6 August 2012
“WE ARE stardust,” sang Joni Mitchell. Caleb Scharf might suggest an update to those lyrics. In Gravity’s Engines, Scharf adds a new insight to the half-century-old idea that most atoms in our bodies were born in exploding stars. He argues that some of those hydrogen atoms were likely touched by radiation from black holes – without which we wouldn’t be here.
Over the past few decades the scientific discussion of black holes – those whirligigs of space-time that generate gravitational fields so extreme not even light can escape – has progressed from debates over their existence to consensus that they occupy the heart of most galaxies. In Gravity’s Engines, Scharf, the director of the Astrobiology Center at Columbia University in New York, deftly recounts this history and clearly explains the science. But his goal is more ambitious: to show how black holes influenced the evolution of the universe.
He begins by explaining black holes as a possible consequence of Newtonian physics: put enough mass in a compact enough space, and gravity would overwhelm everything in its vicinity. After Einstein introduced his general theory of relativity, physicists Subrahmanyan Chandrasekhar and Karl Schwarzschild used the new equations to determine just how much mass was needed and the extent of that vicinity. In the past few decades, astronomers have gathered empirical evidence for the existence of these cosmic omnivores.
In 2002, Scharf was part of a team that observed galaxy 4C41.17, a highly energetic source of X-rays some 12 billion light years away. He and his collaborators collected photons from this galaxy for nearly 40 hours using an X-ray space telescope. The image that emerged showed a bright pinpoint source of X-rays – the black hole at the centre of the system. But it also revealed two jets of gas shooting out from either side. Scharf calculated that together these spanned 300,000 light years, or about three times the diameter of the Milky Way. It was an energy-exchanging stretch of radiation with the capacity to influence when or whether stars and planets could form.
The knowledge that black holes influence the development of individual galaxies is not new. Yet the idea that they are engines that have driven the universe’s development at every stage and scale has the potential to reposition them from the realm of the remote and absurd to the central and essential.
Scharf does not rely on dialogue, action or characters. Instead, he utilises original and often surprising metaphors. Black holes limiting the growth of galaxies are “acting like frustrated farmers trying to keep their weeds under control”. The X-ray sources his team had to sift through to find photons from 4C41.17 become “a cosmic forest full of rustling leaves”. These turns of phrase are often clever, but at times the book feels less like a participatory adventure and more like a lecture, albeit one by an enthusiastic convert to cosmology.
Prior to his encounter with 4C41.17, Scharf admits that he had assumed a massive galaxy in the early universe would be “rather static”. His investigations of black holes instead revealed a universe committing acts of extraordinary violence in its own version of natural selection. The cosmos, too, is red in tooth and claw.