Phenomena · Universe scale
Gravitational wave
A ripple in the fabric of spacetime, propagating at the speed of light, caused by accelerating massive objects. Predicted by Einstein in 1916, directly detected for the first time by LIGO on 14 September 2015.
Strain
h = ΔL / L
GW150914 strain: h ≈ 10−21 — 1/1,000 the diameter of a proton over 4 km
Source: LIGO Scientific Collaboration
Einstein's prediction, a century later
Albert Einstein predicted gravitational waves in 1916 as a consequence of general relativity — his theory that mass curves spacetime. Accelerating masses create disturbances in that curvature that propagate outward as waves at the speed of light. For decades most physicists doubted they could ever be detected given their infinitesimally small amplitude. It took 40 years of detector development and the collision of two black holes 1.3 billion light-years away to prove Einstein right.
The detection of GW150914 opened a new window on the universe — gravitational wave astronomy. Unlike electromagnetic radiation, gravitational waves pass through matter unimpeded. They carry information about the darkest, most dense objects that emit no light at all: merging black holes. By 2026, LIGO-Virgo-KAGRA have detected over 90 compact binary merger events.
Multi-messenger astronomy
On 17 August 2017, LIGO and Virgo detected GW170817 — a neutron star merger — and 70 telescopes across all wavelengths observed the resulting kilonova independently. This was the first multi-messenger event: a single astrophysical event detected via gravitational waves, gamma rays, X-rays, ultraviolet, optical, infrared, and radio simultaneously. It confirmed that neutron star mergers produce heavy elements (gold, platinum) and independently measured the Hubble constant. See our guide to neutron stars and pulsars for more on kilonova physics.
Frequently asked questions
- Any accelerating mass produces gravitational waves, but only the most violent cosmic events produce waves strong enough to detect. The principal sources are: binary black hole mergers (the strongest signals), binary neutron star mergers, asymmetric supernovae, and rapidly rotating neutron stars with slight deformities. Earth orbiting the Sun also produces gravitational waves, but roughly 10³⁶ times too weak to detect — the signal falls off steeply with source mass. LIGO's explanation of gravitational wave sources covers all major categories.
- LIGO (Laser Interferometer Gravitational-Wave Observatory) uses two 4-km laser arms arranged in an L-shape. A passing gravitational wave stretches one arm and compresses the other by an amount roughly 1/1,000 the diameter of a proton. A laser split between the two arms returns to a detector; if the arm lengths differ, the light waves are out of phase and the detector registers a signal. This is the Michelson interferometer principle. Two detectors (Hanford, WA and Livingston, LA) operate in coincidence to reject noise. A third detector, the European Virgo, and Japan's KAGRA improve sky localisation.
- The first detection — GW150914 — was made on 14 September 2015 by LIGO and announced on 11 February 2016. The signal came from two black holes (29 and 36 solar masses) spiralling together and merging 1.3 billion light-years away. The event lasted about 0.2 seconds. During that fraction of a second, the merger radiated about 3 solar masses worth of energy as gravitational waves — briefly making it more luminous than all the stars in the observable universe combined. The discovery earned the 2017 Nobel Prize in Physics for Rainer Weiss, Kip Thorne, and Barry Barish. LIGO's GW150914 page has the full detail.
- The gravitational wave background (GWB) is a diffuse, persistent hum of gravitational waves from millions of unresolved sources — primarily supermassive black hole binaries in the cores of merging galaxies. In 2023, the NANOGrav collaboration and three other pulsar timing arrays announced strong evidence for the GWB using 15–25 years of timing data from millisecond pulsars. The GWB occupies nanohertz frequencies far below LIGO's kilohertz range. Future space-based detectors like LISA (planned 2030s) will probe millihertz frequencies, detecting supermassive black hole mergers in real time. See our guide to neutron stars and pulsars for how pulsar timing arrays work.
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