Traveling at the Speed of Thought: Einstein and the Quest for Gravitational Waves Book

Although the possibility that gravitation propagates with a finite speed was considered by Pierre-Simon Laplace, who realized that this would cause planetary orbits to decay, in the 18th century, modern study of gravitational waves began in 1916 with Albert Einstein's General Relativity. The equations of General Relativity are highly nonlinear, so all nontrivial solutions to them have to be approximate or numeric. In 1918 Einstein came up with an approximate formula relating the power of radiated gravitational waves to the third time derivative of the quadrupole moment of the source. Electromagnetic waves are radiated by dipoles, but, because of conservation of momentum, the gravitational dipole moment of a closed system is constant; only a quadrupole can radiate gravitational waves. During the next half century and more, physicists argued whether gravitational waves even exist, and if so, whether their radiated power is described by the quadrupole formula or something else. Only in the 1970s did astronomers discover a pair of neutron stars, one of which is a pulsar, orbiting each other, which lose energy to gravitational waves, which makes their orbit decay in exact agreement with Einstein's quadrupole formula. The formula has the fifth power of the speed of light in the denominator, which makes detection of gravitational waves very difficult.

This book was published 8 years before gravitational waves were actually detected using a laser interferometer, built using extreme engineering, which was almost 100 years before Einstein described them.