Gravitational Waves
Gravitational waves are propagating changes in spacetime geometry predicted by general relativity. Strong waves are produced by rapidly changing, compact mass systems such as pairs of black holes or neutron stars. Laser interferometers first detected them directly in 2015, opening a way to study cosmic events that is distinct from observing electromagnetic radiation.
Nature and sources
In general relativity, gravity is described through the geometry of spacetime. Gravitational waves are small, travelling distortions of that geometry which propagate at the speed of light. As a wave passes, the separations between freely falling objects change in a coordinated, oscillating pattern.
Changing distributions of mass generate gravitational radiation, but most everyday motions produce effects far too weak to detect. Especially strong sources are compact binaries—two neutron stars, two black holes, or one of each—moving rapidly around one another. As such a pair loses orbital energy to gravitational waves, its orbit tightens; the final inspiral and merger can produce a detectable signal. Some stellar explosions are also potential sources.
How detectors work
Ground-based observatories such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) use laser interferometers. A beam of laser light is split along two perpendicular arms, reflected by mirrors and recombined. A passing gravitational wave changes the arms' relative lengths by an exceedingly small amount, altering the interference pattern of the returning light. LIGO's two U.S. observatories, in Hanford, Washington, and Livingston, Louisiana, each have four-kilometre arms. Comparing signals between geographically separated detectors helps scientists distinguish a cosmic event from local disturbances and estimate its direction.
The measured effect is extremely small: for typical astrophysical sources, the fractional change in arm length (the strain) can be about 10⁻²¹ or smaller. Detecting it requires stable lasers, carefully isolated mirrors and extensive noise control.
From indirect evidence to direct detection
Before detectors recorded a wave directly, astronomers found important indirect evidence in the binary pulsar PSR B1913+16. Measurements showed that its orbit was shrinking in the way general relativity predicts when a system loses energy through gravitational radiation.
The first direct detection, named GW150914, was recorded by both LIGO observatories on 14 September 2015 and announced in February 2016. The signal came from the final fraction of a second in the merger of two black holes, estimated at about 29 and 36 solar masses; the event occurred roughly 1.3 billion years before the signal reached Earth. This observation confirmed a major prediction of general relativity and inaugurated direct gravitational-wave astronomy.
On 17 August 2017, LIGO and Virgo detected GW170817, a signal from two merging neutron stars. Telescopes also observed light from the event, making it the first cosmic event observed in both gravitational waves and electromagnetic radiation. Combining these messengers let researchers study the same event through different physical signals.
What gravitational-wave astronomy adds
Gravitational waves are not electromagnetic radiation: they are disturbances in spacetime itself. Their signals carry information about the motion and properties of massive objects, including systems that may emit little or no detectable light. They therefore complement telescopes rather than replace them. Measurements of waveforms can test predictions of general relativity in strong-gravity conditions and reveal how compact objects behave as they orbit and merge.
The observatories do not photograph a collision. They measure a changing signal and compare it with physical models to infer the source. As detector networks improve, combining observations from multiple sites and, when available, electromagnetic observatories helps refine the location and interpretation of events.
Key facts
- Physical nature
- Travelling distortions of spacetime geometry, not electromagnetic waves or sound.
- Propagation
- In general relativity, gravitational waves travel at the speed of light.
- Strong sources
- Compact binaries, including pairs of black holes and neutron stars, can produce detectable signals.
- First direct observation
- GW150914 was detected on 14 September 2015 by the two LIGO observatories.
- Detection method
- Laser interferometers measure minute changes in the relative lengths of perpendicular arms.
Timeline
1916
Albert Einstein's general theory of relativity predicts gravitational waves.
14 September 2015
LIGO records GW150914, the first direct observation of gravitational waves.
17 August 2017
GW170817 becomes the first gravitational-wave event observed in both gravitational waves and light.
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References
- 1.Gravitational waves / Elementary tour
Einstein Online, Max Planck Institute for Gravitational Physics (Albert Einstein Institute) · other · reliability: high · retrieved 19 October 2026
Supports: Explains gravitational waves as travelling distortions of spacetime geometry and their propagation at the speed of light.
- 2.Gravitational waves / Elementary tour part 2: Making waves
Einstein Online, Max Planck Institute for Gravitational Physics (Albert Einstein Institute) · other · reliability: high · retrieved 19 October 2026
Supports: Describes compact-binary sources and the indirect orbital-decay evidence from PSR B1913+16.
- 3.Observation of Gravitational Waves from a Binary Black Hole Merger
Einstein Online, Max Planck Institute for Gravitational Physics (Albert Einstein Institute) · other · reliability: high · retrieved 19 October 2026
Supports: Summarizes the first direct observation, the changing-mass-quadrupole source, interferometer operation and the small measured strain.
- 4.Gravitational Waves Detected 100 Years After Einstein's Prediction
LIGO Laboratory, Caltech and MIT · other · reliability: high · retrieved 19 October 2026
Supports: Primary collaboration release on GW150914, including its detection date, black-hole merger interpretation and estimated component masses.
- 5.Observation of Gravitational Waves from a Binary Black Hole Merger
American Physical Society, Physical Review Letters; LIGO Scientific Collaboration and Virgo Collaboration · other · reliability: high · retrieved 19 October 2026
Supports: Peer-reviewed original report of the first direct gravitational-wave detection from a binary black-hole merger.
- 6.LIGO Technology
LIGO Laboratory, Caltech and MIT · other · reliability: high · retrieved 19 October 2026
Supports: Describes the engineering systems used by LIGO's laser interferometers.
- 7.What is LIGO?
LIGO Laboratory, Caltech and MIT · other · reliability: high · retrieved 19 October 2026
Supports: Explains the two LIGO observatories, their four-kilometre arms and how the observatory differs from electromagnetic telescopes.
- 8.LIGO's Dual Detectors
LIGO Laboratory, Caltech and MIT · other · reliability: high · retrieved 19 October 2026
Supports: Explains why the Hanford and Livingston interferometers are widely separated, including local-vibration checks, travel-time comparison and source localization.
- 9.LIGO and Virgo make first detection of gravitational waves produced by colliding neutron stars
LIGO Laboratory, Caltech and MIT, with the Virgo Collaboration · other · reliability: high · retrieved 19 October 2026
Supports: Primary collaboration release describing GW170817 and its observation in gravitational waves and electromagnetic light.
- 10.Press release: The 2017 Nobel Prize in Physics
Nobel Prize Outreach; Royal Swedish Academy of Sciences · other · reliability: high · retrieved 19 October 2026
Supports: Provides an independent institutional account of the first detection, the detector's operation and the 2017 Nobel recognition.
References are suggested by the research engine and may be incomplete. Unverified URLs are omitted rather than guessed.
Article history
- Version 119 October 2026 · EditorCreated by Zapia