Known Unknowns

by Graham Farmelo

Of all the scientific advances of the last century, few are more impressive than the discovery that the matter familiar to us on Earth is quite different from the stuff that makes up most of the universe. A whacking 85% of the universe is made of something astronomers called “dark matter”. Its existence was first proposed only in the 1930s: as Lady Bracknell might have said, to have been unaware of a little of the matter in the universe would be unfortunate, to be ignorant of 85% of it looks like sheer carelessness.

Dark matter has proved to be a headache for scientists. Even after decades of careful study, there is still plenty of room for theoretical speculation about how the matter behaves, as Lisa Randall demonstrates. In her book, she sets out a new idea about its behavior and then uses it to offer an explanation as to why the dinosaurs were wiped out some 65m years ago.

This might sound outlandish, but Randall’s impressive record as an imaginative scientist entitle her to a fair hearing. She is a theoretical physicist at Harvard, and has conceived several radical ideas that have become mainstream thinking. Most famously, she co-authored the theory that implies the possible existence of dimensions in addition to the three we are all familiar with, and that these extra dimensions may be observable by means of the Large Hadron Collider at the Cern Laboratory. If they were to be detected, she would be a shoo-in for a Nobel prize.

Randall recently altered the focus of her research from fundamental particles to the work on the solar system that she describes in this book. It is fascinating to read of her initial responses when she began to think deeply about the role of dark matter in our region of the universe. Astronomers have long known that this matter neither emits nor absorbs light, which is why it cannot be directly observed through telescopes. Dark matter does, however, feel the force of gravity, and this has enabled astronomers to observe its effects on ordinary matter in galaxies, as the ordinary matter is tugged across the cosmos by the gravitational pull of unseen dark material. The challenge is to more fully understand dark matter and to map out its presence, often in clusters, across the universe. The achievements of astronomers have been astonishing: it now seems clear that our galaxy, the Milky Way, exists inside an almost spherical “halo” of dark matter.

The first innovation of Randall and her collaborators was to propose–guess–that dark matter might not all be the same. Rather, a small portion of it might interact not only via gravity but also via a new type of force that is similar to electromagnetism. One consequence is the crucial prediction that the Milky Way contains a thin and extremely dense disk of dark matter. The reasoning behind this is complicated, and Randall takes good care to prepare us with a 300-page primer on astronomy and cosmology, during which we scarcely see the swish of a dinosaur’s tail.

But she is a lucid explainer. Without jargon or mathematics, she steers us through astronomical history and is particularly illuminating when briefing us on the cosmic objects that sometimes land on planet Earth with unpleasant consequences. Theoreticians first began thinking about this long ago. In 1742, the French mathematician and philosopher Pierre de Maupertuis recognized that many forms of life on Earth could be wiped out if a comet struck the planet. In the following generation, Pierre-Simon Laplace suggested that “meteorites of great size… would produce a cataclysm that would wipe out entire species”. Today, governments commission scientists to work out how best to deal with the prospect of a direct hit by an asteroid.

Randall’s story draws on a wide variety of disciplines, including biology, palaeontology, meteorology and mathematics. In an excursion to Greece, she happens upon the origins of the word “crater”: a vessel with a large opening that enabled wine to be diluted with water. Yet by far the most compelling part of the book is its climax, where she brings together all the strands of her theory, describes its genesis and looks to the future. She emphasizes the need to risk putting forward unorthodox theoretical ideas–though she is well aware that some of her peers regard the new hypothesis as a “stretch”.

How could dark matter be responsible for killing off the dinosaurs? Roughly speaking, the theory is that as the solar system travels around the Milky Way, the Earth’s dark-matter disk slings comets from a distant region of the galaxy (known as the Oort Cloud) and periodically catapults them towards the Earth–in a pattern that is predictable. It was one of these comets that did for the dinosaurs.

It’s worth remembering that nature has often been found to favour ideas that initially seemed wildly improbable. Randall shows here that there is more than enough scope for theorists to do creative work in the dark.