Space Facts

#38

Just a little update and correction to myself.

sound doesn't move through a vacuum, and everyone knows that space is a vacuum. The thing is, that's not completely true.

Space isn't uniform nothingness. It's full of stuff. In between the stars, there are clouds of gas and dust. These clouds are sometimes the remains of old stars that went out in a blaze of explosive glory, and they're the regions where new stars form. And some of that interstellar gas is dense enough to carry sound waves, just not sound perceptible to humans.

Here's How It Works

When an object moves — whether it's a vibrating guitar string or an exploding firecracker — it pushes on the air molecules closest to it. Those displaced molecules bump into their neighbors, and then those displaced molecules bump into their neighbors. The motion travels through the air as a wave. When the wave reaches your ear, you perceive it as sound.

As a sound wave passes through the air, the air pressure in any given spot will oscillate up and down; picture the way water gets deeper and shallower as waves pass by. The time between those oscillations is called the sound's frequency, and it's measured in units called Hertz; one Hertz is one oscillation per second. The distance between "peaks" of high pressure is called the sound's wavelength.

Sound waves can only travel through a medium if the length of the wave is longer than the average distance between the particles. Physicists call this the "mean free path" — the average distance a molecule can travel after colliding with one molecule and before colliding with the next. So a denser medium can carry sounds with shorter wavelengths, and vice versa.

Sounds with longer wavelengths, of course, have lower frequencies, which we perceive as lower pitches. In any gas with a mean free path larger than 17 m (the wavelength of sounds with a frequency of 20 Hz), the waves that propagate will be too low-frequency for us to hear them. These sound waves are called infrasound. If you were an alien with ears that could pick up this very low notes, you'd hear really interesting things in some parts of space

Black Hole Noises :)

Ripples in interstellar gas, produced by sound waves from a supermassive black hole.
About 250 million light years away, at the center of a cluster of thousands of galaxies, a supermassive black hole is humming to itself in the deepest note the universe has ever heard (as far as we know). The note is a B-flat, about 57 octaves below middle C, which is about a million billion times deeper than the lowest frequency sound we can hear (yes, that's an actual number from actual scientists).

The deepest sound you've ever heard has a cycle of about one oscillation every twentieth of a second. The drone of Perseus' black hole has a cycle of about one oscillation every 10 million years. That's sound on a massive scale, played across deep time.

We know this because in 2003, NASA's Chandra X-ray space telescope spotted a pattern in the gas that fills the Perseus Cluster: concentric rings of light and dark, like ripples in a pond. Astrophysicists say those ripples are the traces of incredibly low frequency sound waves; the brighter rings are the peaks of waves, where there's the greatest pressure on the gas. The darker rings are the troughs of the sound waves, where the pressure is lower.

Hot, magnetized gas rotates around the black hole, more or less like water swirling around a drain. All that magnetized material in motion generates a powerful electromagnetic field. The field is strong enough to accelerate material away from the brink of the black hole at nearly the speed of light, in huge bursts called relativistic jets. These relativistic jets force gas in their path out of the way, and that disturbance produces deep cosmic sound waves.

That deep intergalactic sound carried through the Perseus Cluster for hundreds of thousands of light years from its source, but sound can only travel as far as there's enough gas to carry them, so Perseus' infrasound drone stops at the edge of the gas cloud that fills its cluster of galaxies. That means we can't detect the sound here on Earth; we can only see its effects on the gas cloud. It's like we're staring across space into a soundproofed chamber.