Antimatter – the Ultimate Mirror

17 August 2000

by Gordon Fraser, Cambridge University Press, 0 521 65252 9, £16.95/$24.95.


The correct prediction of antimatter by Paul Dirac is arguably the most astonishing intellectual achievement of the 20th century. By insisting that quantum theory and special relativity must be consistent, he was able to deduce the generalization of the Schrödinger equation to the Dirac equation. By doing that he was able to give a proximate explanation for spin, and to predict a whole new set of particles, antimatter. That the human mind can discover a previously unknown part of the world is a great achievement. (I largely agree with Antonino Zichichi who argued for Dirac as the most important physicist of the 20th century in Physics World in March.) Gordon Fraser’s lively and interesting book provides a broad treatment of this story, and the history, science and implications of antimatter.

This is a very nice book, totally accessible to any curious reader, yet with occasional thought-provoking pieces even for experts. Fraser keeps a fast pace, explaining the science well but taking care not to dwell too long on any difficult aspect. In a few places I didn’t fully agree with his viewpoint or arguments. I will mention some of these as a service to possible readers, but they do not detract from the value of a successful book.

Publishers are notorious for writing anything they please on book jackets and in publicity. Fraser is not responsible for then remark on the jacket that the book is about how science fiction became fact, which is, of course, the opposite of what happened (the remark is taken from the title of chapter 1, but its meaning is different there), or the charming reference to “Hans van der Meer” in the publicity, mixing up Hans Dehmelt (whose work with traps is described in chapter 11) and Simon van der Meer (who figured out how to get antiprotons in sufficient quantities to make a collider.)

Chapter 1 describes the public excitement about the 1995 discovery of antiatoms, and then begins the history. My impression of one bit of the history differs a little here. Fraser says that at first Dirac thought that the antielectron was the proton. He may be correct, but I have heard over the years that people pushed rather hard on Dirac about where the predicted antielectron was – after all, predicting new particles was not normal then. Dirac defensively remarked that perhaps it was the proton, though he knew that that didn’t make sense.

The next chapter introduces the relevant symmetries, charge conjugation, parity and time reversal, and then provides a quick history from Galileo through Newton to Einstein. It includes the Thornhill portrait of Newton without a wig, which I have seen in the Master’s Lodge of Trinity College, Cambridge – Newton looks much more like a physicist there than in his usual wigged appearances. Here and later the book has a nice way of giving brief descriptions that capture the essence of people.

Chapter 3 is a history of the acceptance of atoms, and the discoveries of the electron, nucleus, proton and neutron. Next is a more thorough biographical treatment of Dirac, with some of the many anecdotes, followed by the development of quantum theory and the Dirac equation. Chapter 5 describes the positron discovery, including the opposition of R A Millikan. That opposition helped to make European physicists more aware that Carl Anderson’s CalTech data could be the antielectron than were the US physicists. There is also a (delightful for a theorist) quote from Rutherford of a sentiment that we still encounter: “It seems to be to a certain degree regrettable that we had a theory of the positive electron before the experiments…I would be more pleased if the theory had appeared after the establishment of the experimental facts.”

Fraser then presents a quick discussion of infinities, renormalization and Richard Feynman, and interesting speculations on Dirac and Feynman’s distinctive personalities and the strong influences of their fathers as they were growing up. The story moves to the development of accelerators and the discovery of the antiproton, and then to quarks. (A minor point: the wording of a sentence on p108 suggests that quarks have a known size, but in fact there is only an upper limit and quarks are expected to be far too small to measure their size directly.) Next comes further discussion of parity violation and then CP violation, leading up to Andrei Sakharov’s statement of the conditions required for an explanation of the mysterious baryon asymmetry of the universe.

Particle colliders, which of course, require expertise in handling antimatter, are brought in and some of their discoveries presented. The only typographic error I found was on p175, where the ratio of the top quark mass to the b-quark mass is about 35, not 300. Chapter 13 is basically on antimatter technology, including PET scans and more. Fraser gets somewhat sensational here, beginning the chapter with a survey of the Reagan era “Star Wars” antimissile programme, and then unfairly relating that to the US plans to build the Superconducting SuperCollider, even seeing a connection to antimatter propulsion proposals and personnel for Star Wars. He also laments the loss of the LEAR antiproton beam at CERN, and perhaps misses an opportunity to discuss the difficulties of doing all science projects in times of limited resources, and of deciding which ones to pursue.

Why the universe is matter and not antimatter is still a mystery. The explanation of the evidence in chapter 14 is very clear. However, there are more approaches that could eventually explain this mystery than the book suggests. The problem is that the calculations are very difficult and the underlying theory is not established. Perhaps most fundamentally, we do not yet know the origin and size of the CP-violating effects that are essential to explain the matter asymmetry. One piece of progress is that we do know now that the Standard Model cannot explain the matter asymmetry of the universe, so new physics must enter. It is likely that the phases that lead to the CP violation needed to generate the matter asymmetry arise when string theories are compactified to three space dimensions and when supersymmetry is broken, but these subjects are not yet well understood. If you think these approaches are somewhat far out, you’ll enjoy Fraser’s speculations on this issue even more.

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