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Review: Gravity’s Century

Gravity’s Century: From Einstein’s Eclipse to Images of Black Holes
by Ron Cowen
Harvard Univ. Press, 2019
hardcover, 192 pp., illus.
ISBN 978-0-674-97496-8

Our understanding of the universe changed on May 29, 1919, although no one knew it quite yet. Two teams of astronomers, one in Brazil and the other on the island Principe off the coast of Africa, observed a solar eclipse. Their interest was not with the eclipse itself, but rather with the ability to see stars near the Sun that would otherwise be lost in the Sun’s glare. Months later, the astronomers reported that the positions of the stars had shifted compared to observations taken in the night sky. Those shifts were by the amount expected had the starlight been deflected by the Sun’s gravity as predicted by Albert Einstein’s theory of general relativity.

That verification of general relativity, dethroning Newton’s classical gravity, catapulted Einstein to global fame as word spread (“Men of Science More or Less Agog Over Results of Eclipse Observations” read part of the headline of a New York Times account of the discovery, adding that “Nobody Need Worry” that the stars were displaced.) The discovery also started a century of further work that has reshaped our understanding of gravity and the universe, as expertly told by longtime science writer Ron Cowen in the new book Gravity’s Century.

Much of this concise book (only about 160 pages when excluding the index and endnotes) is about what led up to that 1919 confirmation of general relativity. Cowen follows Einstein’s development of the theory and precursor work by others, as well as British astronomer Arthur Eddington, who organized the 1919 eclipse expeditions to provide observational proof of general relativity. Eddington was not the first to attempt to use an eclipse as a test of Einstein’s theory, just the first successful one: previous efforts were stymied by weather or, in one case, the outbreak of World War I.

Later chapters in the book examine the implications of general relativity, which drove much of astrophysics in the century that followed. That includes the expansion of the universe, black holes, gravitational waves and the concept of quantum gravity, where general relativity combines with quantum mechanics. The final chapter offers a look at the Event Horizon Telescope, the network of radio telescopes established by astronomers to take the first images of a black hole. (The book went to print before the April announcement by the Event Horizon Telescope that they had succeeded in that effort, producing an image of the supermassive black hole in the heart of galaxy M87.)

Throughout the past century, scientists have continued to perform tests—in the lab, on spacecraft, and using distant stars and galaxies—to confirm general relativity. Those tests have all confirmed Einstein, in one case to a precision of between one part in 10 million and one part in a trillion. “With relativity passing every test with flying colors so far, wny continue?” Cowen asks at the end of one chapter of Gravity’s Century. “Because sooner or later, the theory has to fail—and where it does, new physics may emerge.”

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