Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

In relation to this question:

How can a black hole produce sound?

Which notes that the hole "produces" sound. The top answer states that:

What you think of as the hard vacuum of outer space could just as well be seen as a very, very, very diffuse, somewhat ionized gas. That gas can support sound waves as long as the wavelength is considerably longer than the mean free path of the atoms on the gas.

I get that there is "stuff" in space, what I don't get is how sound travels it. I learnt in high school that sound was a wave - as an example, you could fix a shoelace at one end and vibrate the other - voila, a wave forms.

And then sound kind of moves in the same way through, say, air, because of the slight molecular attraction between individual molecules is enough to create a similar waveform.

And then you get to space, and there aren't any molecules nearby to irritate each other, so there's no sound.

If that's the case, why does it matter how long the wavelength is, if there isn't a molecule nearby for the initial molecule to affect, how can sound travel?

I'd understand a little better if the particle a just hit particle b (like a pool ball) and b carried the sound - but that has little to do with waves and wavelengths.

Or is it that the "jet" that is thrown out travels along as a contiguous blob, with a sound wave embedded within it?

share|improve this question

1 Answer 1

up vote 7 down vote accepted

A vibrating shoelace is a poor analogy for a sound wave, because that would be a transverse wave whereas sound is a longitudinal wave.

What this means is your example of molecules hitting each other is perfect. A longitudinal wave is described by the variation of the density of the particles. The Wikipedia page has some nice animations to help visualize what that means. The wavelength in this context is the distance between two periodic fluctuations in the density of the material.

So in the case of the question you linked to, because the sound emitted has a massive wavelength (and consequently an extremely low frequency) that means the distance between the density fluctuations are large. So the particle literally moves and collides with another particle and so on, propagating the wave.

share|improve this answer
It is worth mentioning that this is exactly how sound propgates in air, which in the converse of my statement quoted above can be thought of as a very, very poor vaccum: it's still a bunch of isolated bits (atoms) flying around in empty space, there are just more bits. –  dmckee Jan 10 '13 at 13:54

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.