When two black holes merge, they combine into a single, larger black hole in a violent event that converts a significant portion of their combined mass into pure energy, radiated as gravitational waves. These ripples in spacetime travel outward at the speed of light, briefly releasing more power than all the stars in the observable universe combined. The process unfolds in three distinct stages and can take hundreds of millions of years from start to finish, though the final collision itself is over in a fraction of a second.
How Two Black Holes Find Each Other
Black holes don’t just drift together randomly. Most merging pairs start as binary star systems, two massive stars orbiting each other that both collapse into black holes at the end of their lives. Others find each other in the dense cores of galaxies, where black holes can sink toward the center through gravitational interactions with surrounding stars. When two galaxies collide, their central supermassive black holes are drawn together over time as well.
Once two black holes are gravitationally bound, they begin a long spiral inward. For supermassive black holes in galaxy centers, this inspiral takes roughly 100 to 500 million years, depending on the masses involved and the surrounding environment. Interactions with nearby stars gradually tighten the orbit, and as the black holes get closer, they begin losing energy by emitting gravitational waves. This energy loss accelerates the process: the closer they get, the more gravitational radiation they emit, and the faster they spiral inward.
The Three Stages of a Merger
Physicists break the merger into three phases: inspiral, merger, and ringdown.
During the inspiral, the two black holes orbit each other in a slowly tightening spiral. This is the longest phase by far. As the orbit shrinks, the black holes move faster and the gravitational waves they emit grow stronger and higher in frequency. If you could hear gravitational waves as sound, this would produce a rising chirp, starting low and quiet and climbing in both pitch and volume.
The merger phase is astonishingly brief. After millions of years of inspiral, the two black holes plunge together in a final fraction of a second. Their event horizons (the boundaries beyond which nothing can escape) touch and combine. Spacetime in the immediate vicinity becomes extraordinarily warped, and the gravitational wave signal reaches its peak intensity. This is the moment of greatest energy output.
What follows is the ringdown. The newly formed black hole is initially misshapen, bulging and oscillating like a struck bell. Over the next few milliseconds, it radiates away these distortions as additional gravitational waves and settles into a stable, spinning black hole. The final object is completely described by just two properties: its mass and its spin.
Mass Lost to Gravitational Waves
The final black hole is always lighter than the sum of its parents. The missing mass has been converted into gravitational wave energy, following Einstein’s famous equation relating mass and energy. In the first merger ever detected, GW150914, two black holes of about 36 and 29 times the mass of our sun combined to form a black hole of roughly 62 solar masses. About three solar masses worth of energy, an almost incomprehensible amount, was radiated as gravitational waves in less than a second.
The exact fraction of mass converted into energy depends on the masses and spins of the merging black holes. Physicists have developed formulas that predict the final mass and spin for any combination of parent black holes, and these agree well with computer simulations that solve Einstein’s equations directly. Typically, a few percent of the total mass is radiated away. For equal-mass, non-spinning black holes, the figure is close to 5%. When the black holes are spinning rapidly in favorable orientations, even more energy can be extracted.
The Gravitational Recoil Kick
One of the most dramatic consequences of a merger is that the resulting black hole can be launched through space at enormous speed. Gravitational waves don’t radiate symmetrically in every direction during the final moments of coalescence. If the two black holes have unequal masses or misaligned spins, the waves carry more momentum in one direction than another. This asymmetry gives the newly formed black hole a recoil “kick” in the opposite direction.
For typical mergers, this kick might be a few hundred kilometers per second. But in certain configurations where the spins are large and oriented in specific ways (so-called “superkicks”), the recoil velocity can reach up to 5,000 kilometers per second. That’s fast enough to escape almost any galaxy. A kicked black hole would sail out of its host galaxy entirely, becoming a rogue black hole wandering through intergalactic space. Even moderate kicks of a few hundred km/s can eject black holes from smaller galaxies or globular clusters, which have weaker gravitational pull.
Building Bigger Black Holes Through Mergers
Mergers are one way nature builds progressively larger black holes. The most compelling example came in 2019, when LIGO and Virgo detected an event called GW190521. Two black holes with masses of about 85 and 66 times the sun’s mass merged to form a remnant of approximately 142 solar masses. This was significant for two reasons.
First, the heavier parent (around 85 solar masses) falls in a range where stars aren’t supposed to be able to produce black holes through normal collapse. Extremely massive stars undergo a process that tears them apart before they can form a black hole in the roughly 65 to 120 solar mass range. The most likely explanation is that this 85-solar-mass black hole was itself the product of a previous merger.
Second, the 142-solar-mass remnant is the first strong evidence for an “intermediate mass” black hole. Astronomers had long observed stellar-mass black holes (up to a few dozen solar masses) and supermassive black holes (millions to billions of solar masses) but had very little evidence for anything in between. GW190521 filled that gap and suggested that repeated mergers in dense environments could be a pathway to building ever-larger black holes over cosmic time.
What Mergers Look Like From Earth
Black hole mergers were purely theoretical until September 14, 2015, when LIGO made the first direct detection of gravitational waves. Since then, the LIGO, Virgo, and KAGRA observatories have reported over a hundred gravitational wave signals from merging compact objects, the majority of them from binary black hole systems.
In most cases, a black hole merger produces no light at all. Black holes in empty space have no surrounding material to heat up or glow. But when a merger happens inside a disk of gas, such as around a supermassive black hole in an active galaxy, the story changes. The remnant black hole can launch jets and outflows that slam into the surrounding gas, producing observable flares of light. The GW190521 event, for instance, was tentatively associated with an optical flare detected by the Zwicky Transient Facility. If confirmed, this would represent one of the first cases of seeing both gravitational waves and light from the same black hole merger.
Theoretical models suggest these electromagnetic signals could come from several sources: outflows from the remnant black hole shocking the surrounding disk material, or a jet punching through the gas and creating a hot cocoon that glows in ultraviolet and optical wavelengths. These signals would typically peak in the UV range and fade over days to weeks.
What the Final Black Hole Is Like
After the ringdown, the merged black hole is remarkably simple. No matter how chaotic the merger, the end result is a black hole defined entirely by its mass and spin. All information about the complex dynamics of the collision, the orbital eccentricity, the individual spins and their orientations, is radiated away as gravitational waves.
The final spin depends on a combination of the orbital angular momentum (how fast the black holes were circling each other) and the individual spins of the parent holes. For non-spinning black holes of equal mass, the remnant ends up with a spin of about 0.69 (on a scale where 0 is no spin and 1 is the maximum possible). If the parent black holes were already spinning rapidly in the same direction as their orbit, the final spin can approach the theoretical maximum. If they were spinning in opposing directions, they can partially cancel out, leaving a slower-spinning remnant.
Once settled, the new black hole is indistinguishable from any other black hole of the same mass and spin. It carries no memory of whether it formed from a merger, a collapsing star, or any other process. It simply sits in space, warping the geometry around it, until something else falls in or, on an almost unimaginably long timescale, it slowly evaporates through Hawking radiation.