In my January 23, 2026, “The Universe” column, I wrote about some of the biggest explosions to happen in the universe: exploding stars, hiccupping magnetars, stellar disruptions and colliding black holes. All of these are worth deep diving into, but perhaps black holes are worth diving into the most because, technically speaking, they offer the deepest dive you can physically take. They also create really big bangs, their collisions rapidly releasing almost incomprehensible amounts of energy.
You might think it’s obvious that merger is the ultimate fate of two black holes. After all, the whole point of these things is to swallow the stuff, so the two of them trying to eat each other seems inevitable. However, what happens when they do is not at all straightforward.
Just the fact that they release energy-so Too much energy—seems impossible when they collide. Black holes are black because anything falling into them ultimately travels one way; Nothing, not even light, can escape once a black hole goes inside the event horizon, from where no return is possible. But what happens right before you cross that final “Do Not Cross” sign is where all the action happens.
On supporting science journalism
If you enjoyed this article, consider supporting our award-winning journalism Subscribing By purchasing a subscription, you are helping ensure a future of impactful stories about the discoveries and ideas shaping our world today.
To see why, let’s look at binary stellar-mass black holes, which start out as a pair of massive stars orbiting each other. The stars eventually go supernova, and Their respective cores collapse to form a black hole Whose mass is 100 times more than the mass of the Sun. Such systems are relatively rare to begin with. But rarer still are those with paired black holes that get close enough to each other to eventually collide. If such black holes formed a billion kilometers apart from each other, the merger could take longer than the existence of the universe, approximately 14 billion years old.
If black hole to do Get close enough, perhaps clumped together by the gravity of a passing star, that a very strange effect occurs: They come very close to each other due to gravitational waves They emit. This is what Einstein’s general theory of relativity tells us Any The acceleration of mass creates ripples in the fabric of space-time that travel far at the speed of light, shaking the structure of the universe as they pass. For example, when you stand up you create gravitational waves, but they are extremely low-energy and so faint that they cannot be detected.
but there are black holes extremely Huge, and two of them orbiting each other can move at a large fraction of the speed of light, so they emit abundant, powerful gravitational waves. These waves are formed Outside event horizon, so they are free to spread out into the larger universe. The energy for this comes from the orbital motion of the black hole. As the black holes emit these waves, the energy expended causes their orbits to contract, bringing them closer to each other. This also increases their acceleration, causing more gravitational-wave emissions in a positive-feedback loop (and causing the orbits to become increasingly tighter). In the final few seconds, the black holes rotate around each other at close to the speed of light, emitting more powerful gravitational waves until the two actually merge, combining in a voracious gulp that leaves behind a single, more massive black hole. to date, Astronomers have managed to detect about 300 such mergers Through their respective crescents of gravitational waves.
The key to calculating the amount of energy released is to understand that the mass of the black hole resulting from a merger is not simply the sum of its progenitors. According to relativistic equations, about 5 percent of the combined mass of the merging pair is converted into gravitational waves at that final moment. That transformation is governed by Einstein’s most famous equation, E = mc2Where? m is the mass of the black hole destroyed due to energy and C is the speed of light.
How much energy are we talking about here?
A Very. Suppose, studying the mathematics for the collision of two five-solar-mass black holes, the amount of energy dissipated in less than a second by such a merger would be approximately the same as that emitted into the Sun. seven trillion years. That is, for a brief moment they emit more energy than the black hole light. Arab Galaxies full of stars.
And they were relatively small black holes. There are many others, excess Big.
Supermassive black holes range in size from 100,000 to billions—yes, billions, with “b“—many times the mass of the Sun. They are present at the centers of all large galaxies, including the Milky Way (although our galaxy, called Sagittarius A*, is slightly lighter, at only four million solar masses). How they got so big is still a matter of vigorous debate; they may have been born large and grown even larger, or they may have started out small and grown larger in size as galaxies formed around them.
Supermassive black holes usually live solitary lives, but that may change when galaxies collide and merge. The two supermassive black holes at the center of each galaxy orbit together and, like their stellar-mass cousins, may eventually spiral and combine (though the details are a bit complicated).
Given the enormous mass of these black holes, the ultimate universe-threatening burst of gravitational-wave radiation they emit is far larger. Repeating the above math with a pair of black holes that are, say, each 100 million solar masses, the numbers just go up. The energy they emit in that last second is thousands of times more than the combined energy emitted by all the stars in the visible universe In the same amount of time.
I remember the first time I sat down and calculated it myself. The answer I found for the torrential gravitational waves produced by the collision of stellar-mass black holes was a number so huge I thought I had made a mistake. I checked my numbers, and no, I got it right. Then I realized what it must mean giant Black holes with millions of times more mass are converted into energy. The hairs on the back of my neck stood straight up, and I felt the room spin around me for a moment. The crushing weight of those numbers shattered my soul.
And yet these “largest” explosions in the universe are invisible. Why?
Because gravitational waves themselves are invisible and can occur without any light being emitted. The waves also weaken with distance, and the Great Merger is billions of light years away; Until the waves reach the earth, they cannot be detected. There Is There is some evidence that we have seen a merger, although this has not yet been confirmed. In about a decade, we may have much more data than European Space Agency’s Laser Interferometer Space Antenna (LISA) mission This will prove that these mind-boggling, epic events really do happen.
Even as you read this, gravitational waves from unimaginably energetic black hole mergers are passing through you, diminished to less than a whisper from their unfathomable distance. Considering his power, this is a good thing. Just thinking about them makes me shudder.
