Black holes are absolutely silent as they are pure gravity creatures. But while black holes don’t produce their own sound, they can generate sound waves in their environment.
An ordinary black hole
Black holes are surprisingly simple objects. IN general relativity, which is the framework we use to understand all things gravity, you only need three numbers to fully and completely describe a black hole: its mass, its spin, and its charge. Seriously, it is. If you come across a random black hole in the universe and measure its mass, how fast it’s spinning, and how much electrical charge it has, you’re good to go. That’s all you’ll ever know about a black hole, and all you’ll ever need to understand how a black hole will behave.
Now, our journey to fully understand gravity is not over yet. We know that general relativity is incomplete because it predicts that the centers of black holes contain singularities, which are points of infinite density. Points of infinite density do not actually exist in the universe, but we have no description of strong gravity on small scales. Physicists hope to unravel this someday”quantum gravity” problem, but we don’t have a solution yet.
Read more: Black holes are accelerating the expansion of the universe, cosmologists say
No light or sound
So, while it’s possible that black holes are 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 “don’t have hair ” – i.e. , they are basically bald and very boring.
Black holes are literally made by gravity. They are breaks, defects in the fabric of space-time itself. Unless they carry an electrical 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 affect their surroundings is through their massive 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 pass through. This also means that, again in perfect isolation, the merging process of two black holes releases neither light nor sound.
But luckily for us, black holes don’t live in isolation. They are constantly surrounded by streams of matter (and usually that matter is about to be swallowed up by the black hole). And this matter, however thin it is, can absolutely support sound waves.
Pressure waves
Sound waves are simply pressure waves moving from one place to another in a medium. Anywhere 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 galaxy clusters.
And they have also observed pressure waves in the disks of gas surrounding supermassive black holes. In 2003, astronomers made extensive maps of the region around a black hole located 250 million light years away. The gas was so hot that it glowed in X-rays, and astronomers noticed ripples and ripples in the disk. These were pressure waves, that is, sound waves. The frequency of these waves is about 10 million years, but in 2022 another NASA team raised these waves by 57 octaves (that’s a factor of 144 quadrillion) to bring them within the range of human hearing. The results were…creepy.
Although we couldn’t normally hear these sounds, they were legitimate sound waves, just on a truly astronomical time scale. And while the black hole itself didn’t make the sounds, it did reason them. As the gas made its way from the disk to the black hole, it occasionally collapsed in on itself, which sent ripples of pressure waves reverberating outward. If the black hole wasn’t there in the center to drive the whole thing like a drum, the pressure waves would stop and the giant disc would go silent.