Sound waves from a black hole become audible for the first time

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In 2003, astronomers discovered that a supermassive black hole in the center of the Perseus galaxy cluster, 250 million light-years away, emitted pressure waves that caused ripples in the gas of the galactic cluster. , which could be translated into notes. In musical terms, the pitch (frequency) of this sound is translated into a B-flat, but we can not hear it because the pitch is 57 octaves lower! A new sonication today allows us to hear the sound of this black hole for the first time.

In the room, no one can hear you screaming “, This famous phrase associated with the movie Alien deserves an explanation. In space, we actually hear no sound, but that does not mean that sound waves are not emitted: it simply means that there is not enough substance to transport these waves. . The space consists essentially of vacuum, in which gas and dust particles are dispersed; but these are so far apart that the sound waves they propagate are of extremely low frequency, inaudible to human hearing.

When a sound wave travels around us, it causes atmospheric pressure fluctuations; the time that elapses between each of these oscillations represents the frequency of the sound, and the distance separating the oscillation peaks indicates the wavelength. If the distance between the air particles is greater than this wavelength, the oscillations cease, the sound dies out. In space, sounds must thus have a very long wavelength to pass from one particle to another: this results in a sound that is far too deep for us – the sound that is lowest than a hearing person can detect at a frequency at 20 Hz (i.e. 20 oscillations per second).

Sounds that can affect star formation

The sound emitted by the black hole of the Perseus cluster is about a million billion times deeper than the sounds we can hear: it generates the equivalent of an oscillation every 10 million years! On the occasion of “Black Hole Week”, organized by NASA from May 2 to 6, scientists have produced a new sonication of the phenomenon based on data from the Chandra X-ray observatory, which had made it possible to detect sound waves in 2003.

This sonication is unlike any other done before. The sound waves were extracted in radial directions, i.e. from the center and outward of the black hole, and spread counterclockwise from the center. The signals were then resynthesized to the area of ​​human hearing, 57 and 58 octaves above their actual pitch. Specifically, this means that we hear them thus 144 quadrillion and 288 quadrillion times higher than their original frequency! Hear the result:

In this image, they represent the blue and purple X-ray data captured by Chandra. Radar-like scanning around the image makes it possible to hear the waves emitting in different directions.

It should be noted that the sound waves that propagate inside the galactic clusters, between the galaxies, also constitute a mechanism that makes it possible to heat the plasma from this intragalactic medium, because they transport energy – the gas is also denser there and much warmer. than outside the cluster, in the intergalactic medium. Sound waves can thus play a major role in the evolution of galaxy clusters, as star formation depends on temperature conditions.

A plasma beam transformed into a melody

If the sound coming from the galactic cluster of Perseus is the lowest note in the universe that man has ever discovered, it is not the only one that has benefited from sonication. The supermassive black hole in the Messier 87 galaxy, first photographed in 2019 through the Event Horizon Telescope collaboration, has been the subject of numerous observations from other instruments. This black hole is characterized by a huge beam of plasma that comes out of the core and extends over at least 5000 light years. The data associated with this jet has also been converted to sounds:

Note that these are not pressure waves as in the case of the Perseus cluster, but light waves with different frequencies. This video shows from top to bottom X-ray emissions captured by Chandra, optical light captured by Hubble and radio waves recorded by the Atacama Large Millimeter Array in Chile.

The brightest point on the left in the image corresponds to the location of the black hole, while the structure at the top right represents the jet produced by the material falling into this black hole. Each wavelength is here connected to a different range of audible sounds: radio waves are connected with the lowest tones, optical data with intermediate tones and X-rays with the highest tones. It can be seen that the brightest areas correspond to the loudest sounds of the sonication.

Finally, another sonication project has been launched by a group led by MIT astrophysicist Erin Kara as part of an attempt to map the environment around black holes, just as bats use echolocation to spot obstacles and prey in the dark. Black holes in binary systems occasionally emit X-ray echoes as they devour material from the nearby star: using a “reverberation machine” (Neutron Star Interior Composition Explorer or NICER), Kara and her team converted these echoes into audible sounds, in order to determine the time delays (between the initial emissions and their echoes) and follow the evolution of the black holes each time they absorb substance. Their work helps to better understand the connections between the disc, the beam and the corona in a black hole.

Source: NASA

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