A gravitational wave is a propagating disturbance in the curvature of spacetime, predicted by general relativity, the gravitational component of Einstein’s theory of relativity. Unlike electromagnetic radiation, it is an oscillation of spacetime geometry rather than an electromagnetic field. Gravitational waves travel at the speed of light in vacuum and carry energy away from their sources. Albert Einstein predicted them in 1916; their first direct detection occurred in 2015. They provide a means of observing astronomical systems independently of the light those systems emit. (ligo.caltech.edu)
Physical description
In general relativity, gravity is described through spacetime geometry, governed by the Einstein field equations. Far from a source, where a wave is weak, the geometry can be represented as a background metric tensor plus a small, propagating perturbation. Its measurable effect is a changing relative separation between nearby freely falling objects, rather than an absolute displacement against a fixed spatial background. (link.springer.com)
General relativity predicts two independent transverse polarizations, conventionally called plus and cross. A wave passing perpendicular to a ring of freely falling particles alternately stretches and compresses the ring along orthogonal directions. The two patterns are rotated by 45 degrees relative to one another. These distortions are tidal effects: they depend on differences in gravitational influence across an extended detector. (link.springer.com)
Wave amplitude is commonly expressed as dimensionless strain, the fractional change in separation. Schematically,
where is a reference separation and its wave-induced change; the precise detector response depends on geometry and polarization. Astrophysical strains measured on Earth are extraordinarily small, making gravitational-wave detection a precision measurement problem. (ligo.caltech.edu)
Sources and waveforms
Gravitational radiation arises from changing, nonspherical distributions of mass and motion. In the weak-field, slow-motion approximation, its leading contribution is described by the quadrupole formula, which relates wave amplitude to the second time derivative of the source’s mass quadrupole moment. A perfectly spherical expansion or contraction does not produce gravitational waves. (link.springer.com)
Important sources include orbiting pairs of black holes and neutron stars. As a binary radiates energy, its orbit shrinks and its orbital frequency rises. The resulting signal increases in frequency and usually amplitude, forming a chirp. A black-hole coalescence proceeds through inspiral, merger, and ringdown, during which the remnant settles toward a stationary state. The waveform encodes information about component masses, spins, orientation, and distance. (ligo.caltech.edu)
Other proposed or expected sources include rotating neutron stars with nonaxisymmetric deformations, asymmetric stellar core collapse, and processes in the early universe. Numerous individually unresolved sources can combine into a stochastic gravitational-wave background. Different source populations occupy different frequency ranges, requiring complementary observing methods. (link.springer.com)
Observational milestones
The first strong indirect evidence came from the binary pulsar discovered by Russell Hulse and Joseph Taylor in 1974. A pulsar is a rotating neutron star whose radiation produces regularly observed pulses. Long-term timing showed that the binary’s orbit was shrinking at a rate consistent with energy loss through gravitational radiation. Hulse and Taylor received the 1993 Nobel Prize in Physics for discovering this system. (nobelprize.org)
The first direct detection, GW150914, was recorded on September 14, 2015, by the two detectors of LIGO, in Hanford, Washington, and Livingston, Louisiana. Announced on February 11, 2016, the signal came from merging black holes with estimated masses of approximately 36 and 29 solar masses. About three solar masses’ worth of energy was radiated as gravitational waves. (ligo.caltech.edu)
On August 17, 2017, LIGO and Virgo observed GW170817, associated with a binary neutron-star merger. A gamma-ray burst followed approximately 1.7 seconds later, and telescopes identified an electromagnetic counterpart. This event established a major example of multi-messenger astronomy and tightly constrained the difference between gravitational-wave and light propagation speeds. (ligo.org)
Detection and analysis
Ground-based observatories use laser interferometry to measure differential changes in perpendicular optical paths. LIGO’s arms are four kilometres long; optical cavities repeatedly circulate light to enhance sensitivity. Suspended mirrors and vibration isolation reduce disturbances, while thermal and quantum noise also limit measurement precision. (ligo.caltech.edu)
Searches for compact-binary signals commonly employ matched filtering, comparing detector data with banks of predicted waveforms. Independent methods search for coherent transients without requiring a detailed binary model. Agreement across separated detectors helps distinguish astronomical signals from local artifacts and constrains source position. Bayesian inference is used to estimate source properties and their uncertainties. (ligo.org)
Pulsar timing arrays probe much lower, nanohertz frequencies by measuring correlated deviations in pulse arrival times. In 2023, NANOGrav reported evidence for a background with the angular correlation pattern expected from gravitational waves. An important possible origin is a population of supermassive black-hole binaries, although the evidence did not uniquely establish its source. (nanograv.org)
The planned Laser Interferometer Space Antenna is designed to observe the millihertz band using widely separated spacecraft. Its targets include massive black-hole mergers, compact objects orbiting massive black holes, and binaries containing white dwarfs. (cosmos.esa.int)
Scientific applications
Gravitational-wave observations test relativistic gravity under rapidly changing, strong-field conditions and investigate compact-object populations. Neutron-star signals also constrain dense-matter properties through their effects on the waveform. In cosmology, compact binaries can act as standard sirens: waveform analysis yields a distance estimate that, combined with redshift information, constrains cosmic expansion without relying on the conventional astronomical distance ladder. (link.springer.com)