What are gravitational waves in simple terms?

They are ripples in the fabric of space and time caused by massive objects accelerating, such as black holes spiralling into each other. As they pass, they very slightly stretch and squeeze everything in their path.

When were gravitational waves first detected?

LIGO made the first direct detection on 14 September 2015, from two black holes merging about 1.3 billion light-years away. The discovery was announced on 11 February 2016.

Can gravitational waves hurt us?

No. By the time they reach Earth they are unimaginably weak, changing distances by far less than the width of an atomic nucleus over kilometres. Only the most sensitive instruments ever built can detect them.

How fast do gravitational waves travel?

At the speed of light. The 2017 neutron star merger GW170817 confirmed this when gamma rays arrived just 1.7 seconds after gravitational waves that had travelled for about 130 million years.

LISA is a planned space-based gravitational-wave observatory led by ESA with NASA participation. Its three spacecraft, millions of kilometres apart, will detect lower-frequency waves than ground detectors can, with launch planned for around 2035.

Ripples in spacetime from colliding black holes

Gravitational Waves

Ripples in spacetime · GW150914: ≈ 1.3 billion ly

Gravitational waves are ripples in spacetime, the four-dimensional fabric of space and time described by Einstein's general theory of relativity. When massive objects accelerate, such as two black holes whirling around each other, they send out waves that travel at the speed of light, stretching and squeezing space as they pass. Einstein predicted them in 1916 but doubted they could ever be measured.

A century later, on 14 September 2015, the twin LIGO detectors in the United States caught the signal of two black holes merging about 1.3 billion light-years away. That detection opened an entirely new way of studying the universe: not by its light, but by the vibrations of spacetime itself.

Key facts

Predicted: 1916Albert Einstein, general relativity
First direct detection: 14 September 2015GW150914, announced February 2016
First source: Two black holes≈ 36 and ≈ 29 solar masses
Distance of first source: ≈ 1.3 billion light-years
Speed: Speed of lightConfirmed by GW170817 to extreme precision
Nobel Prize in Physics: 2017Rainer Weiss, Barry Barish, Kip Thorne
LIGO arm length: 4 kmTwo detectors: Hanford and Livingston
Typical strain: ≈ 10⁻²¹A change far smaller than a proton across 4 km
Space detector: LISAESA-led, launch planned around 2035

What are gravitational waves?

General relativity describes gravity not as a force pulling across empty space but as the curvature of spacetime caused by mass and energy. When masses move in certain ways, especially when two compact objects orbit each other rapidly, the curvature changes and those changes spread outwards as waves. A passing wave alternately stretches space in one direction while squeezing it in the perpendicular direction.

Even the most violent events produce astonishingly small effects by the time the waves reach Earth. The first signal changed the length of LIGO's 4-kilometre arms by a tiny fraction of the width of a proton. Gravitational waves pass almost unhindered through matter, so they carry information from places light cannot escape, such as the immediate surroundings of merging black holes.

The chirp

As two black holes spiral together, they lose energy to gravitational waves and orbit ever faster. The wave frequency and strength rise sharply until the moment of merger, then ring down as the new black hole settles. Converted to sound, the signal is a rising 'chirp'. The 3D view shows this inspiral rippling a spacetime grid alongside its waveform.

From prediction to detection

For decades physicists debated whether gravitational waves were real or a mathematical artefact. The first strong evidence came from a pair of neutron stars, one of them a pulsar, discovered by Russell Hulse and Joseph Taylor in 1974. Their orbit was shrinking at exactly the rate expected if the system was losing energy to gravitational waves. Hulse and Taylor received the 1993 Nobel Prize in Physics.

Detecting the waves directly required instruments of extraordinary precision. LIGO, the Laser Interferometer Gravitational-Wave Observatory, uses lasers bounced along two perpendicular arms 4 kilometres long. A passing wave changes the arms' lengths by different amounts, altering how the laser beams combine. After decades of development and a major upgrade to Advanced LIGO, the detectors caught GW150914 just days into their first observing run.

GW150914

The first signal came from black holes of about 36 and 29 solar masses merging into one of about 62 solar masses. The missing three or so solar masses were converted into gravitational-wave energy in a fraction of a second. The Livingston detector saw it about 7 milliseconds before Hanford. Rainer Weiss, Barry Barish and Kip Thorne shared the 2017 Nobel Prize for their roles in LIGO.

GW170817: a collision seen in light and gravity

On 17 August 2017, LIGO and the European Virgo detector recorded the gravitational waves from two neutron stars merging in the galaxy NGC 4993, about 130 million light-years away. About 1.7 seconds after the merger signal, NASA's Fermi telescope detected a short burst of gamma rays from the same part of the sky.

Telescopes around the world then found a fading glow called a kilonova, powered by radioactive elements forged in the collision. The event confirmed that neutron star mergers produce heavy elements such as gold and platinum, showed that gravitational waves travel at the speed of light, and launched 'multi-messenger' astronomy, combining gravitational waves with light across the spectrum.

A global detector network

Today LIGO's two detectors in Hanford, Washington and Livingston, Louisiana work with Virgo near Pisa in Italy and KAGRA, an underground, cryogenically cooled detector in Japan. Together they are known as LIGO-Virgo-KAGRA. Comparing arrival times at several sites lets astronomers locate sources on the sky far more precisely than any single detector could.

Detections have become routine. Each observing run has found more mergers, mostly pairs of black holes, along with neutron star pairs and mixed black hole-neutron star systems. By the end of 2025 the network had reported hundreds of candidate signals. These have revealed black holes in mass ranges once thought unlikely and allowed new tests of general relativity in the strongest gravitational fields known.

Pulsar timing arrays and LISA

Ground-based detectors hear waves with frequencies of tens to thousands of hertz. Much slower waves, from pairs of supermassive black holes in distant galaxies, can be sensed by timing the regular radio pulses of dozens of millisecond pulsars across the Milky Way. In June 2023, NANOGrav and other pulsar timing array collaborations in Europe, Australia, India and China announced evidence for a background hum of such waves washing over the galaxy.

To cover the range in between, ESA is leading LISA, the Laser Interferometer Space Antenna, with NASA as a partner. Three spacecraft flying in a triangle with sides of about 2.5 million kilometres will exchange laser beams to detect waves from merging massive black holes and compact binary stars in our galaxy. The mission was formally adopted in 2024 and is planned to launch around 2035.

Surprising facts about Gravitational Waves

For a fraction of a second, the first black hole merger detected released more power as gravitational waves than all the stars in the observable universe emit as light.
Einstein himself doubted for years whether gravitational waves were physically real, and at one point argued they did not exist.
LIGO's mirrors are suspended from multi-stage pendulums and isolated so well that they can respond to changes far smaller than an atomic nucleus.
GW170817 ruled out many alternative theories of gravity overnight by showing that gravitational waves and light travel at the same speed.
Much of the gold in jewellery on Earth was probably forged in neutron star collisions like the one detected in 2017.

Frequently asked questions

What are gravitational waves in simple terms?: They are ripples in the fabric of space and time caused by massive objects accelerating, such as black holes spiralling into each other. As they pass, they very slightly stretch and squeeze everything in their path.
When were gravitational waves first detected?: LIGO made the first direct detection on 14 September 2015, from two black holes merging about 1.3 billion light-years away. The discovery was announced on 11 February 2016.
Can gravitational waves hurt us?: No. By the time they reach Earth they are unimaginably weak, changing distances by far less than the width of an atomic nucleus over kilometres. Only the most sensitive instruments ever built can detect them.
How fast do gravitational waves travel?: At the speed of light. The 2017 neutron star merger GW170817 confirmed this when gamma rays arrived just 1.7 seconds after gravitational waves that had travelled for about 130 million years.
What is LISA?: LISA is a planned space-based gravitational-wave observatory led by ESA with NASA participation. Its three spacecraft, millions of kilometres apart, will detect lower-frequency waves than ground detectors can, with launch planned for around 2035.

Keep exploring

The 3D views in Ember’s Universe are scientific visualisations: physically based where possible, compressed or exaggerated where the real scales would hide everything.

Sources