We might not be able to hear sound in space, but that doesn’t mean there isn’t any. In 2003, astronomers detected something truly astonishing: acoustic waves propagating through the gas surrounding a supermassive black hole, 250 million light-years away.
We wouldn’t be able to hear them at their current pitch. Emanating from the supermassive black hole at the center of the Perseus cluster of galaxies, the waves include the lowest note in the Universe ever detected by humans – well below the limits of human hearing.
A new sonification (data turned into sound) however, has not only added to the notes detected from the black hole, but brought them up 57 and 58 octaves so we can get a sense of what they would sound like, ringing through intergalactic space.
It’s the first time these sound waves have been extracted and made audible.
The lowest note, the one identified back in 2003, is a B-flat, just over 57 octaves below middle C; at that pitch, its frequency is 10 million years. The lowest note detectable by humans has a frequency of one-twentieth of a second.
The sound waves were extracted radially, or outwards from the supermassive black hole at the center of the Perseus cluster, and played in an anticlockwise direction from the center, so that we can hear the sounds in all directions from the supermassive black hole at pitches 144 quadrillion and 288 quadrillion times higher than their original frequency.
The result is an eerie one, a sort of unearthly (obviously) howling, like many of the waves recorded from space and transposed into audio frequencies.
The sounds aren’t just a scientific curiosity, though. The tenuous gas and plasma that drifts between the galaxies in galaxy clusters – known as the intracluster medium – is denser and much, much hotter than the intergalactic medium outside galaxy clusters.
Sound waves propagating through the intracluster medium is one mechanism whereby the intracluster medium can be heated, as they transport energy through the plasma.
Because temperatures help regulate star formation, sound waves might therefore play a vital role in the evolution of galaxy clusters over long periods of time.
That heat is what allows us to detect the sound waves, too. Because the intracluster medium is so hot, it glows brightly in X-rays. The Chandra X-ray Observatory allowed not only for the detection of the sound waves initially, but for the sonification project.
Another famous supermassive black hole also got the sonification treatment. M87*, the first black hole ever to be directly imaged in a colossal effort by the Event Horizon Telescope collaboration, was also imaged by other instruments at the same time. Those include Chandra for X-rays, Hubble for visible light, and the Atacama Large Millimeter/submillimeter Array for radio wavelengths.
Those images showed a colossal jet of material being launched from the space immediately outside the supermassive black hole, at speeds that appear faster than that of light in a vacuum (it’s an illusion, but a cool one). And now, they too have been sonified.
To be clear, these data were not sound waves to start with, like the Perseus audio, but light in different frequencies. The radio data, at the lowest frequencies, have the lowest pitch in the sonification. Optical data hold the middle range, and X-rays are at the top.
Turning visual data like these into sound can be a cool new way to experience cosmic phenomena, and the method has scientific value, too.
Sometimes, transforming a dataset can reveal hidden details, allowing for more detailed discoveries about the mysterious and vast Universe around us.