One of the four basic interactions that exist in nature is gravity. In addition, it is the oldest, weakest, and most challenging force to quantize. Understanding gravity is crucial for comprehending how things move on Earth, how things move in the heavens, and even how the universe expands. Einstein’s deep insights regarding the nature of space, time, and all astrophysical bodies within it were the result of his study of gravity.

Timothy Clifton examines the evolution of our knowledge of gravity since Kepler’s initial findings and Newtonian theory in this Very Short Introduction. He discusses Einstein’s theory of gravity, which has replaced Newton’s as the dominant theory, and demonstrates how it helps us to understand why light changes in frequency as it passes through a gravitational field, why GPS satellites need to correct their clocks as they orbit the Earth, and why the orbits of far-off neutron stars accelerate. Even the gravitational radiation waves that Einstein predicted have been found today, some 100 years after the publication of his theory of gravity. In his final section, Clifton examines how general relativity is tested and used in astrophysics and cosmology, as well as dark energy and attempts to merge gravity with quantum mechanics, such as string theory.