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Gravitational waves: hearing two black holes merge
6 min readUpdated 8 October 2026
Ripples in spacetime
In 1916, Einstein realised that general relativity predicts waves: when masses accelerate, spacetime starts to vibrate, and the vibration travels at the speed of light. He doubted they could ever be measured, so faint are they. Indirect proof came in the 1970s with the Hulse-Taylor binary pulsar, whose orbit shrinks exactly as predicted.
GW150914, the first signal
On 14 September 2015, the two LIGO detectors in the United States recorded, 7 milliseconds apart, a brief signal: in 0.2 seconds its frequency rose from 35 to about 150 hertz as its amplitude grew. It was the signature of two black holes of about 36 and 29 solar masses spiralling together and merging, 1.3 billion years ago.
The final black hole weighs about 62 solar masses: the equivalent of three solar masses was turned into gravitational waves in a fraction of a second. The discovery earned Rainer Weiss, Barry Barish and Kip Thorne the 2017 Nobel Prize in Physics.
Sound computed in your browser from a simplified model: a Newtonian inspiral for a chirp mass of 30 solar masses, then the damped ringing of the final black hole. It is not LIGO's recording, but it reproduces its shape: a rise in frequency and loudness from 35 to about 150 Hz in 0.2 seconds.
How LIGO works
Each detector is an L-shaped interferometer with 4 km arms. A laser travels down both arms and recombines. When a wave passes, it lengthens one arm and shortens the other, slightly shifting the beams. The length change to measure is about a thousandth of the size of a proton.
Go deeperThe strain
For GW150914, the peak strain reached about 10⁻²¹. On a 4 km arm, that is a change of about 10⁻¹⁸ m. The signal's frequency tells us the masses: the faster it rises, the heavier the pair.
What these waves taught us
- Black hole pairs exist and merge frequently across the Universe.
- Intermediate-mass black holes exist: the GW190521 merger produced a black hole of about 142 solar masses.
- In 2017, a neutron star merger was seen both in gravitational waves and in light, a first.
- The signals agree with general relativity, including the "ringing" of the final black hole just after the merger.
Looking ahead, the European Space Agency is preparing LISA, a space-based detector planned for the 2030s that will catch mergers of supermassive black holes. Meanwhile, explore the black holes we already know.
Frequently asked questions
Can you hear gravitational waves?
Not with your ears: they are distortions of space, not vibrations of air. But their frequency, from tens to hundreds of hertz for stellar black holes, falls in the audible range. So the signal can be turned into sound, as on this page.
How many black hole mergers have been detected?
Hundreds of signals have been recorded since 2015 by LIGO, Virgo and KAGRA, the vast majority from black hole mergers.
Are gravitational waves dangerous?
No. By the time they reach us, they stretch and squeeze Earth by a tiny fraction of an atom's size. It takes the most precise instruments ever built to measure them.
Sources
- LIGO Scientific & Virgo Collaborations (2016), Observation of gravitational waves from a binary black hole merger, PRL 116, 061102
- LIGO Scientific & Virgo Collaborations (2020), GW190521: a binary black hole merger with a total mass of 150 M☉, PRL 125, 101102
- Gravitational Wave Open Science Center, audio files of GW150914
Keep exploring
- Black holes →How black holes form, which types exist, from Gaia BH1 to M87*, and how we photographed and heard them.
- Falling in →Spaghettification, time dilation, crossing the horizon: falling into a black hole step by step, with a gravitational lensing render.
- Quiz →Ten questions about black holes, horizons, Hawking and gravitational waves, with an explanation for every answer.
- Teachers →Ready-to-use activities about black holes: orders of magnitude, time dilation, reading an EHT image.