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I heard in a discovery news video that there is one particle every cubic cm in space. So, if i were to vibrate a circular body of say mass $10^7$kg at $10^{22}$Khz, would i be able to hear a sound from say 1 metre away in space, assuming i will somehow live? Im asking this because there is no documentation anywhere that sound could ever travel in space. Im 14, so I don't know how many zeroes to include in the values. Thanks. If this is true, the big bang would have produced an extremely loud sound.

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    $\begingroup$ Your circular body is called a "diaphragm." Its mass doesn't matter: what matters is its area. I know pretty close to zilch about the propagation of "sound" through the almost-vacuum of deep space, but I bet there is a relationship between the mean free path of the particles out there, their temperature, and the shortest wavelength/highest frequency that can propagate. I'm guessing that the highest frequency would be VERY low. Not cycles per second, but maybe more like cycles per year. $\endgroup$ – Solomon Slow Mar 26 '15 at 20:02
  • $\begingroup$ Also, I'm guessing that in order to detect the sound, you would need a Huge diaphragm---like maybe you could hear it if your ear drum was the size of a planet. Or, I don't know. Maybe even that would not be big enough. $\endgroup$ – Solomon Slow Mar 26 '15 at 20:03
  • $\begingroup$ I expect the big bang produced a loud sound anyway; the universe was a lot denser back then. $\endgroup$ – stanley dodds Mar 26 '15 at 20:03
  • $\begingroup$ See also: physics.stackexchange.com/questions/162184/… and physics.stackexchange.com/questions/49818/… $\endgroup$ – BowlOfRed Mar 26 '15 at 20:27
  • $\begingroup$ Sound waves in the early Universe: en.wikipedia.org/wiki/Baryon_acoustic_oscillations $\endgroup$ – Kyle Oman Mar 26 '15 at 21:22
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Assuming a loose definition of the word "sound," the answer is yes.

Let's consider the experiment you proposed. Suppose you place two large diaphragms facing each other in space. They could be simply large sheets of plastic stretched around a metal rim, like a drum head. And "large" in this context means much larger than the average distance between particles. Initially you would expect both diaphragms to be stationary and flat, since the same number of particles would impact both sides of both diaphragms, and the forces imparted would cancel each other out.

Now suppose you move one of the diaphragms toward the other a couple of feet. That will send more particles heading in the general direction of the second diaphragm than would otherwise be the case. A short time later, some of those particles, or other particles that were struck by those first particles, will impact the second diaphragm, causing a tiny increase in the number of impacts per second compared to the steady-state case. With instrumentation that is sensitive enough, you could detect that change. So it is fair to say that the "sound" has traveled through space.

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