes had emerged and that it manifested itself in the different representational formats.

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   3. 11. Outlook: Feynman, Diagrams, and Virtual Particles (1948–1949)

      • Open Access

      Markus Ehberger

      Abstract

      In Richard Feynman’s work of the late 1940s the virtual particle came to the fore and took centre stage. Following Feynman’s approach to quantum electrodynamics and his quite idiosyncratic methods and conceptions reveals the diverse modifications the concept of the virtual particle underwent in the course of the reconfiguration of quantum electrodynamics. Freeman Dyson’s systematization of Feynman’s approach did not only bring Feynman diagrams to the masses, but in the discussion between Feynman and Dyson a “false prediction” of the diagrammatic format occurred. All the same, the virtual particle became ever more prominent and for Feynman himself took centre stage. We can understand this observation in a historio-philosophical way: When the exemplar, the paradigmatic problem of the theory, changed in the late 1940s one of the most important concepts of the practice of theory, i.e. the virtual particle, changed in its properties and its status.

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8. Analysis, Summary, and Conclusion

   1. Frontmatter

   2. 12. Representations and Practices in the Formation of the Virtual Particle Concept

      • Open Access

      Markus Ehberger

      Abstract

      In this chapter I will pull together the conceptual threads of the historical development. The review of the terminological variants shows that the concept of the virtual particle did not need the backdrop of a stable terminology in and for its development. The tabularic and diagrammatic representations are striking proof of the conceptual shifts discussed in the historical development. I will use the analytical category of a “false prediction” to pinpoint some of the conceptual differences between quantum electrodynamics in the 1930s and the late 1940s, which are strongly interrelated with the difference between virtual transitions and virtual particles. Finally, I will concisely revisit the historical development and argue that the concepts which can be connected to the historical trajectory of the virtual particle shared an important function, namely as the bridging elements in the theoretical description. And, as my argument goes, all of these concepts performed this function by extending the set of possible processes and by making this extended set of possibilities active physical entities.

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9. Backmatter

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