Gravitational waves are ripples in spacetime made by accelerating masses. Einstein predicted them in 1916; LIGO first detected them in 2015. Here is how.
By The Orbital Record editorial teamPublished Last reviewed
Gravitational waves are ripples in the fabric of spacetime, produced when massive objects such as black holes or neutron stars accelerate, and they travel at the speed of light. Albert Einstein predicted them in 1916. LIGO first detected them on September 14, 2015 (GW150914), and the discovery was announced in February 2016.
Ten years later, gravitational waves are a working branch of astronomy with hundreds of detections. This guide explains what they are, how detectors catch them, what they have revealed and what comes next.
Gravitational waves are disturbances in the shape of space and time that spread outward from accelerating masses. In Einstein’s general relativity, completed in November 1915 (Einstein’s field equations), mass curves spacetime; when masses move violently, the curvature changes, and those changes travel outward as waves.
A passing wave alternately stretches space in one direction and squeezes it in the perpendicular direction. Imagine a ring of floating beads in space: as the wave goes by, the ring becomes an oval, then an oval the other way, over and over.
The effect is astonishingly small. The first wave detected, from two black holes 1.3 billion light-years away, changed distances on Earth by about 1 part in 1021 at its peak (the first gravitational wave detection).
Any accelerating mass makes gravitational waves, but only extreme events make waves strong enough to detect:
Gravitational waves are detected by measuring tiny changes in distance, either between mirrors with lasers or between Earth and distant pulsars with radio telescopes.
LIGO has two L-shaped detectors, in Hanford, Washington and Livingston, Louisiana, 3,002 kilometers apart. Each has two 4-kilometer arms. A laser beam is split, sent down both arms, bounced off mirrors and recombined. Normally the two beams cancel out. When a gravitational wave stretches one arm and squeezes the other, the beams fall out of step and a flicker of light reaches the detector.
At its most sensitive, LIGO can detect a change in arm length of 1/10,000 the width of a proton, like noticing the distance to the nearest star change by the width of a human hair. It works as a pair: a real signal must appear at both sites, no more than about 10 milliseconds apart. Our explainer how does LIGO work walks through the lasers, mirrors and noise fighting step by step.
LIGO operates with Virgo, a 3-kilometer detector near Pisa, Italy, and KAGRA in Japan, the first detector built underground with cryogenically cooled mirrors. Together they form the LIGO-Virgo-KAGRA network. A fifth detector, LIGO-India, broke ground on April 23, 2026, in Maharashtra, with the same design as the US detectors (LIGO-India breaks ground). A detector far from the others will sharpen sky positions for telescopes.
Veritasium’s “The Absurdity of Detecting Gravitational Waves”, in our video library, shows why this is so hard.
Pulsar timing arrays use the galaxy as a detector. Millisecond pulsars are dead stars that spin hundreds of times a second and sweep beams past Earth with clock-like regularity. A gravitational wave passing between Earth and the pulsars makes pulses arrive slightly early or late, in a pattern that depends on the angle between pulsars. These arrays are sensitive to waves with periods of years, far below LIGO’s range.
NANOGrav and partner arrays in Europe and India, Australia and China lead this effort.
The Laser Interferometer Space Antenna will fly three spacecraft in a triangle with arms 2.5 million kilometers long, exchanging laser beams. ESA adopted the mission on January 25, 2024, with launch planned for the mid-2030s; NASA contributes the telescopes and ordered an engineering test unit in October 2026 (LISA’s test telescope). LISA will hear merging supermassive black holes and thousands of compact binary stars in the Milky Way.
The first direct detection of gravitational waves was GW150914, recorded on September 14, 2015, at 09:50:45 UTC by both LIGO detectors. The signal swept upward in frequency from 35 to 250 hertz in a fraction of a second, the “chirp” predicted for two black holes spiraling together.
The black holes had about 36 and 29 times the Sun’s mass and merged into one of 62 solar masses about 410 megaparsecs (1.3 billion light-years) away. The missing three solar masses were converted into gravitational-wave energy. The false alarm rate was below once per 203,000 years, more than 5.1 sigma. It was also the first observation of two black holes merging.
Rainer Weiss, Barry Barish and Kip Thorne received the 2017 Nobel Prize in Physics “for decisive contributions to the LIGO detector and the observation of gravitational waves”.
Gravitational waves were predicted, then inferred, then detected, by different people:
GW170817 was the first neutron star merger ever detected and the first event seen in both gravitational waves and light. On August 17, 2017, LIGO and Virgo recorded two neutron stars, with a total of about 2.7 solar masses, spiraling together about 40 megaparsecs (about 130 million light-years) away (GW170817).
Just 1.7 seconds after the merger, NASA’s Fermi satellite detected a short gamma-ray burst from the same patch of sky. Within 11 hours, telescopes found a new glowing source, a kilonova, in the galaxy NGC 4993. Its spectra showed freshly made heavy elements from rapid neutron capture: neutron star collisions forge a large share of the universe’s gold and platinum. More on kilonovae and the other events in neutron star mergers.
The timing also settled a basic question. After traveling for about 130 million years, the gravitational waves and gamma rays arrived 1.7 seconds apart. That limits any difference between the speed of gravity and the speed of light to between -3 × 10-15 and +7 × 10-16 of the speed of light. How that limit was set, and the dark energy theories it ruled out, is explained in do gravitational waves travel at the speed of light?
As of the GWTC-5.0 catalog (May 2026), the LIGO-Virgo-KAGRA network has confirmed 390 gravitational-wave events since 2015. The count has grown quickly as detectors improved:
| Catalog | Observing period | New catalog entries | Cumulative total |
|---|---|---|---|
| GWTC-3 | O1 to O3 (from 2015) | 90 | 90 |
| GWTC-4.0 | O4a (May 24, 2023 to January 16, 2024) | 128 | 218 |
| GWTC-5.0 | O4b (April 10, 2024 to January 28, 2025) | 161 | 390 |
The catalogs use slightly different counting criteria, so the totals are those reported by the collaboration (the GWTC-4.0 catalog; GWTC-5.0 and its 390 events). The fourth observing run, O4, ran from May 24, 2023 to November 18, 2025, and issued about 250 public alerts; the events from its final stretch are not yet in a catalog.
Highlights from O4 include:
The gravitational wave background is a faint, constant hum of long-wavelength gravitational waves filling the universe. In June 2023 NANOGrav reported evidence for it from 15 years of timing 67 millisecond pulsars (NANOGrav’s 15-year evidence). The telltale Hellings-Downs correlation between pulsar pairs appeared at about 3 to 4 sigma, and arrays in Europe and India, Australia and China reported similar evidence the same day.
The leading explanation is the combined signal of many pairs of supermassive black holes slowly orbiting each other in the centers of merged galaxies. More exotic sources from the early universe are not ruled out. The evidence is strong but not yet at the 5-sigma level.
In ten years, gravitational waves have answered questions no telescope could:
The next decade will expand both reach and sensitivity:
For the original story told by the people who made it, Caltech’s “LIGO: The First Observation of Gravitational Waves” is in our video library.
Albert Einstein predicted gravitational waves in 1916. Russell Hulse and Joseph Taylor found the first indirect evidence with a binary pulsar discovered in 1974, earning the 1993 Nobel Prize. The LIGO collaboration made the first direct detection in 2015, and Rainer Weiss, Barry Barish and Kip Thorne received the 2017 Nobel Prize.
Gravitational waves were first detected directly on September 14, 2015, at 09:50:45 UTC, by both LIGO detectors in the United States. The signal, named GW150914, came from two black holes of about 36 and 29 solar masses merging roughly 1.3 billion light-years away. The discovery was announced in February 2016.
Yes. In August 2017 the neutron star merger GW170817 was seen in gravitational waves and, 1.7 seconds later, in gamma rays, after both had traveled about 130 million light-years. That limits any difference between the speed of gravity and the speed of light to about one part in a thousand trillion.
No. By the time gravitational waves from cosmic collisions reach Earth they stretch and squeeze space by about one part in 1021, changing the length of a 4-kilometer detector arm by far less than the width of a proton. They pass through planets and people without any effect we could feel.
No. Nothing that starts inside the event horizon can get out, including gravitational waves. The waves LIGO detects are produced outside the horizons, by the violent motion of the black holes and the warped spacetime around them as they spiral together, merge and settle down into a single black hole.
As of the GWTC-5.0 catalog released in May 2026, the LIGO-Virgo-KAGRA network has confirmed 390 gravitational-wave events since 2015. The catalog covers data up to January 28, 2025. Signals from the rest of the fourth observing run, which ended in November 2025, are still being analyzed.
Because spacetime is extremely stiff. Even the merger of two black holes, one of the most violent events in the universe, produces a stretch of only about 10−21 by the time it reaches Earth. Detecting it means measuring changes of 1/10,000 the width of a proton over kilometers.
Hulse and Taylor, The Astrophysical Journal Letters, 195, L51-L53, 1975
Discovered in 1974 a pulsar orbiting a neutron star every 7.75 hours, later the first evidence of gravitational waves. Hulse and Taylor's Nobel paper.
LIGO Scientific Collaboration and Virgo Collaboration, Physical Review Letters, 116, 061102, 2016
Detected gravitational waves for the first time, from two black holes merging 1.3 billion light-years away. The 2016 LIGO-Virgo GW150914 paper.
LIGO Scientific Collaboration and Virgo Collaboration, Physical Review Letters, 119, 161101, 2017
Detected in 2017 two neutron stars merging 40 megaparsecs away, also seen as a gamma-ray burst and a kilonova. The LIGO-Virgo GW170817 discovery paper.
Pardo et al., Journal of Cosmology and Astroparticle Physics, 2018, 048, 2018
Showed in 2018 that GW170817's gravitational waves travel in 3+1 spacetime dimensions, with no leak into large extra ones. Pardo, Fishbach, Holz, Spergel.
NANOGrav Collaboration, The Astrophysical Journal Letters, 951, L8, 2023
Found evidence in 2023 for a background hum of gravitational waves by timing 67 pulsars for 15 years. The NANOGrav Collaboration's 15-year data set paper.
LIGO Scientific Collaboration, Virgo Collaboration and KAGRA Collaboration, Physical Review Letters, 135, 111403, 2025
Confirmed Hawking's 1971 area theorem and the Kerr nature of black holes with GW250114, the clearest gravitational-wave signal yet. LIGO-Virgo-KAGRA, 2025.
LIGO Scientific Collaboration, Virgo Collaboration and KAGRA Collaboration, The Astrophysical Journal Letters, 2026
Added 128 new gravitational-wave candidates from 2023-2024, bringing the catalog to 218. The LIGO-Virgo-KAGRA GWTC-4.0 catalog of black hole mergers.
Caltech's short account of LIGO's first observation of gravitational waves on September 14, 2015, and the science behind it. A quick introduction to the discovery from the people who made it. caltech
Explains how LIGO's laser interferometers detect gravitational waves that stretch space by far less than the width of a proton, with LIGO physicist Rana Adhikari. Makes an extreme measurement easy to grasp. Veritasium
Covers the 2017 neutron star merger seen both in gravitational waves and by telescopes across the spectrum, including a gamma-ray burst. A short introduction to multi-messenger astronomy. Veritasium
Explains the 2023 announcement, by a large team of gravitational-wave astronomers, of moderately confident evidence for a background of gravitational waves filling the universe, and what could produce it. Detailed and technical. PBS Space Time
Scott Manley explains LISA, the planned trio of spacecraft forming a gravitational-wave detector millions of kilometers across, able to hear supermassive black hole mergers months in advance. Strong on the engineering. Scott Manley