Book Reviewed: On Gravity: A Brief Tour of a Weighty Subject by A. Zee

The cosmic origins of gravity According to Newton, an object falling down to the ground experiences gravitational force from earth. This force represents the earth pulling the object, and also the object pulling the earth up. According to Einstein, the force is not “force” perse, but it is due to the curvature of spacetime (space and time are amalgamated into a malleable spacetime.) Spacetime warps around the object, and when it is falling down to earth, it seeks to extremize its action, that is, find the shortest distance to earth, which is also surrounded by curved spacetime. But according to quantum field theory, the curved spacetime is due to gazillions of gravitons (particles of gravity) sweeping around. When an object is falling, gravitons zing back and forth like crazy between the falling object and the earth. This constant exchange of graviton between the two bodies (falling object and earth) produces the observed gravitational force. This is similar to the constant exchange of photons between two charged particles that produces electromagnetic force. These are two of the four fundamental forces observed in the cosmos. Electromagnetic waves are produced by quantum fields such as light. They consist of both waves and particles. Similarly gravitational fields which are also quantum fields have gravitational waves, and particles called gravitons. The author has a unique style of writing to describe gravitational physics that takes us on a tour of general relativity, quantum physics, quantum field theory, spacetime, gravity and black holes. This math-free book is a good introduction to quantum gravity and has a lot of interesting history. He is a good physicist and a great writer. The book simply demonstrates that our knowledge of spacetime at its most fundamental level is inadequate. This is brutally an honest story written with some passion and humor. Highly recommended to anyone interested in gravity, black holes, and the nature of spacetime.