Gravitational Waves: How LIGO Opened a New Era of Astronomy

On 14 September 2015, a faint ripple in the fabric of space washed across Earth. It had been travelling for about 1.3 billion years, since two black holes spiralled together and merged. Two detectors in the United States, one in Livingston, Louisiana, and one in Hanford, Washington, twitched by a tiny fraction of the width of a proton. That signal, announced in February 2016, was the first direct detection of gravitational waves. The video above features scientists behind the discovery. This article explains what they found in plain English.

What is a gravitational wave?

In 1915 Albert Einstein published his general theory of relativity, which describes gravity not as a force pulling across empty space but as a curving of space and time (space-time) by mass and energy. A year later he predicted that when massive objects accelerate, for example two stars orbiting each other, they should send out ripples in space-time that travel at the speed of light. These are gravitational waves.

As a wave passes, it very slightly stretches space in one direction and squeezes it in the other, then reverses. The effect is incredibly small. Einstein himself doubted it could ever be measured.

Indirect evidence came first

In 1974 Russell Hulse and Joseph Taylor discovered a pair of neutron stars, one of them a pulsar, orbiting each other. Over the following years they showed that the orbit was shrinking at exactly the rate general relativity predicted if the system was losing energy as gravitational waves. They received the 1993 Nobel Prize in Physics. This was strong indirect evidence, but nobody had yet detected a wave arriving at Earth.

How LIGO works

LIGO, the Laser Interferometer Gravitational-Wave Observatory, consists of two L-shaped detectors, each with arms 4 km long. A laser beam is split and sent down both arms, bounced off mirrors and recombined. Normally the light waves cancel out. If a gravitational wave passes, one arm gets very slightly longer while the other gets shorter, and the returning light no longer cancels perfectly.

The changes are around a ten-thousandth of the width of a proton. To measure that, LIGO uses some of the most stable lasers and best vacuum systems on Earth, along with mirrors suspended to isolate them from vibrations. Having two detectors 3,000 km apart lets scientists rule out local disturbances such as trucks or small earthquakes, and the time difference between them helps locate the source in the sky. The project took decades of work: early prototypes date back to the 1970s, and the upgraded “Advanced LIGO” had only just begun operating when the first signal arrived.

The first detection: GW150914

The event, named GW150914 after its date, came from two black holes about 36 and 29 times the mass of the Sun. As they spiralled together they formed a single black hole of about 62 solar masses. The missing mass, roughly three times the mass of the Sun, was converted into gravitational-wave energy in a fraction of a second. At its peak the merger was putting out more power than all the stars in the observable universe combined.

The signal had a characteristic shape called a “chirp”, rising rapidly in frequency and strength as the black holes closed in. Converted to sound, it really does sound like a short chirp. The detection also provided the first direct evidence that pairs of black holes exist and merge.

A Nobel Prize and a new kind of astronomy

In 2017 Rainer Weiss, Barry Barish and Kip Thorne received the Nobel Prize in Physics for their decisive contributions to LIGO and the observation of gravitational waves. More than a thousand scientists worldwide contributed to the discovery through the LIGO Scientific Collaboration and the Virgo Collaboration.

Until 2015, almost everything we knew about the universe came from light and other electromagnetic radiation, plus cosmic rays and neutrinos. Gravitational waves are a completely different messenger. They pass through matter almost unaffected, so they can carry information from places light cannot escape, such as the immediate surroundings of colliding black holes.

Neutron stars and the origin of gold

On 17 August 2017 LIGO and the European Virgo detector in Italy recorded GW170817, the merger of two neutron stars. Just 1.7 seconds later, space telescopes detected a burst of gamma rays from the same region, and telescopes around the world then observed the glowing aftermath, called a kilonova, in a galaxy about 130 million light-years away. It was the first time a cosmic event was seen in both gravitational waves and light, and it showed that such collisions create heavy elements such as gold and platinum.

Where things stand now

A global network now includes the two LIGO detectors, Virgo in Italy and KAGRA in Japan, and a third LIGO detector is planned in India. Hundreds of mergers of black holes and neutron stars have been reported. The European Space Agency’s LISA mission, planned for launch in the 2030s, will place three spacecraft millions of kilometres apart in space to detect lower-frequency waves from supermassive black holes. Meanwhile, pulsar timing arrays, which use pulsars across our galaxy as precise clocks, reported evidence in 2023 for a background of very long gravitational waves.

Why it matters to everyone

Gravitational-wave astronomy is only about a decade old, a bit like where optical astronomy was just after Galileo first pointed a telescope at the sky. Each new detection tests Einstein’s theory in extreme conditions and tells us something about how stars live and die. You cannot observe gravitational waves from your back yard, but LIGO does publish public alerts, and citizen-science projects such as Gravity Spy invite volunteers to help classify noise in the detector data.