In my January 23, 2026, “The Universe” column, I wrote about some of the biggest bangs the universe has to offer: exploding stars, hiccupping magnetars, stellar disruptions and colliding black holes. These all deserve deeper dives, but perhaps black holes deserve one most of all because, technically speaking, they do provide the deepest dive you can physically take. They also make very big bangs indeed, with their collisions rapidly releasing almost incomprehensible amounts of energy.
You might think it’s obvious that merging is the final fate of two black holes. These objects’ whole shtick, after all, is gobbling up stuff, so two of them trying to eat each other feels inevitable. What happens when they do, though, is not at all straightforward.
Just the fact that they release energy—so much energy—when they collide seems impossible. Black holes are black because anything falling into them makes a one-way trip, after all; nothing, not even light, can escape once inside a black hole’s event horizon, its point of no return. But what happens just before crossing that ultimate “do not pass” sign is where all the action is.
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To see why, let’s look at binary stellar-mass black holes, which get their start as a pair of massive mutually orbiting stars. The stars eventually go supernova, and their respective cores collapse to form black holes containing as much as 100 times the mass of the sun. Such systems are relatively rare to begin with. But rarer still are those with paired black holes that are sufficiently close together to eventually collide. If such black holes were to form a billion kilometers from each other, a merger could take longer than the nearly 14-billion-year-old universe has existed.
If black holes do get close enough, perhaps nudged together by the gravity of a closely passing star, a very weird effect takes over: they spiral ever closer together because of the gravitational waves they emit. Einstein’s general theory of relativity tells us that any accelerating mass creates ripples in the fabric of spacetime that expand away at the speed of light, shaking the framework of the cosmos as they pass. You make gravitational waves when you stand up, for example, but they are extremely low-energy and too mushy to detect.
But black holes are extremely massive, and two of them orbiting each other can move at a sizable fraction of the speed of light, so they churn out copious, powerful gravitational waves. These waves are formed outside the event horizon, so they are free to propagate into the greater universe. The energy for this comes from the black holes’ orbital motion. As the black holes emit these waves, their orbits shrink because of the energy expended, drawing them closer together. This also increases their acceleration, driving more gravitational-wave emission (and causing ever tightening orbits) in a positive-feedback loop. In the last few seconds, the black holes whirl around each other at near the speed of light, emitting ever more powerful gravitational waves until the two actually merge, combining in one gluttonous gulp that leaves behind a single, more massive black hole. To date, astronomers have managed to detect about 300 such mergers via their associated crescendos of gravitational waves.
The key to calculating the amount of energy blasted out is realizing that the mass of a merger’s resulting black hole is not simply the sum of its progenitors. According to the relativistic equations, roughly 5 percent of the merging pair’s combined mass converts to gravitational waves in that final moment. That conversion is governed by Einstein’s most famous equation, E = mc2, where m is the mass of the black holes lost to energy and c is the speed of light.
Just how much energy are we talking about here?
A lot. Working out the math for the collision of, say, two five-solar-mass black holes, the amount of energy blasted out in less than a second by such a merger would be roughly the same as the sun will emit in seven trillion years. That is, for a brief moment those black holes emit more energy than the light from a billion galaxies full of stars.
And those were relatively small black holes. Others are much, much bigger.
Supermassive black holes have from 100,000 up to billions—yes, billions, with a “b”—of times the sun’s mass. They exist in the centers of all big galaxies, including the Milky Way (though ours, called Sagittarius A*, is a bit of lightweight at a mere four million solar masses). How they grew so large is still a matter of vigorous debate; they may be born large and grow even bigger, or they may have started small and grown in size as the galaxy formed around them.
Supermassive black holes usually live solitary lives, but that can change when galaxies collide and merge. The two supermassive black holes in the centers of each galaxy fall into orbit together and, like their stellar-mass cousins, can eventually spiral in and combine (though the details of this are a bit complex).
Given these black holes’ huge masses, the final cosmos-quaking blast of gravitational-wave radiation they emit is far, far larger. Repeating the math above with a pair of black holes that are, say, 100 million solar masses each, the numbers simply scream upward. The energy they emit in that last second is thousands of times the combined energy emitted by all the stars in the visible universe over that same amount of time.
I remember when I first sat down and calculated this myself. The answer I got for the torrential gravitational waves spawned by a stellar-mass black hole collision was a number that was so immense that I thought I’d made a mistake. I checked my numbers, and nope, I got it right. Then I realized what this must mean for supermassive black holes, with millions of times more mass converted into energy. The hair on the back of my neck stood straight up, and I think the room spun around me for a moment. The crushing weight of those numbers froze my soul.
And yet these “biggest in the universe” eruptions are invisible. Why?
Because gravitational waves themselves are invisible and can be emitted without any accompanying light. The waves also weaken with distance, and supermassive mergers tend to happen billions of light-years away; by the time the waves get to Earth, they’re nearly undetectable. There is some evidence we’ve seen a merger, though it’s not yet confirmed. In a decade or so, we may get a lot more data from the European Space Agency’s Laser Interferometer Space Antenna (LISA) mission that will prove that these mind-vaporizing, massive events do indeed occur.
Even as you read this, gravitational waves from unimaginably energetic black hole mergers are passing through you, weakened to less than a whisper by their unfathomable distance. Given their power, that’s a good thing. Just thinking about them leaves me shaken.
