Distinguished Favorite in the 2023 Independent Press Awards for Non-Fiction Megan Henry gives us a delightful reading of philosophical ideas behind physics – Einstein’s theory of general relativity mainly among them. The unifying theme of the book is “Mach’s principle”, a principle that is natural to most physicists. As Dr. Henry describes it: “theory should be based on the measurement of observables, efficiently expressing relationships among phenomena.” It is well known that such a theory puts all observers on the same footing, nothing is privileged. This in principle suggests the theory incorporates Mach’s ideas. But does it? This book is the history of this “Incomplete Theory”. Written with excellent teaching skills, the author starts giving us the history of various concepts in physics starting from the ancient Greeks and its passage through the Arab world into the Middle Ages, reaching Copernicus, Kepler, Galileo and Newton, Faraday, Maxwell, Einstein (of course) and key figures of quantum mechanics. She includes many interesting historical notes that make a lively prose easy to read, even when touching on deep subjects. The chapters on frontiers of physics, with the history of the Japanese school of nuclear physics of Yukawa and Nishina, the issue of infinities in quantum field theory, the quest for a theory of everything, cosmology, the Big Bang, dark matter, dark energy and the detection of gravitational waves bring us to modern times, showing how it’s built on shoulders of giants. Megan tells us how she started her own physics reading passion and curiosity, and tying up dead ends and loose ends, poses a challenge for physicists and philosophers: is general relativity compatible with Mach’s principle? She says no, and this could be a clue for advancing the theory – but the theory is still incomplete, as we learn when we finish the book. Jorge Pullin and Gabriela González, Baton Rouge, June 14, 2021 Henry exposes a hole in the conventional understanding of physics in this debut nonfiction work. Gravity is a tricky subject; we know it exists, but not how it operates. The phenomenon is explained by Einstein's theory of relativity, but not by quantum mechanics. The author is an adherent of Mach's principle, which she defines as "the belief that all motion is relational and can only be determined by and described with respect to other objects." She sees this concept as the key to explaining gravity within the framework of quantum mechanics, though doing so would require us to jettison the theory of relativity. With this book, Henry attempts to sketch the history of this "incomplete theory" of gravity, from Aristotle to Bohr and beyond. The major players are all represented here—Copernicus, Kepler, Galileo, Newton—as are the dueling titans of Einstein and Mach. Henry sets forth her view of a Machian universe while identifying persistent holes and nagging loose ends. She chronicles her own journey as she goes, weaving in accounts of the book's composition and revealing how the work—like the theory (and the entire history of physics)—is very much a rocket ship being constructed midflight. Henry's leisurely prose takes on an almost poetic quality, particularly when she is explaining complex ideas: "If one of the spatial dimensions is a circle, it is possible to wrap one of the two dimensions of a membrane around the circle, leaving the other dimension—which looks just like a string—to move around in the remaining nine dimensions." Though not for beginners, this book makes for a wonderful primer on both the history and the unknowns of physics, including discussions of the ways various theories fit (or fail to fit) together. Whether Henry's own notions related to Mach's principle are ultimately borne out feels less essential than the reminder that the path toward a unified theory is populated by false starts, dead ends, and thinkers brave enough to question the status quo. A wide-ranging, open-minded look at the problem of gravity. Kirkus Reviews