Definition
Gravitational waves are propagating ripples in the curvature of spacetime, generated by accelerating masses and travelling at the speed of light.
Einstein predicted them in 1916 as a consequence of general relativity. They were inferred indirectly from the orbital decay of the Hulse–Taylor binary pulsar in the 1970s, and detected directly for the first time on 14 September 2015 by the two LIGO observatories, from the merger of a pair of black holes 1.3 billion light-years away.
Why it matters
How it works
In the weak-field limit, Einstein's equations reduce to wave equations for small perturbations of the spacetime metric. Two transverse polarisations — denoted plus and cross — propagate at exactly the speed of light, stretching and squeezing distances perpendicular to the direction of travel. The strain amplitude h = ΔL / L is typically of order 10⁻²¹ even for nearby astrophysical sources — a fractional change comparable to measuring the Earth–Sun distance to within an atomic radius.
Sources require a time-varying mass quadrupole moment — a spherically symmetric collapse, however violent, does not radiate. The strongest astrophysical emitters are compact binaries (black-hole–black-hole, neutron-star–neutron-star, mixed) in their final inspiral; their orbital frequency rises and the waves "chirp" up in frequency and amplitude until the merger.
LIGO and Virgo are kilometre-scale Michelson interferometers with arms of 4 km (LIGO) and 3 km (Virgo). A passing wave changes the relative arm lengths, shifting the interference pattern at the photodetector. By 2024 the global network had catalogued roughly 100 binary-merger events spanning black-hole masses from a few to ~150 solar masses, plus the landmark binary-neutron-star event GW170817 that pinpointed the origin of heavy elements like gold and platinum.