This week NASA shared some audio that allows us to "hear a black hole".
The terrifyingly low drone soon went viral on social media, as people felt the weight of the sound and pondered their own insignificance amid the vastness of space.
Without getting too existential, let's dive into how NASA captured these sounds, the recordings of which were first released earlier this year, and what else we can "hear" in space.
What does a black hole sound like?
The Perseus galaxy cluster was made famous almost 20 years ago, after sound waves were detected around its supermassive black hole.
NASA shared new audio from the black hole back in May, but posted it on social media again this week.
Here's how it sounds:
Wait, isn't space a vacuum? How can you 'hear' it?
The idea that there is no sound in space is a popular misconception.
Most of space is a vacuum that sound waves can't move through, but galaxy clusters can release a lot of gas, which envelops the many galaxies inside of them, creating a medium for sound waves to travel through.
In 2003, astronomers found that pressure waves sent out by the black hole at the centre of the Perseus galaxy cluster caused ripples in the cluster's hot gas, which could be translated into a note.
The thing is, the note was too low to be heard by humans, because it is 57 octaves (or about seven piano lengths) below middle C, according to NASA.
To create audio that is audible to humans, scientists do something known as sonification, which is the translation of this astronomical data into sound.
How the sound of a black hole was created
NASA says the sonification of the Perseus galaxy cluster is unlike any other done before because it uses the actual sound waves discovered by its Chandra X-ray Observatory space telescope.
Here's the gist of how the sonification was put together, according to NASA:
- The sound waves were extracted outwards, from the centre of the cluster
- The signals were resynthesised into the range of human hearing by scaling them up by 57 and 58 octaves above their actual pitch
- The radar-like scan around the image allows us to hear waves emitted in different directions
The scaling up of the sound signals means that what we are hearing is actually 144 quadrillion and 288 quadrillion times higher than the signals' original frequencies.
For scale, a quadrillion (a million billion) is 1,000,000,000,000,000 — that's 15 zeros.
James Miller-Jones, Professor of Astrophysics at Curtin University, says the very low frequency of the sound is caused by jets of energy from the black hole switching on and off on timescales of tens of millions of years.
While the drones which NASA made audible to humans still sound very low and ominous, the organisation says that wasn't intentional.
"The sound you hear is amplified a lot, and other sounds are interpreted from light data," NASA says.
Professor Miller-Jones says the frequencies of the sound waves are impacted by gases in the galaxy cluster.
"Those sound waves are bumping into regions of dense gas, hotter gas, cooler gas, so they'll move in slightly different speeds in different directions," he says.
"That means they don't have a perfect circular shape. So as they scan around the cluster … it's capturing slightly different pitches."
What else can we 'hear' in space?
NASA has shared a lot of sonifications in the past, including some created using radio waves captured by spacecraft, which it converted into some pretty eerie sound waves.
This compilation of "spooky sounds" includes sonifications of things such as the Sun, Saturn, Jupiter and rhythmic waves of plasma made up of energetic charged particles.
In its sonifications of the black hole in the M87 galaxy, which was first imaged in 2019, NASA created audio using X-ray, optical light and radio wave data from ground and space telescopes.
Here is a sonification of the black hole and its huge jets of energetic particles — it's around 55 million light years away from Earth and has a mass 6.5 billion times that of the Sun.
Earlier this year NASA also released images from the supermassive black hole at the centre of the Milky Way, known as Sagittarius A*.
Its sonification creates an audio representation of the centre of our home galaxy, featuring some plucked and bell-like instruments.
There's a crescendo when the data reaches the bright spot in the lower right of the image in the video below, which is where the black hole Sagittarius A* sits among huge clouds of gas and dust.
What can we learn from sound waves in space?
While sonifications use different types of data collected from telescopes, NASA's viral Perseus sonification is unique because it also uses sound waves.
"This is the only one that I've seen that is really translating real sound waves into the sonification, and to me that's just a beautiful demonstration of what is going on. It's quite powerful," says Professor Miller-Jones.
"It tells us a lot about the cluster, and how energy is transported through it."
Sound waves are very important in astronomy, and can carry a lot of information, including about the internal structure of stars and how they vibrate, he says.
"Sound waves do travel anywhere there's not a perfect vacuum," he said.
"So we can use them to get a variety of information out about a variety of things from cosmological scale through to galaxy clusters and stars, and I think that's fascinating."
