We Heard It Anyway

On sonification, silence, and how we hear what we can't perceive

As a kid, I used to think the stars weren’t made only of light and heat, but of voice, too. In my imagination, their voices called out from distances my mind couldn’t begin to hold. Every summer night, I’d lie stretched out on the grass, trying to focus hard enough to catch it. I pictured a vast, terrifying choir somewhere above me, waiting for something so immense it might shake the bones of the world. The kind of sound that makes you want to cover your ears, even while part of you needs to hear it. I liked that feeling, the way wonder and fear blurred into each other.

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Hearing Sound

Sound, when you get down to it, is just vibration moving through something. Air, water, whatever happens to be there to carry it along. When something starts vibrating inside that medium, it sets the particles around it in motion, and they set the next ones moving too, until the disturbance spreads outward as a wave. That’s all your voice is doing when you speak. Your vocal cords tremble a little, the air takes it from there, and the vibration keeps going until it fades out or reaches someone else’s eardrum. And suddenly, whatever you said exists for them too.

Experience the Sound

We’re told there’s no sound in space because there’s nothing there to carry it. No air. No water. No medium. And most of the time, that’s true. But space isn’t always empty. Not really.
Think about that. Some people can’t hear certain sounds. Some can’t hear any at all. They can only imagine what thunder might be like, or music, or the way a voice sounds when someone says their name. And still, in their own way, they feel it through vibration, through pressure, through the way the world moves when something passes through it. They don’t hear the sound itself so much as experience what it does.
So maybe what we’re really talking about isn’t sound itself, but the experience of it. A few years back, NASA released an audio file that lets you “hear” what’s happening around the supermassive black hole at the center of the Perseus galaxy cluster, roughly 240 million light-years from Earth:

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Out there, the black hole sits inside an enormous cloud of superheated gas, and that gas can move, ripple, and carry pressure waves much the same way sound moves through air down here. Back in 2003, observations from the Chandra X-ray Observatory showed those ripples spreading through the cluster as disturbances moving through the plasma.

Sonification

The trouble is, they’re far too slow for us to hear. Their pitch falls well below anything the human ear can pick up. So NASA didn’t record a sound drifting through the vacuum of space so much as translate the data. They took those pressure waves and shifted them into a frequency range we can actually listen to. A sonification. Data turned into tone. Signals converted into something we can hear, not because they were ever meant to be heard, but because hearing them helps us understand what we’re looking at.
Something like that happened again in 2016, when the first detection of gravitational waves was announced, produced by two black holes spiraling into each other and merging far beyond anything we could ever see directly:

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A New Angle

When we listen to a black hole, we’re not hearing it the way we’d hear thunder or a passing car in the night. What we’re listening to is a model, a mapping, something that lets our senses catch up, in their own limited way, with what they were never built to perceive.
It’s just a matter of shifting perspective a little, changing the angle, lying back on the grass again, and letting yourself be surprised by what we’re capable of, moving past the limits of our senses and catching a glimpse of something that still feels almost impossible.

Sometimes that’s all it takes.

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Thanks for reading, guys.

Michael

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