On September 14, 2015, a tremor rippled through two massive laser installations in Louisiana and Washington state, altering the length of their four-kilometre tunnels by less than one-thousandth the width of a proton. That microscopic twitch was GW150914 the literal shudder of two black holes colliding over a billion light-years away. For the first time in human history, we didn't just look at the universe. We heard it roar.
What followed that morning was a century in the making. Einstein had predicted gravitational waves in 1916, a consequence so wild that he himself doubted they could ever be measured. It took a hundred years, four kilometres of evacuated steel tubing, and lasers stable enough to detect motion smaller than any instrument ever built to prove him right. Since that first detection, our global network of interferometers has catalogued over 300+ confirmed events and the number keeps climbing.
What are gravitational waves?
General relativity tells us that mass warps the fabric of spacetime around it the heavier the object, the deeper the warp. When those massive objects accelerate, orbit each other, or collide, they don't just sit still in their warped pockets. They send ripples racing outward across the universe at the speed of light, compressing and stretching spacetime as they travel. These are gravitational waves.
The LIGO detectors work by shooting a laser beam down two perpendicular four-kilometre vacuum tubes and bouncing it off mirrors at the far ends. Under normal conditions, the beams return and cancel each other out perfectly. When a gravitational wave passes through, it stretches one arm and compresses the other by roughly one-thousandth the diameter of a proton. The interference pattern that results is the signal. To put that sensitivity in perspective: if you scaled the LIGO mirrors up to the distance between Earth and the nearest star, they could detect a displacement the width of a single human hair.
The global network now consists of the two LIGO detectors in the United States, the Virgo detector near Pisa in Italy, and the KAGRA detector buried underground in Japan. Triangulating signals across multiple detectors lets astronomers localise the source in the sky essential for pointing optical telescopes at the right patch of space when the event produces light as well as gravitational waves.
Visualization: thescientificdrop.com | Data: GWOSC (gwosc.org)
The three major event classes
Since GW150914, the global detector network has catalogued over 90 confirmed events across four observing runs. They fall into three distinct classes, each representing a different flavour of cosmic catastrophe.
Binary Black Hole (BBH) mergers
These make up the overwhelming majority of detections. Two black holes locked in a gravitational death spiral lose energy to gravitational radiation, tightening their orbit over millions of years until they finally merge. In the final fraction of a second before coalescence, they are radiating more energy than all the stars in the observable universe combined. The gravitational wave signal sweeps upward in frequency as the orbit tightens from a low rumble to a sharp chirp. That rising chirp, when slowed down and played as audio, sounds uncannily like a brief "bloop." It is the sound of spacetime being torn and stitched back together.
Binary Neutron Star (BNS) collisions
Neutron stars are the ultra-dense collapsed cores of massive stars that ended their lives in supernovae a solar mass of material crushed into a sphere roughly 12 kilometres across. The density is extraordinary: a teaspoon of neutron star material would weigh roughly a billion tonnes on Earth. When two of these objects spiral into each other, they don't merge silently. They shatter themselves apart in a violent spray of debris, producing not just gravitational waves but gamma-ray bursts, X-ray afterglows, and a kilonova a luminous explosion that synthesises heavy elements in the extreme conditions of the collision.
Neutron Star–Black Hole (NSBH) mergers
The most asymmetric class. A black hole swallows a neutron star whole. Depending on the mass ratio and the spin of the black hole, it either tears the neutron star apart tidally as it spirals in briefly producing electromagnetic counterparts similar to a kilonova or it simply consumes it cleanly, leaving no light signature whatsoever. The gravitational wave signal alone distinguishes these events from binary black hole mergers.
Why GW170817 changed astronomy forever
On August 17, 2017, LIGO and Virgo caught the inspiral of two neutron stars in the galaxy NGC 4993, roughly 130 million light-years away. The signal, designated GW170817, lasted nearly two minutes in the detector band far longer than any black hole merger. Then, 1.7 seconds after the gravitational wave signal ended, NASA's Fermi satellite detected a short gamma-ray burst from the same location.
What followed was the most coordinated astronomical observation in history. Over 70 telescopes on the ground and in space pivoted to NGC 4993. They watched a blue kilonova bloom and fade over days, producing a cloud of freshly minted heavy elements. Spectroscopic analysis confirmed the presence of strontium and, indirectly, gold, platinum, and uranium. The gold in your jewellery was forged in an event like this, scattered across the galaxy and eventually incorporated into the solar system 4.6 billion years ago.
GW170817 also confirmed that gravitational waves travel at exactly the speed of light to a precision of one part in 1015 a measurement that ruled out an entire class of modified gravity theories in a single observation. It also marked the birth of multi-messenger astronomy: for the first time, a single cosmic event had been detected simultaneously in both gravitational waves and light.
Notable gravitational wave observations
The table below outlines some of the most historic detections captured by the global network of interferometers. Distances are given in millions of light-years (Mly) and billions of light-years (Gly). Component masses are in solar masses (M☉).
| Event | Date | Type | Masses (M☉) | Distance |
|---|---|---|---|---|
| GW150914 | 14 Sep 2015 | BBH | 36 + 29 | 1.3 Gly |
| GW170817 | 17 Aug 2017 | BNS | 1.48 + 1.26 | 130 Mly |
| GW190425 | 25 Apr 2019 | BNS | 2.0 + 1.4 | 520 Mly |
| GW190814 | 14 Aug 2019 | Mass Gap | 23.2 + 2.6 | 790 Mly |
| GW200105 | 5 Jan 2020 | NSBH | 8.9 + 1.9 | 900 Mly |
The GW190814 mystery
One event deserves special attention. GW190814 involved a 23-solar-mass black hole merging with an object of just 2.6 solar masses. That secondary object sits in a peculiar no-man's land between the most massive neutron star ever confirmed (around 2.3 solar masses) and the lightest known black hole (around 5 solar masses). Astronomers call this the mass gap, and GW190814's secondary sits squarely inside it. Whether it was an exceptionally heavy neutron star or an exceptionally light black hole remains an open question and either answer rewrites our understanding of how these objects form and what sets the upper limit on neutron star mass.
What comes next: Einstein Telescope and LISA
Ground-based interferometers are approaching fundamental limits. Seismic noise from traffic, ocean waves, and geological activity imposes a floor on sensitivity at low frequencies. The next generation of detectors is designed to break through that floor entirely.
The Einstein Telescope, planned for construction deep underground in Europe, will use a triangular 10-kilometre geometry and cryogenically cooled mirrors to achieve sensitivity roughly ten times beyond current LIGO. It will detect stellar-mass black hole mergers across virtually the entire observable universe, turning gravitational wave astronomy from a catalogue of notable events into a continuous census of the cosmos.
More ambitiously, the Laser Interferometer Space Antenna (LISA) will place three spacecraft in an equilateral triangle 2.5 million kilometres on a side, trailing Earth in its orbit around the Sun. Free from seismic noise, LISA will open a new low-frequency window sensitive to the slow inspiral of supermassive black holes at galactic centres, the gravitational hum of millions of compact binaries in the Milky Way, and potentially echoes of phase transitions in the very early universe. We began this field by detecting one event in 2015. We are moving toward an era in which gravitational wave detectors run continuously, cataloguing hundreds of mergers per year, mapping the black hole population of the universe the way optical surveys once mapped its galaxies.
The universe has always been loud. We are only now learning how to listen. For a sense of the distances involved the 1.3 billion light-years that GW150914 travelled to reach us see how our own radio transmissions compare in our post on Earth's Radio Bubble. And for the general relativistic framework that makes all of this possible, see our deep dive into Gravity: From Newton to Einstein.
General Relativity Series
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Read the Full GR Series →Frequently Asked Questions
What are gravitational waves?
Gravitational waves are ripples in the fabric of spacetime produced when massive objects accelerate particularly during the inspiral and merger of compact binary systems like black holes or neutron stars. They travel at the speed of light and were first directly detected by LIGO on September 14, 2015, confirming a prediction Einstein made in 1916.
How does LIGO detect gravitational waves?
LIGO uses laser interferometry. A laser beam is split and sent down two perpendicular 4-kilometre vacuum tubes. When a gravitational wave passes through, it stretches one arm and compresses the other by roughly one-thousandth the diameter of a proton. The returning beams fall slightly out of phase, and that interference pattern is the detection signal.
Who discovered gravitational waves?
The direct detection was achieved by the LIGO Scientific Collaboration in September 2015 and announced publicly in February 2016. The discovery earned Kip Thorne, Barry Barish, and Rainer Weiss the 2017 Nobel Prize in Physics. Einstein had mathematically predicted their existence in his 1916 paper on general relativity.
What is a kilonova?
A kilonova is a luminous transient explosion produced when two neutron stars merge. The extreme neutron fluxes of the collision drive rapid neutron capture nucleosynthesis, forging heavy elements including gold, platinum, and uranium. The 2017 event GW170817 was the first gravitational wave detection accompanied by a confirmed kilonova, directly observed by over 70 telescopes worldwide.
What is the mass gap in gravitational wave astronomy?
The mass gap refers to a range of compact object masses roughly 2.5 to 5 solar masses where neither confirmed neutron stars nor confirmed black holes have been found through electromagnetic observations. The event GW190814 detected a 2.6 solar mass secondary object sitting directly in this gap, leaving open whether it was the heaviest neutron star or the lightest black hole ever observed.
Further Reading
On The Scientific Drop
- Earth's Radio Bubble: Every Signal We've Ever Sent Into Space: For a sense of the distances involved how far 1.3 billion light-years truly is compared to humanity's own transmissions.
- Gravity: From Newton to Einstein: The general relativistic framework that predicted gravitational waves a century before we could detect them.
- Where Is Voyager 1 Now?: Another original NASA data visualisation this time tracking humanity's most distant spacecraft using JPL Horizons.
Official & Academic Sources
- GWOSC: Complete Gravitational Wave Event Catalogue: The full list of every confirmed detection, with parameters and data releases.
- Abbott et al. (2016): Original GW150914 Discovery Paper: The Physical Review Letters paper that announced the first direct detection to the world.
- Einstein Telescope: Official Project Site: Plans, timeline, and science goals for the next-generation underground detector.
- ESA LISA Mission Page: Full overview of the space-based gravitational wave observatory planned for the 2030s.
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