Abstract
Niels Bohr’s most-noted contributions to contemporary thought center around his work with quantum theory. His atomic model of 1913, his complementarity approach to quantum mechanics, and his ensuing lifelong debate with Albert Einstein have all received extensive attention from physicists, historians, and philosophers alike. Yet Bohr’s scientific creativity was not limited to atomic phenomena and quantum mechanics. He continued to blaze new paths into uncharted territory during the 1930s and 1940s as an early leader in the developing field of nuclear physics. This collection of original work, however, has garnered far less attention from Bohr scholars than has his well-known work with non-relativistic quantum mechanics.
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Notes
Some of this work may be found in the pair of volumes, Brown and Harré (1988) and Saunders and Brown (1991). Along with this attention has come some debate about how ‘new’ the problems of field theory actually are: some philosophers maintain that the philosophical puzzles of quantum field theory are the same as those already treated in analyses of non-relativistic quantum mechanics.
A not-quite-complete list would include, e.g., Wallace (1968), Feinberg (1972), Redhead (1980), Freundlich (1980), Pickering (1982), Cushing (1983), Capek (1984), Cushing (1985), Hones (1987), Gavroglu (1988), and Hones (1991). Nearly all of these articles discuss either the adequacy of our concept of “elementary particle”, with reference to containment and Geoffrey Chew’s “bootstrap hypothesis” of the early 1960s, or methodological patterns of high-energy physics theory evaluation.
For the purposes of this historical account, I date the ‘beginning’ of particle physics as a distinct sub-discipline of physics at around 1950. This is somewhat arbitrary, since much work which was done during the 1920s and 1930s could be labeled retroactively as “particle physics”. In fact, according to the Oxford English Dictionary, the term “elementary-particle physics” dates from 1946. (Cf. Hovis and Kragh, 1991, 781.) Still, 1950 provides an easy benchmark, corresponding to the first Rochester conference. For more on the early history of particle physics, see especially Brown and Hoddeson (1983), and Brown, Dresden, and Hoddeson (1989).
For more on Bohr’s contributions to the early history of CERN, cf. Moore (1966), 428–430, Amaldi (1985), Hermann et. al. (1987), and Pais (1991), 519–522. Aaserud (1990) treats a similar example of Bohr’s fund-raising and political support for, yet lack of scientific involvement in, the experimental biophysics program.
Friedman and Weisskopf (1955), 134. The essay is reprinted in Weisskopf (1972). In remarks added to the original essay, Weisskopf explained that “Bohr’s compound-nucleus picture is more useful and applicable than the remarks at the end of this [1955] article imply ...[M]ost of Bohr’s conclusions about the compound nucleus, in particular its decay, remain valid today”. (Weisskopf, 1972, 166 – 7). It was Weisskopf himself who helped to produce the refined picture which eventually replaced Bohr’s initial compound nucleus model. Weisskopf’s work with Herman Feshbach and Charles Porter in 1953–4 is often referred to as the “cloudy crystal ball” model. Cf. Feshbach, Porter, and Weisskopf (1953) and (1954), and Weisskopf (1972), xii–xiii, 18–19.
This brief historical treatment thus relies upon a distinction I have made elsewhere between ‘historical influences’ and ‘philosophical aspects’ (cf. Kaiser 1992a). My interest here is not to trace subtle and variegated ‘routes of transmission’ from Bohr’s 1936 address to the particle physics studies of the 1950s or of today, which would no doubt involve physicists such as Weisskopf, but rather to note the continuity of several philosophical aspects of Bohr’s work with contemporary studies.
The “resonance” terminology prevalent throughout the 1950s and early 1960s in particle physics, as Andrew Pickering explains, was imported directly from nuclear spectroscopy studies. Cf. Pickering (1984b), 33, 120, and Hones (1991), 62–63.
Cf. Blatt and Weisskopf (1952), 340, and Friedman and Weisskopf (1955), 139. The limits to the validity of Bohr’s assumption of independence are discussed in Blatt and Weisskopf (1952), 340 – 342. It was an investigation of these limits which led to Weisskopf’s refined picture in the “cloudy crystal ball” model.
Breit and Wigner (1936). This paper was received by the Physical Review on 15 February 1936, two weeks following Bohr’s address to the Copenhagen Academy; it is doubtful whether or not Breit and Wigner were aware of Bohr’s model at the time they wrote their paper. There is little question, however, that the conceptual legacy of the compound nucleus model derives primarily from Bohr’s work: rather than presenting a new fundamental way of viewing nuclear reactions, the Breit-Wigner paper treated one particular instance of resonance phenomena (the capture of slow neutrons). Their mathematical analysis was soon generalized by others, whereas Bohr’s qualitative approach remained relatively intact and unchanged until 1953.
Cf Krieger (1992), 58–59. For examples of Dirac’s earlier use of the concept of “plenitude”, cf. Kragh (1990), 270–274.
For more on particle physics phenomenology, cf., e.g., Kane (1987) and Renton (1990). This account overlooks the rich history behind the “Standard Model” of electroweak interactions, including the prolonged concerns during the 1950s and 1960s about which kinds of coupling (scalar, vector, tensor, axial vector, or pseudoscalar) are exhibited in nature. Other papers dating from the same historical period as Bohr’s 1936 address have also been very influential in shaping today’s view of particle interactions, such as Hideki Yukawa’s 1935 work on the mediation of forces by exchange particles, and Enrico Fermi’s 1934 point-like, or current-current, model of weak interactions. Still, it is significant philosophically that our present ideas about particle interactions reflect Yukawa’s and Bohr’s approaches, incorporating two-step parametrizations for reactions, rather than Fermi’s one-step model.
Victor Weisskopf (personal communication) has pointed out an important distinction between Bohr’s use of intermediate states and those currently used in particle physics phenomenology: for Bohr the compound nucleus states were real, whereas most of the contemporary particle physics states are thought of a virtual. (My thanks to Professor Weisskopf for sharing his comments on an earlier draft of this paper.) Yet the dependence of particle physicists upon these intermediate states still points to an inheritence from Bohr: even as the character of the intermediate state has changed, physicists have tacitly insisted upon parametrizing reactions in terms of Bohr’s 1936 postulates.
Equations (8), (9), and (10) are written for the ‘inclusive’ reactions of interest. In other words, the proton and anti-proton will produce the various bosons plus other particles to conserve such quantities as electric charge. Only the particles involved with the rest of the reactions (either H,W (83), or W R) are written. These equations could also be written as ‘exclusive’ reactions, in terms of the proton’s and anti-proton’s constituent quarks. The (83) refers to the mass of the standard model W boson, so as to distinguish it from the much heavier (hypothetical) W R.
Bullock, et al. (1991a) and (1991b). The “handedness” of the taus refers to their helicity: “right-handed” means that the spin of the tau is parallel to its momentum, whereas ‘left-handed’ means that the spin is antiparallel to its momentum.
Kaiser (1993) for a derivation of this observable asymmetry.
From an unpublished version of Bohr’s 16 September 1927 Como lecture. The manuscript was mailed to G.C. Darwin on 16 October 1927, and is reprinted in Works 6, 89–948. Cf. Kaiser (1992a), 226 – 231.
N. Bohr, unpublished “Post Scriptum”, dated 13 August 1957; quoted in Kaiser (1994). Emphasis added.
Cf. Hones (1991) for a discussion and critique of Hacking’s positions in the context of experimental high energy physics.
The establishment of the ontological existence of neutral currents in 1973–4 similarly revolved around such questions of extracting (statistical) signals from backgrounds. For more on the history of their ‘discovery’, cf. Pickering (1984a), and Galison (1987), Chap. 4. These two historical accounts differ in their ontological approaches: whereas for Pickering the effects which physicists eventually interpreted as arising from neutral currents were constructions of the physicists’ own making, Galison highlights the interaction between physicists and material resistances which refused to “make these effects go away” (in the words of one physicist). Cf. also Hones (1987). Although Pickering elaborated his construction-model further in Pickering (1984b), his more recent work appears to lead in the direction of Galison’s approach. Cf. Pickering (1989).
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Kaiser, D. (1994). Niels Bohr’s Conceptual Legacy in Contemporary Particle Physics. In: Faye, J., Folse, H.J. (eds) Niels Bohr and Contemporary Philosophy. Boston Studies in the Philosophy of Science, vol 153. Springer, Dordrecht. https://doi.org/10.1007/978-94-015-8106-6_11
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