At the beginning of last May, NASA published some audio that contained what was supposed to be the sound of a black hole . Publications like this, which are tremendously interesting for the scientific community and allow us to discover another aspect of the studied stars, can be confusing for the general public . Does a black hole really sound like this? Isn’t it supposed that sound doesn’t propagate in space ? These are some of the questions that you have probably asked yourself when listening to the audio published by NASA (included in the video at the beginning of this article and in the references section at the end of it).
The black hole from which these sounds were collected was the one at the center of the Perseus cluster of galaxies , a cluster about 240 million light-years away containing more than 1,000 galaxies surrounded by a gigantic cloud of gas . This is one of the most massive structures known in the universe. Surrounding the central region of this cluster is the galaxy NGC 1275 , which itself contains a supermassive black hole at its core . This black hole causes disturbances in the gas of the galaxy that contains it and in the gas cloud that surrounds the entire cluster. These disturbances take the form of pressure waves that travel over millions of years through the hot gas.
These pressure waves were first detected in 2003 by a team led by Andrew Fabian of the University of Cambridge using the Chandra Space Telescope . This telescope is specially designed to observe the universe in X-rays and among its many discoveries and observations is this one that we mentioned. These pressure waves are analogous to those that produce sound in the air of the atmosphere. However, the similarities end there, because this pressure wave would have an oscillation period of more than 9 million years . That is, the note it would produce would be about 57 octaves below middle C on a piano . An octave is nothing more than the distance that separates a particular note from the next or previous iteration of itself . It would therefore be the separation between a do and the following do (after going through re, mi, fa, sol, la and si in between). The distance between the lowest and highest note on a conventional piano is just over 7 octaves , and the range of hearing for humans is about 10 octaves.
So hearing a sound 57 octaves lower than middle C on a piano would be impossible without some kind of artificial modification . This is exactly what NASA has done and published recently. Using observations of these pressure waves in NGC 1275 and the surrounding gas, they have generated a sound wave and raised it the required 57 octaves to be comfortably audible to the human ear.
Waves of this type cannot be observed for any object, since normally the stars are not surrounded by large clouds of gas and dust, but by the vacuum of space. This void, which is never really empty but contains a few particles per square meter, is incapable of transmitting sound . The sound needs a material medium, because it needs the atoms or particles to collide with each other, transmitting the wave. This is not possible in the interstellar medium that surrounds the solar system or in the medium that surrounds the planet Earth, but it has been possible for this black hole in the center of NGC 1275 and for other objects, such as the supermassive black hole located in the center of the galaxy M87 . This was the black hole of which the first direct photograph was obtained, published by the Event Horizon Telescope in 2019.
Therefore we can conclude that the sound published by NASA is not a real sound , in the literal sense of the term, since it would be a thousand billion times more serious than what a human could hear. If we could somehow travel to the vicinity of NGC 1275 that would not be what we would hear . In fact, we would probably hear absolute silence, because although the gas clouds that surround the galaxy have a certain density and can transmit some sound, these will undoubtedly be too serious. Of course, NASA itself has not assured at any time that these are sound recordings , but have announced it as the sonification of observations taken from the X-ray part of the electromagnetic spectrum.
These sounds are therefore similar to those that were recorded in the 1970s by the Voyager 1 and 2 probes and more recently by other probes such as Cassini or Juno, which collected radio waves from some planets in the solar system, and then transform them into sound waves , similar to the method we use to transform radio waves into the songs and talk shows we hear on the radio.
References:
Data Sonification: Black Hole at the Center of the Perseus Galaxy Cluster (X-ray), 2022, NASA, https://www.youtube.com/watch?v=ioR5np1fmEc
