Gravitational wave
A propagating ripple in the curvature of spacetime, traveling at the speed of light.
Definition
A gravitational wave is a disturbance in the geometry of spacetime that propagates outward from its source at the speed of light, carrying energy and momentum. In the weak-field regime it can be described as a small perturbation h_{μν} of the flat Minkowski metric, g_{μν} = η_{μν} + h_{μν} with |h_{μν}| ≪ 1, obeying a flat-space wave equation. The wave is transverse — its physical effect lies in the plane perpendicular to its direction of travel — and it stretches and squeezes proper distances between freely-falling test masses as it passes, an effect quantified by the dimensionless strain h ~ ΔL/L.
Gravitational waves are produced by accelerating mass-energy with a time-varying quadrupole moment. There is no monopole radiation (mass is conserved) and no dipole radiation (momentum is conserved), so the lowest-order source is the changing mass quadrupole — for example two compact objects orbiting one another. Because the gravitational coupling 8πG/c⁴ is extraordinarily small, even violent astrophysical events produce waves of minuscule amplitude: the first directly detected signal, GW150914, reached Earth with a peak strain of about 10⁻²¹, a length change of less than the width of a proton over LIGO's four-kilometer arms.
Einstein first predicted gravitational waves in 1916 and doubted their reality as late as the 1936 Einstein–Rosen episode, when the difficulty of distinguishing real waves from coordinate artifacts nearly led him to deny their existence. The 1957 sticky-bead argument established that the waves carry extractable energy; the Hulse–Taylor binary pulsar gave decisive indirect evidence in the 1970s–80s through orbital decay; and LIGO's direct detection on 14 September 2015 confirmed the prediction a century after it was made.