Particles, fields, space-time: From Thomson's electron to Higgs' boson

Martin Pohl
Particles, fields, space-time: From Thomson's electron to Higgs' boson
CRC Press, ISBN 978-0-367-34723-9

Some highlights

In the following, I give some examples of the topics I found particularly interesting or illuminating in the different parts of the book:

• In the introductory and background chapters, the “tree diagram” of a “reductionist view of the development of particle physics”, as Martin Pohl calls his position, or the many cases he discusses where advances in craftsmanship and technology laid the ground for discoveries in physics (e.g. glass blowing and vacuum pumps for cathode ray tubes in the second half of the 19^th century).
• In chapter 9 on the standard model, the very clear and, at the same time, short account of the development of electroweak theory, in particular of the Wu parity violation experiment and more generally of the symmetry violations by the weak interaction (indeed useful for teaching purposes), as well as a mainly conceptual but thorough treatment of the main elements of strong interaction theory (self-interaction, running coupling constant, asymptotic freedom, confinement, hadronization, etc.).
• In the “outlook” chapter 10, e.g. the discussion of the Hillas diagram for classifying sources of high-energy cosmic rays according to their size R and magnetic field B (based on the Larmor radius, making accessible a topic of current particle astrophysics even at a high-school level), and in general the fascinating extension of accelerator particle physics to cosmic ray observatories and the Alpha Magnetic Spectrometer (AMS) on the ISS.
• Many interesting historical considerations and excursions within the individual chapters, e.g. about Bruno Pontecorvo, his contributions to neutrino physics, and his “somewhat mysterious” life as a communist and spy for the Soviet Union. Another interesting example concerns the Gargamelle experiment, which in 1973 provided evidence for neutrino elastic scattering of the type νe^¬- → νe^¬-, as predicted by electroweak theory. Pohl emphasizes that “[t]o filter such rare events out of the many thousand (mostly empty!) bubble chamber pictures is an impressive achievement” and gives the names of the all-female team who achieved this feat (which the original publication doesn’t). The acknowledgement of the too long neglected work of women in science, at this and other places in the book, makes with its sober matter-of-factness a quite convincing impact.
• To the historical perspective the entire chapter devoted to “War time physics” adds the description of a kind of historical turning point, which was scientifically extremely prolific – whether you want it or not – and politically decisive for today’s world.
• Last but not least, the entire chapter on “Enabling technologies”, putting the focus on their interplay with discovery and progress in physics as mentioned above, starting from pioneering work on cosmic rays, through extended and insightful treatment of detector and accelerator physics, to failed projects, which are also historically most interesting.

Of course, the above perspective is completely personal, and tainted by not being a specialist (as intended by the author). Moreover, I am working in physics education and physics teacher education, and thus in some way responsible for the “next generation” of physics students and – hopefully – interested laypeople. This has clearly had some influence on the perspective I have taken towards the book and the selection of smaller and larger “gems” I found within it. Analogously, this applies also to the next para­graph.

Some points to discuss, minor improvements, and 1½ points of gentle disagreement

A few points that might merit re-consideration are to be mentioned. A minor question is why the term ‘confinement’ appears nowhere; if there is a reason for this, it would be interesting to know it. Then, the very clear focus box 9.17 on the cross section of the e^-e⁺ annihilation into hadrons could be completed by the famous graph showing the “jumps” of the cross-section at the thresholds given by the quark masses, a graph that has several educational merits: a reasoning accessible to high school students, but going right to the heart of the standard model; an example of a “precision test” of the model; and an example of the impressive collective gathering of data in the international community of physicists [3; fig. 52.2]. The following point might help to better follow the book’s line of thought: The notions of decay width, peaks and resonances appear as early as on p. 139, then at several other places, but are only properly introduced in box 9.12 on p. 204; this box should be moved to sect. 7.3 (alongside box 7.7 on decay widths), where cross-sections and decay widths are introduced as main observables for the square of the invariant amplitude.

I turn to my very few points of disagreement with the book. The first ½ point concerns the author’s view on the history of science, which he says has often bored him, as “who exactly published what detail first” is not really interesting. Truly so, but I think this perception does not really do justice to the history of science as it is practised today. Rather, it is about the evolution, revolutions and interactions of scientific thought and their driving forces, in some sense the “dynamics”, not just the “kinematic record” of the history of science, exactly in the way of the historical aspects treated in Martin Pohl’s book, and this is interesting. In fact, it is also illuminating and can be helpful for the understanding of scientific content, not only for motivation (see e.g. [4]). If researchers in this field base their conclusions on meticulous validation of historical data and facts, this should appear to physicists as being rather akin to their own approach.

The other point of respectful disagreement is on the use of the so-called “natural units” ℏ = c = 1 etc. While it is clear that this is very convenient for experts, there are three objections to this: First, an expository text should be oriented towards the convenience of the reader (learner), not the convenience of the author (expert). Second, learners lose a powerful method of error checking and of order-of-magnitude reasoning [5], as illustrated e.g. in the marvellous “Search for Simplicity” series by Victor Weisskopf [6]. These faculties are even more important in a new, unfamiliar field. It is difficult enough to train students for them, so an educational text should support rather than hamper their development. Third, the educational potential of dimensional considerations is replaced by an opaque assertion. A physical quantity is given by the product of a number and a unit [7]; changing the unit changes the number but does not make the whole product disappear. So, what the so-called “choice of natural units” in terms of natural constants actually does is to set the numbers to 1 and supress the units, which amounts to suppressing the constants altogether [8,9].

By way of conclusion

Beyond its depth, breadth and inclusion of historical aspects, there is another key feature of the book I found very interesting: the very close treatment of experiments, alongside with the theory and the technology behind them. This reminds me of the great “classic” by a E. Bodenstedt on “Experiments in nuclear (and particle) physics and their interpretation” [10], a reference work in the area during the studies of this reader; in some sense the book by Martin Pohl is a modern exemplar in the same insightful style on “Experiments in Particle Physics and their Interpretation and Technology”.

Coming back to its broad scope, “Particles, fields, space-time: From Thomson's electron to Higgs' boson” contains a very stimulating bibliography of almost 700 references, including pioneering papers, reviews, popular, historical, philosophical and political accounts, and more – one sees how a lifetime of thought has gone into the book.

And finally, a figure of thought I liked very much: the large parenthesis between the reductionist credo in ch. 1 and a pensive quote by Freeman Dyson with which the book ends: “A reductionist philosophy, arbitrarily proclaiming that the growth of understanding must go only in one direction, makes no scientific sense. Indeed, dogmatic philosophical beliefs of any kind have no place in science.”

Andreas Müller, Université de Genève

[1] Heisenberg, W. (1958). Physics and philosophy: The revolution in modern science. New York: Harper & Row, Publishers.
[2] Anderson, P. W. (1990). Some Thoughtful Words (Not Mine) on Research Strategy for Theorists. Physics Today, 43(2), 9–9.
[3] Particle Data Group (2020), Review of Particle Physics, Prog. Theor. Exp. Phys., 083C01; academic.oup.com/ptep/article-pdf/2020/8/083C01/34673722/ptaa104.pdf .
[4] Matthews, M.R. (Ed.) (2014). International Handbook of Research in History, Philosophy and Science Teaching. Dordrecht: Springer.
[5] Robinett, R. W. (2015). Dimensional analysis as the other language of physics. Am. J. Physics83(4), 353-361.
[6] Weisskopf, V. (1985 - 86). Search for Simplicity, Am. J. Physics53(1-12); 54(1-2).
[7] International Bureau of Weights and Measures (BIPM) (2019). The International System of Units (SI) (9^th ed.) Paris: BIPM; www.bipm.org/en/publications/si-brochure/ .
[8] Desloge, E. A. (1984). Suppression and restoration of constants in physical equations," Am. J. Phys. 52(4), 312-315.
[9] In fact, what is really behind “ℏ = c = 1” can be put in clear terms, and it is something different from a mere choice of units [8]; interestingly the underlying mathematical structure is that of a vector space which can even be used to turn an opaque, ill-formulated procedure into an interesting exercise for students; Maksymowicz, A. (1976). Natural units via linear algebra. Am. J. Physics44(3), 295-297; Ansmann, G. (2015). Natural units and the vector space of physical values. Eur. J. Physics, 36(3), 035008.
[10] Bodenstedt, E. (1978, 1979). Experimente der Kernphysik und ihre Deutung (3 volumes). Mannheim, Wien, Zürich: Bibliographisches Institut.

[Published: January 2022]