On Time - Jim Al-Khalili *****

By a small margin Jim Al-Khalili's The World According to Physics is my favourite of his books, both because the 'three pillars' concept at its heart works so well - but also because it's short. On Time is somewhat heavier, but thankfully not a wrist-buster. It might seem at first sight odd that a book on a subset of physics is considerably longer than one covering the whole thing, but time is one of those subjects that inevitably involves considerable complexities and needs depth.

As Al-Khalili establishes, it's hard to think of any other scientific concept where our perception of it - what he refers to as manifest time or psychological time - is so different from the view from physics. Some physicists go so far as to suggest that time doesn't exist at all. In the book, Al-Khalili sets out (to my mind quite successfully) both to explain why they think this is the case, but also why it's not the best interpretation of time. I'd certainly note that I've never yet come across a time-denying physicist who doesn't recognise the existence of dinner time.

We begin with what's meant by time (not as easy to establish as you might think), which leads through to predictions about the future (in essence what most science involves). There's plenty here on the distinction between past, present and future and what each of these really is. We then get relativistic, treating time as a dimension, introducing spacetime and the implications of first the special and then the general theory of relativity. (John Gribbin would be irritated that Al-Khalili refers to these as special and general relativity rather than special and general theories.) We then get onto entropy and its definition of time's arrow - with implications for a beginning and end of time - before taking in time as an emergent property and concluding that time is real.

If I have one criticism it is that throughout Al-Khalili keeps things at a descriptive level rather than allowing any mathematics to enter the fray: I'm not expecting equations, but there can be an in-between stance where we understand that the verbal description doesn't quite hack it (and, for instance, I've always thought it's a good idea to have at least an appendix with a mathematical example of time dilation, as the maths is so simple and makes the unlikely outcome more clear).

This extends to a degree to some of the examples given where, without a bit more depth what's said sounds a touch unlikely. So, for instance, we are given examples of changes that don't involve time: 'one can easily think of examples involving things that change, but which are not necessarily changing over time. Atmospheric pressure changes when you climb to higher altitudes, the speed of a car changes when you press your foot on the accelerator pedal...' Yet these examples happen over time: neither can be instantaneous. 

There's also the usual description of the way that systems evolving with time don't have a direction, because 'such equations run equally well forwards and backwards in time'. But (at least at this point in the book) there is no consideration given to this being 'spherical cow' rather than real world physics. A classic example is a pair of billiard balls colliding. You can run the 'movie' of what you see backwards and it's indistinguishable. But this is only true on a non-existent frictionless table - and if you cherry pick an isolated system in time, excluding the bits where the motion starts and ends - though if, like me, you find this cheat very irritating, it is resolved in the final chapter.

These are small moans, though. Al-Khalili takes us through mind-bending subject with panache, making what could be a dense and difficult topic approachable and engaging. 

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Review by Brian Clegg - See all Brian's online articles or subscribe to a weekly email free here