Intermediate · 1:51

Two black holes merge

How two black holes spiral together, shake space itself and were heard on Earth 1.3 billion years later

Each is simulated from real physics and measurements; anything no eye could see, like magnetic fields and radio waves, is drawn and labelled as drawn.

Below are the 9 chapters of this simulation in Cosmic Scale: each one's caption, then the fuller explanation the app shows under “Read more”. Colours, arrows and lines mentioned in the text belong to the 3D scene in the app.

Open Cosmic Scale then the Phenomena tab

1. A pair of black holes

Two black holes, of 36 and 29 Suns, circle each other 1.3 billion light-years away. They are 10,000 km apart and go round once every 2.1 seconds, shown here at their real speed.

Each is what is left of a massive star. The larger hole's horizon is about 210 km across and the smaller's about 170 km, no wider than a city, yet around each the light of the stars behind is bent into arcs and rings. How two such holes came to circle each other is still debated: they may have been born as a pair of giant stars, or have met in a crowded cluster of stars.

Nothing pulls them together from outside. Their orbit loses energy to ripples it makes in space itself, so it shrinks, slowly at first and then ever faster. At this distance they have about fifty minutes left.

2. Ripples in space

As they circle, they shake space itself. Gravitational waves spread out at the speed of light, stretching space one way and squeezing it the other, twice each orbit.

Albert Einstein's general theory of relativity predicted these waves in 1916. Any mass that changes its motion sends them out, but only huge masses moving very fast make waves strong enough to matter. Here they are drawn on the plane of the orbit, gold where space is stretched and blue where it is squeezed; each part shows what the pair was doing when that part of the wave set out.

The waves carry energy away, and it comes out of the orbit. Yet even a million kilometres away they stretch space by only about one part in a million. These slow waves, rising and falling about once a second, are too slow for any detector on Earth to feel.

3. Faster and faster

Time sped up: as the orbit shrinks it speeds up, and the waves grow stronger. In their last fifty minutes the holes close from 10,000 km apart to about 900, whirling round 17 times a second.

The closer the holes, the faster they move and the stronger the waves they send out, so the orbit loses energy faster still. The spiral runs away with itself: most of these fifty minutes are spent in wide, slow turns, and the last thousand kilometres take less than a third of a second.

The waves' pitch is twice the orbit's turns a second. When it passes about 30 cycles a second, fast enough for LIGO's detectors to feel, the holes are moving at a third of the speed of light relative to each other.

4. The last fifth of a second

Slowed down about seventy times: in their last fifth of a second the holes go round five more times, closing to about 320 km apart at more than half the speed of light.

This is the part of the waves LIGO heard: about ten cycles in a fifth of a second, their pitch rising from about 35 to 150 cycles a second and their strength growing all the while. Turned into sound, it rises like a bird's chirp, and that is what scientists call it.

So close together, the holes bend space so strongly that the simple formulas for the spiral break down. Only supercomputers solving Einstein's equations can follow them to the end. Scientists found the masses, 36 and 29 Suns, by comparing what LIGO measured with thousands of such simulations.

5. Two become one

The horizons touch and merge. Three Suns' worth of mass has now turned into gravitational waves, most of it in the last fifth of a second; at their peak they carry more power than all the stars in the observable universe.

At the peak the waves carry off about 3.6 × 10⁴⁹ watts, some fifty times the light of every star in the observable universe put together, though only for a few thousandths of a second. In all, 36 and 29 Suns became a hole of 62: the missing three left as waves.

The merging itself is drawn simply here. Supercomputer simulations show what really happens: the two horizons join into a single, lopsided one, which quickly settles into shape. Through all of it, nothing but the waves escapes: two black holes merging in empty space give off no light at all.

6. One black hole

What is left is one black hole of 62 Suns, its horizon turning about 100 times a second. It rang like a struck bell, at about 250 cycles a second, and fell silent almost at once.

The ringing is the new horizon settling into shape. Its pitch and how quickly it dies away depend only on the hole's mass and spin, just as general relativity predicts, and what LIGO heard matches.

Stephen Hawking showed in 1971 that the total area of black holes' horizons can never shrink. Here the two horizons had about 230,000 km² between them; the new one has about 370,000 km², more, although three Suns of mass were lost. In 2025 a louder merger, GW250114, confirmed this more clearly than ever.

7. Out into the universe

The waves race outwards at the speed of light, fading as they spread. In just over a second they have gone as far as the Moon is from Earth; in eight minutes, as far as the Sun.

Their strength falls in step with distance: ten times farther, ten times weaker. Unlike light, they pass through stars, gas and dust as if nothing were there, so they arrive just as they left, only fainter.

They travelled for 1.3 billion years. When they set out, life on Earth was still mostly single cells. On the way the universe grew, and stretched them with it: they arrived 9% longer and lower in pitch than they left, which scientists allow for when they work out the masses.

8. LIGO hears it

On 14 September 2015 at 09:50:45 UTC, the waves swept across Earth from the southern sky. LIGO's detector in Livingston, Louisiana felt them first; the one in Hanford, Washington, 3,000 km away, 7 thousandths of a second later.

Each LIGO detector is an L of two arms 4 km long, with laser light running up and down each. A passing wave stretches one arm while squeezing the other; at the peak these arms changed length by about a thousandth of the width of a proton, and LIGO measured it.

Because the waves travel at the speed of light, the 7 ms between the detectors placed the source on a ring in the southern sky; two detectors could say no more. It was night in America: 4:50 in the morning in Louisiana. Here time runs about sixty times slower.

9. Hundreds more

It was the first time anyone had detected gravitational waves, a century after Einstein predicted them. By 2025 detectors had heard about 300 black hole mergers, one every few days.

The discovery won Rainer Weiss, Barry Barish and Kip Thorne the 2017 Nobel Prize in Physics. LIGO now listens together with Virgo in Italy and KAGRA in Japan: with three or more detectors, a source can be placed far more precisely in the sky.

The heaviest pair heard so far, GW231123, made a black hole of about 225 Suns. In 2017 two neutron stars were heard colliding, and telescopes then caught the explosion's light, where heavy elements such as gold are forged.

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