Have we really been able to listen to the sound of the black hole that lies in the middle of our Milky Way?

“Nasa reveals the sound made by a black hole”, “this black hole emits a painful sound”, “listen to the sound produced by this unique phenomenon”. Following the historic image of the black hole located in the center of our galaxy, revealed on Thursday, several media outlets repeated the murmur it was supposed to emit, from a video produced by NASA and visible on the website of the Chandra X-ray space telescope. This type of press headline also followed the May 4 publication of a “sonication” of the black hole located in the center of the Perseus cluster of galaxies. On YouTube, it has been viewed nearly 2 million times.

But is it really the sound we could hear if we could get closer to these objects located at impassable distances (27,000 light-years for the central black hole of the Milky Way) that these media suggest? We asked the question to Philippe Zarka, a radio astronomer at the Paris Observatory, who has done a specialty in “translating the stars’ light into sound”. They are available on his website Les chants du cosmos.

Gas around black holes could carry sound

The discussion begins with a brief reminder of physical laws: “A sound comes from an acoustic wave, which is propagated by collisions of matter”. For example, when a conversation partner is talking to another person, the air molecules that separate them collide and will transport the vibrations so that they can be heard over a certain distance. But if you slide a thickness of vacuum between the two, the sound no longer propagates, you no longer hear anything. It is for this reason that “from Earth we can not receive sound coming from outside the atmosphere because space is so empty that there is no collision of particles”, explains Philippe Zarqa.

If you put your head in the photosphere of the Sun, there would be things to hear, it moves, it grooves, it pulsates! But in the galactic center you have to be more patient, it’s longer, more boring

Now the gas orbiting a black hole can be in sufficient quantity to carry the acoustic waves, the astronomer assures. “If we put our head in this gas where the vibrations occur, and of course imagine that we are not being destroyed by the gravity of the black hole, we could perceive this sound directly and locally, provided it is at a frequency where our ears are sensitive (from 20 Hz to 20 kHz), “says Philippe Zarka. But disappointment could be at the meeting, where the phenomena in the galactic center vary very slowly and at extremely low frequencies.” If you put your head in the Sun’s photosphere, there would be things to hear, it moves, it grooves, it pulsates! But in the galactic center you have to be more patient, it’s longer, more boring, ”smiles the scientist.

A “completely random” sound image

Let’s go back to the document produced by NASA about the “sonication” of the black hole in the center of our galaxy and how it was performed. In general, the sound can be represented by a diagram, with on the vertical axis the frequencies and on the horizontal axis the time that passes. A score is one of the simplest forms. ‘What has been done for the black hole is to take a rotating beam that starts at noon and goes all the way around. They decided that the position of this beam corresponds to the passing time and the direction along its axis, to the frequencies. This makes it possible to represent the variation in the brightness of the corona around the black hole as a sound that has a dimension of time and frequency ”, suggests Philippe Zarka.

The sound obtained is certainly neat and friendly, but it has no physical relation to what is happening in the phenomenon which is this black hole.

But according to him, “the sound image produced is totally arbitrary: Let’s imagine that the rotating beam starts at three o’clock and no longer at noon, we move the time. Or if we prefer a vertical line that sweeps the image from left to right, we get a sound with a completely different structure. ”In short, according to our scientist, the sound obtained is“ admittedly beautiful and pleasant, but it is absolutely artificial and without any physical relation to what happens in it. phenomenon, which is this black hole “. On YouTube, NASA only presents the video as” a sonication – translation to sound – “of the image of the supermassive black hole located in the middle of the Milky Way. However, this leaves a lot of room for interpretation.

Risk of “spreading false ideas”

Philippe Zarka has also done a specialty in adding sound to celestial phenomena. But he makes it an honor to only work on images that already present this time-frequency structure. He works on “dynamic spectra”, which show radio waves coming from the Sun or Jupiter, for example. “I only transpose this time and this frequency to the human-audible domain to get a sound image that is true to what is observed in radio waves, in visible light, in X-rays, etc.,” he assures.

Philippe Zarka makes it possible to “listen” to radio emissions from the Sun produced by relativistic electrons (moving at a third of the speed of light) through the corona and disrupts the local “plasma” (Screenshot Philippe Zarka)

But for what exactly? “I do radio astronomy, and when I talked about my work at conferences, I showed dynamic spectra. It’s so pointless for people that I used to jokingly say that those who had not gone after five minutes had fallen asleep! , Philippe Zarka rewinds. “I wondered how to faithfully respect the physical structure of the signal, without betraying its meaning, while offering a medium that is attractive enough for people to come and say to me, ‘what is it?’ I want to understand! “”. The result of his experiments sounds “strange”, “attractive”, but which is not “true”, the researcher immediately warns on his website. “I suggest them for the sake of popularization, whereas what appears on Chandra’s website with the black hole is rather a communication medium that risks spreading a lot of misunderstandings,” he concludes.

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