First Gravitational Wave Detection (2015): LIGO's GW150914 Paper

Gravitational waves were first detected directly on September 14, 2015, by the two LIGO observatories in the United States. The signal, GW150914, came from two black holes of 36 and 29 solar masses merging about 1.3 billion light-years away. The LIGO and Virgo collaborations announced it in this paper on February 11, 2016.

The question before the paper

Einstein’s general relativity predicts that accelerating masses send ripples through spacetime, stretching and squeezing distances as they pass. The Hulse-Taylor binary pulsar had shown indirectly, from the 1970s on, that such waves carry energy away. But by the time they reach Earth the distortions are absurdly small, about one part in 10²¹. Could an instrument ever measure one directly?

What the paper found

• A signal in both detectors. At 09:50:45 UTC on September 14, 2015, the LIGO detectors in Hanford, Washington and Livingston, Louisiana recorded the same brief signal.
• The shape of a merger. The signal swept upward in frequency from 35 to 250 Hz, with a peak strain of 1.0 × 10^-21, matching general relativity’s prediction for two black holes spiraling together and merging.
• Unmistakably real. The signal-to-noise ratio was 24, and the false alarm rate was below one in 203,000 years, a significance above 5.1 sigma.
• Black holes nobody had seen. Black holes of 36 and 29 solar masses merged into one of 62 solar masses at about 410 megaparsecs (roughly 1.3 billion light-years). The missing 3.0 solar masses were radiated as gravitational-wave energy.

How gravitational waves were first detected

Each LIGO detector is an L-shaped laser interferometer with two arms 4 kilometers long. A laser beam is split, sent down both arms, bounced back by mirrors and recombined. A passing gravitational wave stretches one arm while squeezing the other, changing how the two beams line up when they recombine. Having two widely separated detectors lets scientists reject local noise: a real signal appears in both, a few milliseconds apart. The measured waveform was then matched against relativity simulations of merging black holes to read off masses and distance.

Why it mattered

A century after Einstein predicted them, gravitational waves were detected directly, opening an entirely new way to observe the universe. GW150914 was also the first observation of a pair of black holes merging, proof that such binaries exist and merge within the age of the universe. Rainer Weiss, Barry Barish and Kip Thorne received the 2017 Nobel Prize in Physics for decisive contributions to LIGO and the observation of gravitational waves.

What we know now

Gravitational-wave astronomy is now routine. In 2017 the network caught the first neutron star merger, GW170817, also seen in light. As of the GWTC-5.0 catalog (May 2026), the LIGO-Virgo-KAGRA network has confirmed 390 events; see GWTC-4 for the first part of the fourth run. In January 2025 the detectors recorded GW250114, a near twin of GW150914 and the clearest signal yet, which confirmed Hawking’s area theorem. See our gravitational waves guide.

Who wrote it

The paper is signed by the LIGO Scientific Collaboration and the Virgo Collaboration, with individual authors starting with B. P. Abbott and R. Abbott. LIGO was conceived, built and is operated by Caltech and MIT. The paper has about 4,000 citations in Semantic Scholar as of October 2026.

Read the original

LIGO Scientific Collaboration and Virgo Collaboration, Abbott, B. P., Abbott, R., et al. (2016). “Observation of Gravitational Waves from a Binary Black Hole Merger.” Physical Review Letters 116, 061102. doi:10.1103/PhysRevLett.116.061102. Free on arXiv: 1602.03837.

Questions people ask

Who discovered gravitational waves?

The LIGO Scientific Collaboration and the Virgo Collaboration made the first direct detection on September 14, 2015, announced in February 2016. Rainer Weiss, Kip Thorne and Barry Barish won the 2017 Nobel Prize for it. Earlier, Hulse and Taylor’s binary pulsar gave indirect evidence, recognized by the 1993 Nobel Prize.

When were gravitational waves first detected?

Gravitational waves were first detected directly on September 14, 2015, at 09:50:45 UTC, by both LIGO detectors. The signal, named GW150914 after its date, came from two merging black holes. The discovery was announced on February 11, 2016, a century after Einstein predicted the waves.

Why are gravitational waves so hard to detect?

Because spacetime is extremely stiff. Even the violent merger of two black holes produced a peak strain of only about 10^-21 at Earth: each 4-kilometer LIGO arm changed length by a tiny fraction of the width of a proton. Measuring that requires isolating mirrors from every other vibration.