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