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So I've read various articles about the sound emitted from the Perseus Cluster black hole, and I understand that the way NASA was able to detect the drone 57 octaves below middle C being emitted by the black hole was by studying the effects on the surrounding gas clouds, and by converting the x-rays emitted as a sound we can interpret; so in other words it was all indirect measurement.

However the Gizmodo article stated that "Sound waves can only travel through a medium if the length of the wave is longer than the average distance between the particles." The thing I don't really understand here, and isn't really addressed in any of the info I've found online about this is:

If the length of the wave needs to be longer than the distance between each particle in the medium, why can't incredibly low frequency infrasound travel through deep space or regions where density of particles is say 1 atom per cubic metre or more? According to the Wikipedia page about the Perseus Cluster, is says "No human will actually hear the note, because its time period between oscillations is 9.6 million years" So this would mean the hertz of this wave must be incredibly small, and thus the wave must be very long allowing it to transfer between even very spread out particles in space?

So essentially I'm asking why can't infrasound travel though the imperfect vacuum of space where particles are very spread out, but are still present meaning the space isn't completely empty? Why is it that the sound waves will stop traveling at the edge of the cluster, and not continue through these very spread out particles between galaxies/clusters/superclusters?

Is it that the wave has lost it's energy long before it reached earth? At what point is it decided that infrasound can no longer travel through gas? Because if it can travel through the gas cloud in the cluster, where does it suddenly stop and decide that it can propagate no longer.

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    $\begingroup$ Related: physics.stackexchange.com/q/48574 $\endgroup$ – dmckee Jan 7 '18 at 18:17
  • $\begingroup$ Since sound is a collective phenomenon and the wavelength is large, you'd need a highly sensitive and astronomically large "microphone" to detect this sound (possibly comparable to the size of the galaxy). Plus you'd have a clock synchronization issue in such a large microphone.. $\endgroup$ – safesphere Jan 8 '18 at 2:17
  • $\begingroup$ Ok, so basically it comes down to technical limitations of not being able to detect the infrasound from the black hole, and without being able to detect it we can't necessarily prove that it can travel through relatively empty space, but still that doesn't mean it's not theoretically possible? $\endgroup$ – Some Random Jan 8 '18 at 4:51
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Sound waves are exactly mechanical movements of the particles of the medium transfering sound. For example in metals where you have crystal lattice, sound is just movement of tiny displacement of metal atoms. If one of such atom changes his position, it force, by interatomic interactions to move its neighbours and so on. The same phenomenon occurs in various fluids, in this case incompressible ones.

In gas and in compressible fluids, it looks different. In that case displacement of atoms is actually local increase or decrease of density, which means increase or decrease of pressure. This is exactly what we hear: our ears are sensitive to changes of the pressure.

Sound of various characters are , mechanical changes of various media. So sound waves are collective phenomenon, there's no sound if you have only one or two particles. So if length of the sound wave is very short, oscillations of pressure are very quick. But in order to transfer such oscillations from one place to another, other particles has to be able to move! If oscillations are too short, it cannot change other particles position etc or even if doing so, it is not coherent change, meaning such kind of discrepancy cannot travel in medium. It just have to stay local and disappeared after short time.

Theoretical limit of duch behavior is exactly mean distance between particles in given body, medium, gas or fluid.

So in a case of extremely low density, where medium distance between particles is about kilometres, even if you changed state of one atom, it is very low probability it will have any influence on another. They just have to collide, by coincidence, to exchange its momentum. Of course it may even happen, theoretically, but then there should be another collision of other pairs and another, and do on, and in such small density gas it is just nearly impossible to transfer any discrepancies in state at one end of the system to any other on large distance.

So such kind of medium may be treated as continuous, if wavelength is big enough, and here extremely long length should be considered. Such excitation may transfer energy and it is mechanical discrepancy of the medium, so even intergalactic gas may transfer sound, at least if we consider it as mechanical wave by definition.

It can transfer density waves caused by external field, like gravitational tidal waves or, if particles are ionised, which usually is the case, electromagnetic fields as well. But this is case of external field acting on separated medium's particles.

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    $\begingroup$ Even the intergalactic medium can transmit sound below certain frequencies. While the cutoff frequency is extremely low, it's the same phenomenon when we hear a musical instrument, a voice, or a far-off train: it's "sound". $\endgroup$ – David Hammen Jan 7 '18 at 17:00
  • $\begingroup$ I am not sure if it is good idea to call it sound. It reminds in some aspects a light vs very low frequency radio waves. Of course it is a mechanical, kinetic energy transfer. But in order to be sound, medium should allow approximation as continuous media. I suppose it have very peculiar characteristic which does not fit well into this picture. $\endgroup$ – kakaz Jan 7 '18 at 17:23
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    $\begingroup$ At those extremely low frequencies / extremely long wavelengths, even the very sparse intergalactic medium appears to be continuous. It's "sound". $\endgroup$ – David Hammen Jan 7 '18 at 17:25
  • $\begingroup$ Ok, it may be mainly question of terminology. Do you believe it is possible to have sound wave of length comparable to two galaxy distance? I don't know if it gives us anything useful. But of course, mechanical excitation of this size can exist, and such energy transfer is possible. Agree. $\endgroup$ – kakaz Jan 7 '18 at 17:31

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