What does a black hole sound like?
Actually, since sound waves don't propagate in the near-vacuum of outer space, we can't hear black holes. But if we could, they might sound somewhat similar to the static from a badly tuned TV set.
That's the word from the authors of a study published Friday in the journal Science Advances. In addition to giving us a sense of what black holes might "sound" like, the study sheds new light on the behavior of accretion disks. Those are the disk-shaped collections of matter, such as gas and dust, that surround black holes.
Accretion disks are often used as tools to study black holes since, unlike black holes themselves, they give off light.
"Since black holes cannot be observed directly it is only because of the existence of these disks that we can infer what a black hole might 'sound' like," Dr. Simone Scaringi, lead author of the study and a Humboldt research fellow at the Max-Planck-Institute for Extraterrestrial Physics in Germany, said in an email. "It is important to realize, however, that because space is almost a vacuum, there is no real sound. What we did is observe brightness variations over time for accreting black holes (and other systems too). I then converted these light variations into sound variations."
In other words, Scaringi and his colleagues took observations of the shifting light patterns from accretion disks (taken by NASA's Kepler space telescope, ground-based telescopes, and the European Space Agency's XMM-Newton satellite) and converted them into sound waves.
For instance, if the light intensity from an accretion disk fluctuated 10 times a second, it was converted to a sound wave of 10 cycles per second, or 10 Hertz, Space.com reported.
Scaringi then had to "translate" the sounds into a range that humans can hear.
"[T]he variations we see in accreting systems are very low or very high in frequency, depending on the size of the system, and would fall outside of the human hearing range," Scaringi said. "Because of this I had to shift the 'sounds' of accreting systems into the human audible range for us to listen to."
The researchers said that they were surprised to find similar brightness variations not only in accretion disks surrounding black holes but also in disks surrounding other celestial objects, including white dwarf stars and young stellar objects.
The research explains parallels among different astrophysical objects, scientists said.
"The major discovery in this research was that the way in which the amount of light coming from the accretion disk varies can be explained in the same way for accretion disks around stellar mass black holes, supermassive black holes, white dwarfs and young stellar objects, objects on their way to becoming stars," Dr. Tom Maccarone, associate professor of physics at Texas Tech University and a co-author of the study, told The Huffington Post in an email.
The seemingly random variations of brightness in disks around those objects appeared to be all remarkably similar -- except for their tempo. The noise-like brightness variations in larger accretion disks were slower than the fast variations in smaller disks.
"This is a really exciting result," Dr. Christian Knigge, professor of physics and astronomy at the University of Southampton in England and a co-author of the study, said in a written statement. "It suggests that the process by which astronomical objects grow is fundamentally the same, regardless of the type, mass or size of the object."
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