Ah, the sounds of late summer. Pass a pool, and hear the happy yelps of kids splashing around. Sit outside at night, and bask in the soothing buzz of cicadas hidden in the trees. Open the internet, and hear the terrifying howling of outer space.
Thank NASA for that last one. The space agency recently shared a clip online of sound coming from a cluster of galaxies about 250 million light-years from Earth. NASA, always eager to show off its capacity to produce cosmic wonder, presented the audio enthusiastically, as if to say, Wow, check out this cool thing! And although the transformation of space phenomena into something detectable by our human ears certainly seems like an exciting exercise, the reality is—well, have a listen.
The noise sounds like a ghostly wail, or the horror-movie music just before a jump scare, or, as several people have pointed out, the cries of countless souls trapped in eternal darkness. Just nothing good; less awe-ful, and more awful. Does space really sound this scary?
The answer is, sort of. And there is a perfectly horror-free explanation for it. Some parts of space are full of hot gas, including the medium between the distant, sparkly galaxies huddled together. In 2002, when a NASA space telescope named Chandra studied the Perseus cluster, it detected wavelike movements in the gas, propagating outward like ripples in water. The ripples, scientists determined, were produced by the supermassive black hole in the cluster’s central galaxy. When the black hole sucks in cosmic material, it burps some out—explosive behavior that pushes around the gas nearby. The resulting waves, astronomers concluded, were sound waves, with a frequency much too deep for any of us to hear.
It wasn’t until recently that Kimberly Arcand, Chandra’s visualization scientist, decided to shift those impossibly low cosmic notes into the audible range. She wanted the public, and particularly those who are blind or have reduced vision, to be able to experience the wonder of the Perseus cluster with senses besides sight. Arcand told me she was inspired by Wanda Díaz-Merced, a blind astrophysicist who developed a program to convert sunlight into sound so that she could hear a solar eclipse sweeping across the United States in 2017. Arcand and her team extracted the sound data from Chandra’s observations and then, with some mathematical work and sound editing, brought them into the range of human hearing, a couple hundred quadrillion times higher than the original frequency. The result: a spooky, cosmic wail.
Arcand and her team at Chandra have previously made a variety of celestial images into music through a process known as sonification, but those projects were based on light, not sound. Consider the glittering, star-filled center of our Milky Way galaxy. To hear it, scientists assigned different sonic features to the cosmic material in a snapshot of the galaxy. Stellar stuff at the top of the image corresponds to higher pitches; the brightest bits play at top volume. Short notes represent stars, and a drawn-out hum indicates clouds of gas and dust. The image features observations in multiple wavelengths, which scientists used to make a more beautiful song: xylophone for X-rays, violin for optical light, piano for infrared.
The melody of our galactic center sounds lovely, peaceful. Most of the sonifications in Chandra’s library do too, even the clips that lack instrumental elements and feature only a jumble of notes. They are nothing like the primal scream of the Perseus cluster, which the Chandra team released in May this year. “The Perseus one is perhaps the most evocative because it is actually based on sound waves,” Arcand said. It’s more objective, which makes the noise feel a bit more real. At the same time, the cosmic wail wouldn’t sound exactly like this if you could hang out in the Perseus galaxy cluster with a helmet and superpowered hearing. The spooky audio is a combination of the sound waves emanating from the central galaxy in different directions, not a single scream in time. Still, “this is as close as we know how to get,” she said.
When I asked Arcand what she thought about the sound freaking people out, she cracked up. “I just feel bad,” she said. Arcand grew up singing in choirs, and for her, the Perseus audio is musical, like a dramatic tune from an emotional, sweeping Hans Zimmer track. She has worked on the Chandra mission for more than two decades, and being so intimately familiar with the data, she was unlikely to be spooked by it, even when it sounds like, you know, that. “I didn’t hear anything scary in it,” she said, but “I totally understand that other people have a different perspective.” Scientists and sound engineers could certainly edit the clip to make it less creepy, mixing in some chimes or nice harp chords. But this is space putting on a little performance, and we may as well experience it as the artist intended.