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Review: Gravity’s Engines


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Gravity’s Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos
by Caleb Scharf
Scientific American/Farrar, Straus and Giroux, 2012
hardcover, 272 pp., illus.
ISBN 978-0-374-11412-1
US$26

Black holes: what are they good for? Not quite absolutely nothing, but since their discovery less than a half-century ago (the term “black hole” itself didn’t gain popular acceptance until a lecture by physicist John Wheeler in 1967) these objects have been treated primarily as astrophysical curiosities. Their existence proved the theoretical models that showed how a sufficiently massive object could collapse upon itself to the point where nothing, not even light, could escape its gravitational pull. Beyond that, though, black holes seemed more influential as fodder for science fiction or even prog rock: “Cygnus X-1” by Rush is named after one of the first black holes discovered, its lyrics warning that “all who dare to cross her course are swallowed by a fearsome force.”

However, Caleb Scharf argues in Gravity’s Engines that black holes are more than just astronomical oddities. Scharf, an astronomer at Columbia University, writes that black holes, in particular the supermassive ones weighing millions of times more than the Sun, play an essential role in shaping the universe we live in today, including making life itself possible. “Black holes are gravity’s engines—the most efficient energy generators in the cosmos,” he writes in the book’s preface. “And because of that, they have played a key role in sculpting the universe we see today.”

“Black holes are gravity’s engines—the most efficient energy generators in the cosmos,” Scharf writes. “And because of that, they have played a key role in sculpting the universe we see today.”

Black holes as “energy generators” might strike some people as odd: after all, aren’t black holes objects that swallow up anything that comes too close, never to be seen again? As Scharf explains in the book, though, that energy generation comes from outside the black hole’s event horizon, where material gathers in accretion disks with tremendous amounts of kinetic energy from the black hole’s intense gravitational field. That energy is released in, among other ways, giant bubbles of hot gas, expanding outward into space.

Such bubbles created by supermassive black holes in the cores of galaxies play a major role in regulating their growth. These expanding bubbles push against the flow of gas heading into the cores of galaxies, limiting the amount of gas that can cool and condense into stars. “If they weren’t big and efficient at pushing back,” he writes of these black holes, “we would see far more cooling gas converted into stars, and it would be a very different universe if that had been the case.”

Black holes also play a role in the formation of life, he argues. The Milky Way, like most galaxies, has a supermassive black hole at its core. But, cosmically speaking, it’s rather a small one, weighing in at only about four million solar masses (the one in the core of the nearby Andromeda galaxy, by comparison, is estimated to be about 100 million solar masses.) It is an active black hole, but fortunately spews that energy away from the Earth, sparing us from bursts of X-rays and gamma rays that are not conducive to life. Galaxies like the Milky Way, which “did not spend their past building colossal black holes and fighting the demons unleashed in the process,” may be the ones best suited for life, he concludes.

Scharf writes for the educated layperson in Gravity’s Engines, assuming little background about black holes and building up that knowledge through liberal use of analogies and some historical asides. For someone already familiar with the basics of black holes, that can initially make for some slow reading, but the wait is worth it once he gets into the heart of his argument about black holes being the “engines” that have shaped the universe we know today. Black holes are clearly more than astronomical curiosities that stimulate the imagination. They are objects that have made the universe we live in possible.


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