Dark Matter and the Dinosaurs - Lisa Randall ****

I did my PhD in galactic dynamics - which is an awkward subject when people want to know what its relevance to the 'real world' is. So I was excited when Clube and Napier's book The Cosmic Serpent came out, around the same time, because it provided me with a ready-made answer. It argued that the comets which occasionally crash into Earth with disastrous results - such as the extinction of the dinosaurs - are perturbed from their normal orbits by interactions with the large-scale structure of the galaxy.

I was reminded of this idea a few years ago when there was a flurry of media interest in Lisa Randall's "dark matter and the dinosaurs" conjecture. I was sufficiently enthusiastic about it to write an article on the subject for Fortean Times - though my enthusiasm didn't quite extend to purchasing her hardback book at the time. However, now that it's out in paperback I've remedied the situation - and I'm glad I did.

Dark matter is believed to exist in large quantities in the galaxy, but it's very difficult to detect because it only interacts with ordinary matter through gravity. Most astrophysicists assume it only interacts with itself in this way, too, which means it has no way of dissipating energy. Randall, on the other hand, posits that a subset of dark matter can somehow dissipate energy - causing it to collapse down into an extremely thin disc. The gravitational effect of this disc then produces a perturbing effect on cometary orbits. According to Randall, only this model - rather than one involving ordinary matter or 'conventional' dark matter - is capable of providing a perturbation of the necessary magnitude.

This is the kind of science I find fascinating - involving a long chain of argument from one observable phenomenon, via a whole bunch of unobservable things, to another, seemingly unrelated, observable effect. It's pure speculation, of course (and probably wrong) but at least it's logical. That's what sets it apart from the kind of superficially similar speculation you find in sci-fi movies and Velikovsky-style pseudoscience.

My only criticism of the book is that it's too long. It's almost three books in one, in fact. First there's a popular science primer on cosmology, dark matter and galactic structure. Then there's a second popular science book - this time on asteroids, comets, impact events and mass extinctions. Finally there's the detective story tracing the cause of these extinctions to an exotic new kind of dark matter. Personally, I only wanted to read the third book. Even with some essential scene-setting at the start, that could easily have been achieved in half the book's 412 pages. The result would have been more exciting and more unique - and I probably would have given it five stars instead of four.

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Review by Andrew May