by Jeff Foust
On Gravity: A Brief Tour of a Weighty Subject
by A. Zee
Princeton Univ. Press, 2018
hardcover, 192 pp., illus.
Gravity can seem like a powerful force at times. It holds everything down and, here on Earth, makes it difficult to escape the planet and go into space. At its extreme, the gravitational force of black holes prevents even light from escaping, and at large scales shapes the universe itself.
But gravity, as it turns out, isn’t that strong, relatively speaking. “Gravity is absurdly weak compared to the electromagnetic force,” writes Anthony Zee in one of the early chapters of On Gravity. How much weaker? “Electromagnetism is stronger than gravity by a factor of 1036.” That’s… a lot.
Zee offers insights like this throughout the book, a slender volume devoted to one of the four fundamental forces of the universe. In less than 200 pages (a total that includes an appendix and notes), he goes through the comparison of gravity with the other three fundamental forces—that “absurd” weakness of gravity is what made it so challenging to detect gravitational waves, he notes—and works his way through special and general relativity, ending with the efforts to understand dark matter and dark energy and develop a quantum model for gravity.
With On Gravity, Zee decides to shoot for a middle ground in terms of level of sophistication. “One motivation for this book is to help people bridge the gap between popular books and textbooks on Einstein gravity,” he writes in the preface. Having written both popular books and an advanced textbook, he decided to write On Gravity to “cross that gap” between the two.
The result is a book with the brevity of a popular book (if not shorter) but one that doesn’t shy away from some mathematics, including calculus. The most detailed math is reserved for the appendix; if you can easily understand that, he advises, you may be ready for a more advanced textbook on the subject. If not, though, the main sections of the book, even with the occasional equation, are still easily comprehensible, with some history and anecdotes thrown in.
By aiming for that middle ground, Zee runs the risk of finding only a limited audience for the book. But for those who want to move beyond popular books about gravity, but aren’t ready for a massive textbook with advanced mathematics, On Gravity does provide more insights about how gravity works without getting too complex. Gravity may be absurdly weak, but that doesn’t mean it’s easy to fully understand, or overcome.