Prof. Dr. Klaus Hentschel

What is today’s mental model of the photon? Naively materialistic projectile or corpuscular interpretations of light were defended not only by Isaac Newton and his many nineteenth-century acolytes but by important experimental physicists from the following century, such as Johannes Stark or Arthur Holly Compton. Thomas Young, Augustin Fresnel and many other scientists of the nineteenth and early twentieth century countered it with an equally naively absolutist wave theory of light underpinned by Maxwell’s equations for electromagnetic radiation. Neither of these interpretations can account for the wave-particle duality established in 1909. That’s why Sects. 9.1–9.2 argue in favor of an ontologically restrained, instrumentalistic interpretation and advise against naive realism or the illegitimate assignment of locality. Sect. 9.4 goes further to warn against our tendency to individualize reality. Only thus can the Bose-Einstein statistics be duly accounted for. All of this is so counterintuitive to us, schooled by macroscopic experience, that the light quantum-alias photon-must remain a“mysterious Cheshire ca” or “elusive beast”.

How are these developments reflected in textbooks and scientific instruction? Charts and examples show how eight significant episodes in the history of the introduction of the light quantum are barely adequately or insufficiently treated in over 100 analyzed science textbooks and almost 40 practical instruction manuals. Coming generations of scientists are repeatedly confronted with historically false myths, barring the way toward a deeper understanding of the historical processes but also to sufficient comprehension of the concepts involved, such as the light quantum a.k.a. the photon. Support is offered for the claim that professional instruction in the history of the science and technology must form a part of good scientific training.

Proceeding from the introductory propositions about semantic accretion and folds or ‘convolutions’ of meaning, we now treat the conceptual history of light quanta or photons. The twelve semantic layers forming the notion are presented: (1) the particle model of light, starting with Newton into the nineteenth century; (2) light propagation having a finite high velocity; (3) the emission and absorption of light particles by matter; (4) the transfer onto matter of momentum as well as (5) of energy by light; (6) the energy-frequency proportionality E ∼\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim $$\end{document} v in the photoelectric effect; (7) strict energy quantization E = hv; (8) wave-particle duality, first intimated by Einstein 1909 and developed further by Louis de Broglie and others; (9) the distinction between spontaneous and induced emission, introduced by Einstein in 2 talks held 1916–17, forming the theoretical basis of the modern laser; (10) internal angular momentum (spin 1) carried by light quanta; (11) indistinguishability among light quanta, ultimately leading to Bose-Einstein statistics; (12) the photon as a virtual exchange particle of QED that thanks to its lack of mass can transport electromagnetic forces over infinitely large distances.

Photon experiments in quantum mechanics (since 1945) are discussed. Sect. 8.1 begins with photon clumping since Hanbury Brown and Twiss 1955–57. Sects. 8.2–8.3 report about beam-splitter experiments on single photons, and single-photon interference by Taylor 1909 up to Grangier et al. (1986). Sects. 8.4–8.5 cover Alain Aspect’s experiments on EPR photon-photon correlations 1980ff. and Wheeler’s delayed-choice and which-way experiments, which later developed into quantum entanglement and quantum teleportation (in Sect. 8.8). Sect. 8.6 covers photon bunching, i.e. a clustering of photons which as spin-1 particles are quantum statistically bosons. Then the experimentally verified Hong-Ou-Mandel dip is followed by photon antibunching in resonance fluorescence. The final section of Chap. 8 outlines the high-energy scattering experiments conducted with lead ions in the Large Hadron Collider (LHC) at CERN, publicized in February 2017, perhaps the first direct observation of photon-photon scattering hitherto only be testable by a number of indirect indicators.

Planck’s and Einstein’s steps toward quantization are discussed, including a historical comparison of these two very different thinkers, their motives and heuristics. Sect. 2.2 studies Albert Einstein’s arguments up to the 1905 paper and how the many important publications from this annus mirabilis and shortly afterwards until 1909 are interconnected. Max Planck’s second quantum theory 1909–13 serves as a contrast: with it Planck attempts to retract his hesitantly introduced quantization of energy from 1900 by blaming it on the material resonators inside an idealized radiator at equilibrium (a ‘black body’) and thereby uphold the theory of a continuous radiation field satisfying Maxwell’s equations. Other papers by Einstein banished this interpretative option by showing that Planck’s formula for the distribution of radiation already presupposes quantized energy in the radiation field itself. Sect. 2.5 discusses the variety of terms used by Einstein and his contemporaries during this work-in-progress. Most of the alternatives considered then, such as ‘light corpuscles’ or ‘energy projectiles’, have been forgotten since. Sect. 2.6 statistically retraces the slow rise of the term ’photon’ in common usage generally attributed to the physical chemist Gilbert N. Lewis (1926).

The notion of the ‘light quantum’ is interpreted as a conceptual fusion in the sense of Gilles Fauconnier’s and Mark Turner’s ‘conceptual blending’ (2002). The notion of a black hole can be interpreted as a superpositioning of the everyday notion of a hole in the ground (into which objects, such as golf balls, can fall and disappear) onto the mathematical concept of a space-time singularity. Analogously, photons can be interpreted as a superpositioning of the notion of corpuscularity (from the Newtonian projectile model of light) onto notions of quantized energy of those ‘light quanta’ (derived from entropy and fluctuation considerations). Figs. 7.1–7.3 provide three variously finely resolved depictions of this interpretation which can only be performed in historical retrospect, unlike my prior diachronic reading described in Chap. 3.

If Einstein himself could not fully cope with his concept of light quanta, it is no wonder that his contemporaries would not enthusiastically rally behind it from the start either. The early reception of the concept of light quanta starts with initial skepticism by almost the entire scientific community (including many of Einstein’s strongest supporters, e.g., Max Planck and his pupil Max von Laue). Sect. 5.2 interprets the Compton effect 1922/23 as a watershed and Sect. 5.3 discusses the theory by Niels Bohr, H.A. Kramers and John Slater (1924). This BKS theory was the last attempt at a quantum theory without energetically quantized photons—the correlation experiments by Walther Bothe and Hans Geiger (1924–25) refuted it.

Why could it be useful to think about the complex history of a concept like the photon? The dense stratification of a dozen layers of meaning, which are fused together in this concept is still a live issue. For a deeper understanding it is instructive to study the history behind the concept and the cognitive obstacles faced by some of the world’s most brilliant physicists. Participation in those profound, sometimes heated debates opens deep insights into the way in which our conceptual apparatus operates, into the genesis of new concepts and new mental models. This introduction explains the method employed by this study, particularly the general historiography of scientific terminology and the treatment of mental models from cognitive psychology. Sect. 1.2 draws clear defining lines between the labels ‘term’, ‘concept’ and ‘mental model’. Sect. 1.3 interprets the way a concept develops as layered semantic accretion. Specific preliminary studies by the author and other historians of science on individual historical episodes are described with full citations. Chap. 1 closes with the author’s acknowledgements to publishers, institutions and individuals.

The mental models by early actors are presented. Sect 4.1 discusses Isaac Newton’s “globuli of light” along with amendments by some of his important adherents; the next two sections cover Einstein’s mental model of light quanta around 1909 as singularities in the radiation field, along with his doubts about it 1910–15. Sects. 4.4–4.6 review the mental models of three influential experimental physicists: Johannes Stark’s light quanta, J.J. Thomson’s model of hard x rays, and W.H. Bragg’s neutral pair model for -rays. Sect. 4.7 treats the mental model argued by Planck, Peter Debye and Arnold Sommerfeld to relegate the quantization of energy and momentum to the material resonators in a black body. Sect. 4.8 deals with Gilbert Lewis’s mental model of photons by which this American physical chemist introduced the term ‘photon’ in his reflections about the temporal symmetry between emission and absorption but still attaching to it the completely wrong notion of photon number being conserved.

During Einstein’s lifetime, the special and general theories of relativity were quite frequently interpreted by philosophers. Most of these interpretations actually were misinterpretations. Even today interpretative statements about relativity theory are often false or highly misleading. Why is this so? In my Ph.D. dissertation (Hentschel 1990a), I analyzed (mis)interpretations by 10 different philosophical schools active in the early twentieth century which widely differed in their approaches, emphasis and blind spots. Many of these interpreters – including philosophers of high standing such as Ernst Cassirer, Moritz Schlick or Joseph Petzoldt – had studied the theory intensely and many even had close contact with Einstein himself or with one of the members of his “protective belt” of close friends and allies. Rather than declaring all of these (mis)interpreters as either luminaries or idiots (which would be implausible, if not downright silly), I show structurally how these misunderstandings arose and why they were kind of unavoidable, even for highly qualified and often well-informed interpreters. More popular texts about relativity theory were often second-order interpretations of these first-order accounts, thus multiplying the first-order errors of misinterpretations. I will give a few characteristic examples but my focus will rather be on structural characteristics of these misinterpretations. I will discuss how to analyze them historically by means of interpretational frames. A link will also be made to Ludwik Fleck’s thesis that socially and cognitively stabilized “thought collectives” (“Denkkollektive”) exert strong constraints on human thinking and interpretation (“Denkzwang”). My concept of interpretational frames is one method to formalize and analyze the complex interrelations between different assumptions and inferences within such a frame of thinking (“Denkstil”). Semantic frames and word clouds are also discussed as alternative approaches but both are discarded as unsuitable for the purpose of reconstructing interpretative frames.

This paper is a summary of my doctoral dissertation on philosophical interpretations of Einstein’s special and general theories of relativity, submitted to the Dept. for History of Science, Univ. of Hamburg, in 1989, which was recently published in the Series Science Networks at Birkäuser.2 After a brief overview of its content I will focus on a discussion of the method employed to analyse philosophical interpretations of a physical theory.My analysis is based- firstly on about 2500 contemporary published texts about the theories of relativity written both by scientists and philosophers. These texts have not been of particular interest to historians or philosophers of science up to now; this is understandable from the fact that many of diem contain gross oversimplifications, misinterpretations and incorrect statements about the theories of relativity.

Die Relativitatstheorien (RT) Einsteins gehoren zu den meistdiskutierten Theorien der Physik des zwanzigsten Jahrhunderts. Nach der Formulie­ rung der sog. 'speziellen Relativitatstheorie' (SRT) im Jahr 1905 nah­ men zunachst nur einige Spezialisten von ihr Kenntnis, bis mit ungefiihr fiinf Jahren Verspatung dann auch zunehmend Nicht-Physiker sich mit ihr zu beschaftigen begannen, angeregt durch populiirwissenschaftliche, all­ gemeinverstiindliche 'Einfiihrungen' von Kollegen Einsteins wie z. B. Paul Langevin in Frankreich oder Max von Laue in Deutschland. Diese Pha­ senverschiebung zwischen fachwissenschaftlichem Ausbau der Theorie und offentlicher Notiznahme wiederholte sich bei Einsteins 'allgemeiner Theorie der Relativitat und Gravitation' (ART). Zwischen 1913 und 1915 in ihren wesentlichen Ziigen ausformuliert, wurde sie erst nach einer spektakuliiren experimentellen Bestatigung im Jahr 1919 einem breiterem Publikum be­ kannt. In meiner Arbeit werde ich den Facettenreichtum der Ausdeutungen, die beide RT erfuhren, zunachst zu referieren und durch repriisentative Zi­ tate aus der Literatur der Zeit zu belegen haben. Der Umfang dieser Arbeit geht wesentlich auf das Konto dieser ausgewahlten Belege - davon verspre­ che ich mir, dafi nicht nur bislang unveroffentlichte Dokumente, sondern auch entlegene Texte hier in ihren zentralen Passagen leicht zuganglich ge­ macht werden. Fernerhin werde ich aber auch zu analysieren haben, warum derartig vielfaltige, einander mitunter diametral entgegengesetzte Interpre­ tationen einer wissenschaftlichen Theorie vorgelegt wurden.

This paper discusses facets of 19th-century scientific photography as a visual culture. The example of spectral research and documentation is particularly well suited, because prismatically diffracted light from the sun or from luminous gases was one of the most frequently examined phenomena of that century. The results were significant not only for physics but also for analytical chemistry and astrophysics. The spectrum also served as an ideal test object for checking the effectiveness of a wide array of photochemically sensitizable surfaces to the various color regions. Scientific photography became the most important experimental technique in the infrared and ultraviolet. H. A. Rowland's spectrum charts are discussed as an example of the transition from comprehensiveness in documentation to fetishism. The discussion of the Lippmann process, one of the first methods of color photography, addresses the associated training of the eye. Issues of authenticity and the much averred „mechanical objectivity” are raised with regard to retouching. The overriding theme of visual science cultures leads furthermore to unanticipated interdependencies with other scientific fields, such as geography, and draws the importance of practitioners into the foreground