How LIGO Detected Gravitational Waves From Merging Black Holes

On 14 September 2015, at 09:50:45 UTC, two detectors in the United States picked up a faint, rising "chirp" lasting about a fifth of a second. It was the first direct detection of gravitational waves, ripples in spacetime that Albert Einstein's general theory of relativity had predicted almost exactly a century earlier. The signal, named GW150914, came from two black holes spiralling together and merging roughly 1.3 to 1.4 billion light-years away.

The LIGO and Virgo collaborations announced the discovery on 11 February 2016, after months of checks. Below is what was detected, how the instruments work, what it did and did not prove about Einstein, and how the field has grown since.

What Exactly Was Detected?

Gravitational waves are disturbances in the shape of space and time itself, produced when massive objects accelerate. They travel at the speed of light and stretch and squeeze everything they pass through, but by an almost unimaginably small amount.

The source of GW150914 was a pair of black holes estimated at about 36 and 29 times the mass of the Sun. (A later reanalysis gives about 35 and 30.) In the final fraction of a second they were orbiting each other at between a third and more than half the speed of light, only a few hundred kilometres apart. They merged into a single black hole of about 62 solar masses.

That arithmetic is the remarkable part. The missing three or so solar masses were converted into energy and radiated away as gravitational waves, around 5 × 10⁴⁷ joules. For a brief instant, the peak power of the merger exceeded the combined light output of all the stars in the observable universe.

The sky position could only be narrowed to a broad patch of the southern sky, about 600 square degrees, roughly in the direction of the Magellanic Clouds but vastly farther away. With just two detectors, pinpointing the source more precisely was not possible.

How LIGO Measures a Ripple in Spacetime

LIGO, the Laser Interferometer Gravitational-Wave Observatory, consists of two identical instruments: one in Hanford, Washington, and one in Livingston, Louisiana, about 3,000 kilometres apart. Each is an L-shaped interferometer with arms 4 kilometres long.

A laser beam is split and sent down both arms, bounced back and forth by mirrors, and then recombined. Normally the two returning beams cancel each other out. When a gravitational wave passes, it lengthens one arm very slightly while shortening the other, and a flicker of light appears at the detector.

The change is tiny. GW150914 altered the effective arm length by about a thousandth of the width of a proton. To measure that, engineers had to isolate the mirrors from earthquakes, passing trucks and even ocean waves, and keep the beams in an ultra-high vacuum.

Two sites matter because local noise affects one detector but not the other. A real cosmic signal should appear in both, a few milliseconds apart. GW150914 reached Livingston about 7 milliseconds before Hanford, consistent with a wave moving at light speed. Its statistical significance was more than 5.1 sigma, the standard threshold for a discovery in physics.

The signal's shape also mattered. Its frequency swept up from about 35 to 250 hertz over around eight cycles, then rang down. That pattern matched computer models of two black holes inspiralling, merging and settling into one, which is what made the interpretation so convincing.

Did It Prove Einstein Right?

In 1916 Einstein showed that his new theory of gravity implied the existence of gravitational waves. The detection confirmed that prediction directly, and the waveform matched general relativity's description of strong gravity with impressive accuracy. In that sense, headlines saying "Einstein was right" were fair.

A few nuances are worth keeping:

Einstein himself was doubtful. He regarded gravitational waves as a mathematical result and thought they would be far too weak ever to measure. At one point in the 1930s he even questioned whether they were physically real.
He was also sceptical of black holes. In 1939 he argued that the extreme collapse they require would not happen in nature. GW150914 was also the first direct observation of a pair of black holes merging, so it confirmed something Einstein did not expect.
It was not the first evidence. In 1974 Russell Hulse and Joseph Taylor discovered a binary pulsar whose orbit was shrinking at exactly the rate expected if it were losing energy to gravitational waves. That indirect evidence earned them the 1993 Nobel Prize in Physics.
One test does not settle all of physics. General relativity passed this test, but it still cannot be reconciled with quantum mechanics, and questions such as the nature of dark energy remain open.

The achievement belongs to many people. Thousands of scientists and engineers in the LIGO Scientific Collaboration and the Virgo Collaboration contributed. In 2017, Rainer Weiss, Barry Barish and Kip Thorne received the Nobel Prize in Physics for decisive contributions to the LIGO detector and the observation of gravitational waves.

Common Myths About the Discovery

"It happened recently." The waves left their source roughly 1.3 billion years ago, when life on Earth was still almost entirely microscopic. They simply arrived in 2015.

"The black holes collided at the speed of light." They were fast, but not that fast. Their orbital speed rose from about 30 percent to around 60 percent of light speed in the final moments.

"We can hear black holes." Gravitational waves are not sound and cannot travel through air to our ears. The famous chirp is the detector signal converted into audio, which happens to fall in the range of human hearing.

"The detection was a single lucky blip." LIGO had just begun its first observing run with upgraded "Advanced LIGO" detectors. Many more events followed within months, and Virgo in Italy, which was offline for upgrades in September 2015, joined later.

What Gravitational-Wave Astronomy Has Found Since

GW150914 opened a new way of observing the universe. Instead of only collecting light, astronomers can now "listen" to the motion of mass itself.

In 2017 the detectors caught the merger of two neutron stars, GW170817, which was also seen by telescopes across the electromagnetic spectrum. It showed that such collisions forge heavy elements such as gold and platinum, and it confirmed that gravitational waves travel at the speed of light.

By September 2025 the LIGO-Virgo-KAGRA network had reported about 90 merger detections from its first three observing runs and roughly 220 more during the fourth run. On 10 September 2025 the collaboration announced GW250114, a merger similar in mass and distance to GW150914 but recorded far more clearly. Its quality allowed researchers to test Stephen Hawking's 1971 area theorem, which states that the total surface area of black hole horizons cannot shrink, and the result agreed with his prediction.

Future plans include more sensitive ground detectors and the space-based LISA mission, which is designed to pick up lower-frequency waves from supermassive black holes.

How to Read Science News Like This

Big physics stories are easier to judge when you separate a few things. First, the date of the event and the date of the announcement are different: here, September 2015 versus February 2016. Second, numbers such as "36 solar masses" are estimates with error bars. Third, a confirmed prediction supports a theory without proving every claim linked to its author.

If you want to go to the source, the discovery paper, "Observation of Gravitational Waves from a Binary Black Hole Merger", is free to read, and its abstract summarises the result in a single paragraph. LIGO's own website also has plain-language explainers and audio of the chirp.

Sources: LIGO and Virgo collaborations, Observation of Gravitational Waves from a Binary Black Hole Merger (2016); Wikipedia: First observation of gravitational waves; LIGO: GW250114 and 10 years of gravitational-wave astronomy; original story via I Heart Intelligence.

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