Non-Fiction Reviews
Charge: Why does gravity rule?
(2024) Frank Close, Oxford University Press, £16.99, hrdbk, 255pp, ISBN 978-0-198-88505-4
As former Head of Theoretical Physics at the Rutherford Appleton Laboratory (the UK’s leading particle physics research centre) and Head of Communications and Public Understanding at CERN, Frank Close is very well-placed to take the lay reader through the intricacies of the so-called ‘Standard Model’ which covers three of the four known fundamental forces: electromagnetic, strong and weak nuclear. What is left out, of course, is gravity, the weakest of all and Close’s overall message is that were it not for the balance achieved between the other three, gravity would have not been able to get enough of a grip on matter to form stars and galaxies, or planets and, of course, us.
He begins with a down-to-Earth but eyebrow-raising conundrum: with every breath we take in 1,000 lightning bolts-worth of electric charge, in the form of electrons, so why aren’t we zapped several times over? The answer is that the electrons are captured by the atoms of our bodies and their negative electric charge is balanced out by the positive charge of the protons in the nuclei. But that then generates a further puzzle: how is it that this delicate balancing act came about, when electrons and protons are so different, not least when it comes to their mass? (Editorial note: very roughly an electron has approximately a two thousandth the mass of a proton.)
In seeking answers to these questions, Close takes us through the development of elementary particle physics over the last century or so. The stage is set with the chapter on ‘The Nuclear Atom’ but here, I think, the account of quantum theory in terms of waves fitting into electron orbits is a surprising mis-step – these are not your regular waves, as Close himself briefly acknowledges.
Things are much more sure-footed in the next chapter on electromagnetism, which takes us from Maxwell’s equations to Dirac’s quantum electrodynamics and the possibility of magnetic monopoles. However, there also exist other forms of charge, such as those which ‘spawn’ (p. 51) the ‘weak’ and ‘strong’ nuclear forces. The former is associated with radioactive beta decay and as Close points out, it has similar features to its electromagnetic counterpart. Where it differs, however, is in the violation of ‘parity’, a kind of mirror symmetry and why nature is asymmetric in this way remains a further mystery (p. 57 and pp. 159-161).
The strong nuclear force is then considered in Chapter 5, titled simply ‘Quarks’. Originally proposed as a kind of book-keeping device, these are now accepted as constituents of the protons and neutrons that form atomic nuclei. Given Close’s background and his other works, including The Cosmic Onion: Quarks and the Nature of the Universe (Heinemann Educational, 1983 – since revised), it should come as little surprise that this, and the following chapter on quarks’ colour charge, are two of the clearest and most engaging in the book. The ‘Janus-faced’ nature of these particles is then explored in Chapter 7, where this is explained in terms of their carrying one or other of two different forms (labelled ‘up’ and ‘down’) of the ‘flavour’ charge which gives rise to the weak nuclear force. Here again we see a ‘profound similarity’ (p. 105) between quarks and particles such as the electron and we are given the beginnings of an answer to our initial mystery: it is because the average charge of two up quarks and one down quark is 1/3, together with the fact that quarks cluster in threes that we have a perfect electrical balance between positively charged protons and negatively charged electrons (p. 106). Of course, that still leaves the further, more fundamental, mystery as to why there is this similarity to begin with.
Hopefully one of the ‘Grand Unified Theories’ on the (distant) horizon and covered in Chapter 8 will provide an answer. The problem then is to find evidence for such theories and Chapter 9 sketches some recent experiments in this area, none of which have come up with the goods – so far.
The final chapter, ‘Mysteries’, returns to the opening puzzle and asks how sure are physicists that the charges on the electron and the proton are indeed identical (albeit opposite in sign). The answer is, pretty sure! Here Close presents an interesting and, for me, all too brief, survey of the cosmological consequences were these charges to differ, even if only very slightly. As it stands, then, and as Close acknowledges, the mystery remains unresolved.
Despite that indeterminate conclusion and an overall imbalance within the book itself, generated by the focus on the theoretical side of things, this remains a clear and engaging account. To cover such a broad sweep of modern physics in just 170 pages takes a fair bit of skill and there are precious few folk around capable of pulling it off as nimbly as this.
Steven French
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