When two black holes collide, they spiral into each other at increasing speed, warp the fabric of space and time around them, and ultimately merge into a single, larger black hole. In that final moment, roughly 5 percent of their combined mass converts into pure energy, radiated outward as gravitational waves. That may sound modest, but for objects weighing tens or hundreds of times more than our Sun, it represents an energy release greater than the combined light output of every star in the observable universe, compressed into a fraction of a second.
The Three Stages of a Merger
A black hole collision unfolds in three distinct phases: inspiral, merger, and ringdown. During the inspiral, the two black holes orbit each other in a tightening spiral. As they get closer, they move faster, eventually reaching a significant fraction of the speed of light. Each orbit radiates gravitational energy, which causes the orbit to shrink further, accelerating the process in a runaway feedback loop.
The merger itself is the violent climax. The two event horizons (the point-of-no-return boundaries around each black hole) touch and combine. Spacetime in the immediate vicinity becomes so wildly distorted that only supercomputer simulations can model what happens. This phase lasts only milliseconds for black holes in the range of tens of solar masses, yet it produces the most intense burst of gravitational waves.
Finally, the ringdown. The newly formed black hole is not yet “settled.” It vibrates like a struck bell, its shape oscillating as it radiates away the last of its excess energy in gravitational waves. Within milliseconds, it relaxes into a stable, spinning black hole described by just two properties: its mass and its spin.
Gravitational Waves and the Shaking of Spacetime
The collision produces ripples in spacetime called gravitational waves. These travel outward at the speed of light in all directions, stretching and compressing space itself as they pass. By the time they reach Earth, billions of years later, the effect is extraordinarily subtle. A gravitational wave from a distant merger stretches a four-kilometer detector by less than one-thousandth the width of a proton.
Yet that is enough to detect. The LIGO and Virgo observatories use laser beams bouncing between mirrors to measure these impossibly small distortions. Since the first detection in 2015, the LIGO-Virgo-KAGRA network has observed about 300 black hole mergers in total, with more than 200 of those recorded during the current fourth observing run alone. What was once a theoretical prediction has become routine observation.
How Much Energy Gets Released
The mass of the final black hole is not simply the sum of the two that merged. About 5 percent of the combined mass is converted entirely into gravitational wave energy, following Einstein’s equation E=mc². For the first merger ever detected, known as GW150914, two black holes of roughly 36 and 29 solar masses merged to form a single black hole of 62 solar masses. The missing 3 solar masses were radiated as gravitational waves in a fraction of a second. Three solar masses may not sound dramatic, but that amount of mass converted to energy equals the total energy output of trillions upon trillions of stars.
For supermassive black holes weighing millions or billions of solar masses, the energy release scales proportionally. A merger between two supermassive black holes could radiate energy equivalent to hundreds of millions of solar masses, making it one of the most powerful events possible in physics.
The Gravitational Kick
One of the stranger consequences of a merger is the recoil, or “gravitational kick.” When the two black holes have unequal masses or their spins are misaligned, the gravitational waves they emit are not perfectly symmetric. This asymmetry creates a net push on the final black hole, like a rocket exhaust. The resulting kick can exceed 1,000 kilometers per second, fast enough to eject the newly formed black hole from the galaxy that hosted it. Some merged black holes may be drifting through intergalactic space right now, flung from their home galaxies by the force of their own birth.
When Supermassive Black Holes Collide
Nearly every large galaxy has a supermassive black hole at its center. When two galaxies merge (which happens regularly on cosmic timescales), their central black holes eventually find each other and begin spiraling inward. This process is far slower than mergers between smaller black holes. In the galaxy NGC 7727, two supermassive black holes sit only about 1,600 light-years apart, and simulations estimate they will merge in roughly 130 million years.
On a cosmic scale, these events are not rare. In the local universe, major galaxy mergers happen at a rate of roughly one per 100 billion years per galaxy. That sounds slow, but across the billions of galaxies in the observable universe, supermassive black hole mergers are happening regularly. At earlier points in cosmic history, when galaxies were packed closer together, the rate was ten to a hundred times higher.
Current ground-based detectors like LIGO are tuned to catch mergers of smaller black holes, typically between 5 and 100 solar masses. Supermassive mergers produce gravitational waves at much lower frequencies, too low for LIGO to pick up. A planned space-based observatory called LISA will orbit the Sun and use laser beams stretched across millions of kilometers, giving it the sensitivity to detect mergers of black holes weighing tens of millions to billions of solar masses, even from the very early universe.
What Happens to Anything Nearby
Despite the colossal energy involved, a black hole merger poses no danger to anything at a safe distance. Gravitational waves weaken rapidly as they spread, and by the time they travel even a few light-years, their effect on matter is negligible. A planet orbiting a distant star in the same galaxy would feel nothing.
Close to the merger, the story is different. Within a few hundred times the size of the black holes’ event horizons, spacetime is violently churned. Any matter, gas, or stray stars in that region would be subjected to extreme tidal forces. For supermassive black hole mergers, surrounding gas could be heated and accelerated to produce bright electromagnetic flares, potentially visible with conventional telescopes. Detecting light alongside gravitational waves from the same merger is a major goal of modern astronomy, because it would reveal details about the environment around these collisions that gravitational waves alone cannot show.
What the Final Black Hole Looks Like
The black hole that emerges from a merger is fully described by its mass and spin. For GW150914, the final black hole had a spin of about 0.67 (on a scale where 0 is no rotation and 1 is the maximum allowed by physics). Mergers tend to produce rapidly spinning black holes, because the orbital motion of the original pair gets converted into rotational energy.
If the merger involved a gravitational kick, the new black hole may be moving at high speed relative to its surroundings. In extreme cases, it could leave the core of its host galaxy and settle into a wider orbit, or escape entirely. For most mergers, though, the kick is modest enough that the black hole remains near the galactic center, where it can grow further by absorbing gas or merging again with other black holes. Some of the black holes LIGO has detected may themselves be products of earlier mergers, building up mass over successive collisions in dense stellar environments.