Gravitational Waves: Ripples in Spacetime Explained
Gravitational waves are ripples in spacetime from accelerating masses. LIGO first detected them in 2015 from two merging black holes 1.3 billion light-years away.
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Gravitational waves are ripples in the fabric of spacetime, produced when massive objects accelerate. They travel at the speed of light and stretch and squeeze space itself as they pass. Albert Einstein predicted them in 1916 as a consequence of general relativity, but he doubted they could ever be detected. On 14 September 2015 the LIGO detectors in the United States recorded the first one, a signal from two black holes that merged about 1.3 billion light-years away. It opened a new way of observing the universe.
What are gravitational waves?
In special relativity, nothing travels faster than light; general relativity extends this to gravity. A mass bends spacetime around it, and when the mass moves, the change in curvature spreads outward as a wave rather than appearing everywhere at once. Any accelerating mass radiates gravitational waves, including you waving your arm, but the effect is absurdly small. Only the most violent cosmic events — merging black holes or neutron stars, supernovae — produce waves strong enough to detect from Earth.
As a wave passes, distances between free objects oscillate: space is stretched in one direction while squeezed in the perpendicular direction, then the pattern reverses. The change is described by the strain, the fractional change in length. For the first detection the strain was about one part in 1021, so a 4-kilometre detector arm changed length by far less than the width of a proton.
How does LIGO detect them?
LIGO, the Laser Interferometer Gravitational-Wave Observatory, has two sites, at Hanford in Washington state and Livingston in Louisiana. Each is an L-shaped Michelson interferometer with 4-kilometre arms. A laser beam is split into two, sent down the arms, reflected from 40-kilogram mirrors suspended as pendulums, and recombined. With no wave, the light is arranged to cancel at the output. A passing wave changes the arm lengths slightly differently, so a tiny flicker of light appears.
Two sites matter because a real signal must show up in both, a few milliseconds apart, while local noise such as trucks and earthquakes does not. The Virgo detector in Italy and KAGRA in Japan join the network, and together the detectors can narrow down where in the sky a source lies.
What did the first detection show?
The signal, named GW150914, came from two black holes of about 36 and 29 solar masses spiralling together and merging into one black hole of about 62 solar masses. The missing three solar masses were radiated away as gravitational-wave energy in a fraction of a second, briefly outshining all the stars in the observable universe combined. The waveform matched the predictions of general relativity in detail. It was also the first direct evidence that stellar-mass black holes come in pairs that can merge. Rainer Weiss, Barry Barish, and Kip Thorne received the 2017 Nobel Prize in Physics for the detection.
Earlier indirect evidence already existed: the binary pulsar discovered by Russell Hulse and Joseph Taylor in 1974 loses orbital energy at exactly the rate general relativity predicts for gravitational radiation.
Listening to neutron stars: multi-messenger astronomy
On 17 August 2017 the detectors recorded GW170817, two neutron stars merging. Within about two seconds a gamma-ray burst was detected from the same patch of sky, and telescopes then watched a kilonova, a glow powered by freshly made heavy elements. Observations indicated that such mergers are a major source of heavy elements such as gold and platinum. It was the first event seen in both gravitational waves and light, a milestone in multi-messenger astronomy.
What comes next?
Ground-based detectors hear waves with frequencies of tens to thousands of hertz. Lower frequencies need other methods. Pulsar timing arrays such as NANOGrav reported evidence in 2023 for a background hum of nanohertz gravitational waves, likely from supermassive black-hole binaries. The European Space Agency's LISA mission, three spacecraft in a triangle millions of kilometres across, is planned to detect millihertz waves from space. Each band shows a different part of the cosmos, and well over a hundred merger candidates have already been catalogued.
Tags
astronomy black holes general relativity gravitational waves spacetime
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