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duminică, 29 iulie 2012

A trustworthy guide to black hole astronomy

Black holes are some of the most bizarre objects predicted by theoretical physics that actually exist in the cosmos. While many of the more exotic ideas about black holes coming from string theory and other quantum gravity models are far from testable, the existence of astrophysical black holes is uncontroversial. Even if you insist on separating out the observed black holes from the theoretical black hole properties (including everything that lies inside the event horizon, the boundary beyond which nothing can escape), we still have reason to believe they are the same thing. Black holes are real.

In Caleb Scharf's forthcoming book, Gravity's Engines, the focus is on the observed and observable properties of astrophysical black holes. The subtitle—How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos—may be a trifle grandiose, but the book itself is an excellent overview of the state of black hole research. In fact, to explain why black holes are so important, Scharf provides a tour of much of modern astronomy and cosmology along with some requisite history, an impressive feat for such a relatively short book.

After a conceptual introduction to black hole theory, Scharf begins with the fascinating (albeit incorrect) "dark star" model by 18th century mathematicians John Mitchell and Pierre-Simon Laplace. Modern black holes, based on general relativity, are introduced via Karl Schwarzschild's original calculation (performed in the trenches of the Russian front in World War I) and the famous paper by J. Robert Oppenheimer and Hartland Snyder. (Scharf doesn't mention Snyder, though admittedly he says Oppenheimer was "one of those who developed the physics" leading to our modern understanding of gravitational collapse.) Along the way, Scharf describes stellar evolution, white dwarfs, and neutron stars, to show that stellar-mass black holes—as opposed to supermassive black holes (SMBHs) found in the centers of galaxies—arise as the logical continuation of those other stellar remnants.

However, Scharf's main goal is to explore how the biggest black holes evolved with their galaxies, and how this shaped the larger environment. To accomplish that goal, he delves into cosmology: the contents and history of the Universe. Since galaxies and galaxy clusters arise from dark matter structures, and SMBHs help shape both of those things, the understanding of structure formation is a part of the tale. Scharf weaves the disparate strands of this tale together very well, so even though many pages may go by without a single mention of black holes, by the time the main thread appears again, it's clear why the other strands were introduced.

Scharf is very fond of metaphors like the "weaving" one in the paragraph prior. Sometimes those metaphors are very effective: his "cosmic bag" analogy for describing the contents of the Universe in the proper proportions is wonderful. (I'm reminded of Edward Harrison's "cosmic box" metaphor, which I have used on many occasions.) Similarly, Scharf's "map of forever", which includes both space and time to show how intricately linked every aspect of cosmology is, provides a wonderful metaphor.

On other occasions, Scharf lets his metaphors run away with him: when describing how black holes feed back into their environment, he uses a runaway car, a soufflé, and a bowl in the same paragraph as analogies for the same process. While it's probably true that no single metaphor is sufficient for complex physical systems, mixing several together can be distracting. However, this is a minor quibble: for the most part, Scharf uses his analogies effectively, and (in my opinion at least) metaphor is a necessary means to the end of explaining high-level science to a general audience.

After describing how black holes shine brightly and pump particles back out into space, Scharf turns his attention to the work to which he contributed. While his current job as Columbia University's Astrobiology Center mostly deals with exoplanets, his earlier work involved studying SMBHs in early galaxy clusters. Particularly, he brings this forward to argue that certain types of SMBHs in galaxies may make them habitable for life, an interesting and somewhat controversial theory. It's definitely true that central black holes and their host galaxies evolve together in a fascinating and complicated way, so Scharf's idea may turn out to be correct. As always in science, it's about the evidence for the argument, not the elegance behind it, but it's an intriguing idea that will make readers think.

The book assumes no strong prior knowledge of black holes or astrophysics, but by the time it ends, I think readers should have a good idea of the state of the art. While Scharf's approach is not the same as that of Stephen Hawking or Brian Greene, his explanations are complementary to theirs. Anyone left wondering whether black holes are just too exotic to be real from the more quantum approach will find good answers in Gravity's Engines.


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