Mass - Jim Baggott *****

Jim Baggott is one of the UK's best popular science writers and never disappoints. As the book's name suggests, Mass is about what seems at first sight a straightforward and ordinary aspect of matter. It's just a property that stuff has that makes it behave in a certain way. But the further we get into the book, the less obvious the nature of mass becomes - as a reader, it can feel a little like following Alice down the rabbit hole.

We begin with a run through the history of our growing understanding of what matter is, and the nature of mass. Apart from repeating the myth that Giordano Bruno was burned at the stake for supporting a heliocentric cosmology, this is fairly straightforward stuff, but then Baggott makes the interesting step of not just making the transition from a philosophical view to a scientific one, but continuing with the philosophy to include, for example, Kant's 'Ding an sich' or 'thing-in-itself' concept that underlines the way that we can't actually know reality, only our sensory responses and the models we build. There was a time when scientists were on the attack as far as philosophy is concerned (Stephen Hawking infamously declared philosophy to be 'dead' in The Grand Design), but in practice, with a concept like mass, that philosophical consideration is important and useful.

As he continues, Baggott takes us through relativity and its implications for mass to be dependent on frame of reference and quantum theory to underline our growing understanding of what stuff is, before coming to his coup de grace, where we find that mass is not that fundamental aspect of stuff that it appears to be, but is rather a combination of the interaction of quantum fields and an effect produced by energy. It's a neat inversion of our usual way of looking at mass and matter - beautifully well presented. Along the way, Baggott manages his usual trick of going into the physics to a slightly deeper level than is common in popular science coverage - for example, in his description of what is involved in the renormalisation used to get rid of infinities from quantum electrodynamics - while keeping the text mostly approachable.

If I have any criticism, I felt the skip through relativity didn't quite do the subject justice. It was too summary to really get a feel for it, but too detailed to be scene setting, making it one of the less interesting parts of the book, particularly if you've read anything on relativity before. That balance seemed to be handled better with quantum theory. I'd also say that towards the end, where we get into abstruse matters, there isn't quite enough explanation, so the reader is occasionally left thinking 'I don't see how you make that leap.' This seemed particularly true when talking about spontaneous symmetry breaking. There's a diagram showing how ice has lower symmetry than water that confuses rather than helps, and we are told that for spontaneous symmetry breaking to occur when water freezes 'we need to add something (impurities or inhomogeneities, in this case) to encourage it to happen.' A very reasonable reader response is 'If you have to add something, it certainly isn't spontaneous' - a problem that isn't addressed.

Despite a few points like this towards the end, for me the book was interesting throughout (I liked the business book style 'five things we learned' at the end of each chapter) and it encourages the reader to really think about the nature of matter and how something as apparently straightforward as mass is not what it seems. That delight in revealing the unexpected typifies, for me, the joy of physics.

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Review by Brian Clegg