No, the sound of black holes cannot be heard

No

Black holes and sounds, what's real? Science fiction cinema has accustomed us, with a few rare exceptions, to thunderous explosions in space: the Death Star that explodes in Star Wars: episode IV makes noise. The shots of Klingon spaceships make noise in Star Trek The Motion Picture. The asteroid exploding in Armageddon makes noise. Yet, to always remain in the sci-fi environment, "in space, no one can hear you scream". It was the slogan for the promotion of Alien; and that's it, since no sound can propagate in space.

Content This content can also be viewed on the site it originates from. in sounds emitted by the most disparate celestial bodies. There are the ethereal sounds emitted by the rings of Saturn, the disturbing ones of the Jovian magnetosphere. The chirping of two colliding black holes. The latest one that is shooting these days is the alleged sound of a black hole in the Perseus galactic cluster. But let's take it slow.

Sound Sound is a vibration that propagates through what is called a transmission medium. Whether it is air, or water (or a solid as in the case of earthquakes), the vibration propagates by making the particles of the medium vibrate in sequence. That vibration is what we call an acoustic wave and is therefore a vibration of the medium: if there is no medium, or it is too rarefied, there is no sound, because there is nothing to vibrate.

Representation of sound waves propagating in a medium. Credits: Dan Russell

This is the biggest difference compared to light waves, because light instead gives its best in a vacuum, where it does not interact with matter and is therefore free to propagate at its maximum speed. But they are still waves, and therefore also acoustic ones, just like light waves, are characterized by a frequency: the higher it is, the sharper the sound, the lower it is and the more serious the sound. These are concepts that we experience all the time: when we speak or sing, our vocal cords vibrate at a lower or higher frequency and trigger the propagation of a sound wave in the air. It is therefore evident that in space, mostly empty, sound cannot be propagated.

Sonifications Those "the music of the rings of Saturn" or "listen to two colliding black holes" or the thousand varieties that are found on the net, they are therefore misleading titles, because they suggest that they are real sounds that we could actually hear if we were in space. Often what we forget to point out is that that sound does not exist and does not propagate, it is a way of representing data known as sonification. It means that the frequencies of other waves - be they light or gravitational - are taken and converted into acoustic frequencies. It is a very popular practice both to present some data that are difficult to visualize, and also from a strictly artistic point of view, to create music. It must not be seen as something incorrect: after all, even when we put data on a graph we are only choosing a way of representing them that relies on other senses than hearing, but it is still an arbitrary choice.

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Often, as far as planets are concerned, sonifications refer to the radio emissions of their magnetic fields. The charged particles trapped in the magnetic fields of planets such as Jupiter or Saturn emit radiation at radio frequencies, the conversion of which is quite good because the human ear can perceive acoustic frequencies between 20 and 20 thousand Hertz, an interval that includes, in luminous frequencies , just the radio waves. But many times the intervals are modified precisely to make listening possible: this is the case, for example, of the InSight probe, whose frequency has been amplified by 60 times.

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Something similar has also been done for the two colliding black holes, memory of that first observation of gravitational waves in history: in the video of the sonification of these signals you can perceive the difference between the sound without amplification - low and dark - and then the amplified sound - a chirping.

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There are many more cases beautiful and elaborate, like the one with which NASA has sonified some images of the Hubble Space Telescope combined with the data of the Chandra X-Ray Observatory: a vertical line sweeps the pillars of creation like a music box, interpreting the luminous frequencies of of the stars and those of the gas that it encounters on two different musical tracks. One sonifies Hubble's visible light, the other Chandra's X-rays.

Content is bouncing around the net these days but is a bit different. Published by NASA last May but based on Chandra's 2003 data, this sonification concerns the black hole located in the Perseus galactic cluster. This is a peculiar operation compared to the others because it actually starts from some real acoustic waves.

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The cluster of Perseus galaxies is one of the most massive objects we know of in the entire universe: there are thousands of galaxies immersed in an immense cloud of gas. The material around the black hole at the center of this cluster is quite dense, enough to allow for the propagation of acoustic waves. But of course: even if we were inside the gas cloud, we would be far from being able to hear this sound, because its frequency is extremely low. To make it audible, NASA has therefore increased it by a million billion times, to put it in musical terms, by 57 octaves below middle C. This sonification is published along with many others on the A Universe of Sound website.