Gravitational Waves

Ripples in spacetime

Einstein's general theory of relativity describes gravity not as a pull but as a bending of space and time, joined together as spacetime. Heavy objects curve spacetime, and other objects follow those curves.

When massive objects move and speed up, they can send waves through spacetime, a bit like a boat sending ripples across a lake. These are gravitational waves. As one passes, it stretches space in one direction and squeezes it in the other, over and over, at the speed of light.

The effect is tiny. Even a wave from two colliding black holes changes the distance between Earth and the Sun by less than the width of an atom. Only the most violent events in the universe make waves we can hope to measure.

From prediction to proof

Einstein worked out in 1916 that gravitational waves should exist, but he thought they would be far too weak to detect. For decades many physicists even argued about whether they were real.

Indirect proof came in 1974, when Russell Hulse and Joseph Taylor found a pair of neutron stars orbiting each other. Over the years their orbit shrank at exactly the rate expected if they were losing energy as gravitational waves. The discovery earned the 1993 Nobel Prize in Physics.

Directly catching a wave on Earth took another 40 years and some of the most sensitive instruments ever built.

How LIGO, Virgo and KAGRA work

LIGO, the Laser Interferometer Gravitational-Wave Observatory, has two detectors in the United States, at Hanford, Washington and Livingston, Louisiana. Each is shaped like an L with arms about 2.5 miles long.

A laser beam is split and sent down both arms, bounced off mirrors and recombined. Normally the two beams cancel out. If a gravitational wave stretches one arm and squeezes the other, the beams fall slightly out of step and a flicker of light appears. The change in length is less than one ten-thousandth the width of a proton.

Having several detectors far apart helps rule out local shaking, like trucks or earthquakes, and lets scientists locate a source on the sky. LIGO works with Virgo in Italy and KAGRA, an underground detector in Japan. Together they form a global network.

The first detection

On September 14, 2015, both LIGO detectors recorded a short signal, a rising chirp lasting a fraction of a second. It came from two black holes, about 36 and 29 times the mass of the Sun, spiraling together and merging about 1.3 billion light-years away.

In that final instant, about three Suns' worth of mass was turned into gravitational-wave energy. The result was announced in February 2016, and in 2017 Rainer Weiss, Barry Barish and Kip Thorne received the Nobel Prize in Physics for their roles in LIGO.

By the end of the fourth observing run in late 2025, the network had recorded more than 300 candidate signals. Most are black hole mergers, with a smaller number involving neutron stars.

What gravitational waves tell us

In August 2017 detectors caught waves from two neutron stars colliding. Within seconds, space telescopes saw a burst of gamma rays, and observatories around the world then watched the fading glow, called a kilonova. The debris contained heavy elements, strong evidence that collisions like this forge much of the universe's gold and platinum.

Gravitational waves reveal black holes that would otherwise be invisible, and show they come in a wider range of masses than expected. They let scientists test general relativity where gravity is strongest, and even give an independent way to measure how fast the universe is expanding.

Because the waves pass through dust and gas almost untouched, they carry information straight from the heart of events that light cannot show.

Listening from space: LISA

Ground detectors hear high-pitched signals from fairly small objects. To catch the slow, deep waves from merging supermassive black holes in galaxy centers, a detector needs much longer arms than any on Earth.

The European Space Agency adopted the LISA mission, built with NASA, in 2024. Three spacecraft will fly in a triangle about 1.6 million miles on each side, linked by lasers, trailing Earth around the Sun. As of 2026 launch is planned for around 2035.

Meanwhile, radio astronomers use pulsar timing arrays, watching the steady ticks of many pulsars, and in 2023 they reported evidence of a gentle background hum of very long gravitational waves. You cannot see gravitational waves with a telescope, but public alert services share new detections, often within minutes.