Black holes are absolutely silent, as they are creatures of pure gravity. But while black holes produce no sound of their own, they can generate sound waves in their environment.
A Simple Black Hole
Black holes are surprisingly simple objects. In the general theory of relativity, which is the framework we use to understand all things gravity, you only need three numbers to completely and totally describe a black hole: its mass, its spin and its charge. Seriously, that’s it. If you encounter any random black hole in the universe and measure its mass, how fast its spinning, and how much electric charge it has, you’re done. That’s all you’ll ever learn about the black hole, and all you’ll ever need to understand how the black hole will behave.
Now, our journey towards a complete understanding of gravity is not yet done. We know that general relativity is incomplete, as it predicts that the centers of black holes contain singularities, which are points of infinite density. Points of infinite density don’t actually exist in the universe, but we don’t have a description of strong gravity at small scales. Physicists hope to someday untangle this “quantum gravity” problem, but we do not yet have a solution.
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No Light or Sound
So, while it’s possible that black holes may be more complex than general relativity suggests they are, some theorists have proposed the “no-hair theorem,” stating that even in advanced theories of gravity, black holes “have no hair” – that is, they’re essentially bald and very boring.
Black holes are literally made of gravity. They are punctures, defects, in the fabric of spacetime itself. Unless they carry an electric charge (which they usually don’t, because if they did they would quickly neutralize themselves by attracting matter of opposite charge), the only way they can influence their surroundings is through their enormous gravitational pull.
In perfect isolation, it would be impossible to hear a black hole. Hearing requires the presence of sound waves, and the presence of sound waves requires a substance to travel through. This also means that, again in perfect isolation, the process of two black holes merging releases no light and no sound.
But fortunately for us, black holes do not live in isolation. They are constantly surrounded by streams of matter (and usually that matter is on its way to being swallowed by the black hole). And that matter, as thin as it might be, can absolutely support sound waves.
Sound waves are just waves of pressure traveling from one place to another in a medium. Anywhere where there’s a medium, there’s a chance for pressure waves, which means there’s a chance for sound. Astronomers have observed pressure waves in all sorts of interesting situations, from the surface of the Sun to giant gas clouds that fill clusters of galaxies.
And they’ve also observed pressure waves in the disks of gas surrounding supermassive black holes. In 2003, astronomers took extensive maps of the region around a black hole sitting 250 million light-years away. The gas was so hot that it glowed in X-ray radiation, and the astronomers noticed ripples and waves in the disk. These were pressure – that is, sound – waves. The frequency of these waves was something like 10 million years, but in 2022, another team at NASA upped those waves by 57 octaves (that’s a factor of 144 quadrillion) to bring them into the range of human hearing. The results were…creepy.
Even though we couldn’t normally hear those sounds, they were legitimate sound waves, just on a truly astronomical timescale. And while the black hole itself didn’t make the sounds, it did cause them. As gas funneled from the disk onto the black hole, it occasionally rammed into itself, which send ripples of pressure waves reverberating outwards. If the black hole wasn’t there at the center to drive the whole thing like a drum, the pressure waves would cease, and the giant disk would fall silent.