GRAVITY'S ENGINES

An intelligent explanation of a weird but essential feature of the universe.

Black holes are bodies so massive that they are invisible because their light cannot escape. Although permitted by relativity, nobody, Einstein included, believed they existed. It turns out they are everywhere, writes Scharf (Extrasolar Planets and Astrobiology, 2008), director of Columbia University’s Astrobiology Center. As stars age, cool and shrink, most, like our sun, will become white dwarves: tiny and immensely dense but still shining. As larger stars shrink, their greater gravity squeezes them into even smaller, denser neutron stars. In stars more than three times our sun’s mass, gravitational collapse continues, distorting space-time so much that the star’s light curls back on itself, producing a “singularity,” an infinitely small point containing the entire mass. Physicists dislike infinities, so there may be a better explanation, but there’s no denying that black holes happen. Their gravity attracts material, including stars, which disappear inside. Other material orbits in a huge “accretion disk” whose high-speed interactions generate torrents of radiation. Supermassive black holes at the center of every galaxy may pour out more energy than a billion stars. This energy plays a vital role in controlling the size of galaxies and the formation of stars, which means, ultimately, the formation of planets and life.

Written for educated laymen, this should not be treated as an introduction to cosmology (for that try Brian Clegg’s Gravity or Chris Impey’s The Living Cosmos), but Scharf provides a rich, satisfying and usually comprehensible account of an extraordinary phenomenon.