Sound can travel through space after all - but we can't hear it

It's a fact well-known enough to be the tagline to the 1979 sci-fi horror blockbuster Alien: "In space, no one can hear you scream." Or to put it another way, sound can't be carried in the empty vacuum of space - there just aren't any molecules for the audio vibrations to move through. Well, that is true: but only up to a point. As it turns out, space isn't a complete and empty void, though large swathes of it are. The interstellar gas and dust left behind by old stars and sometimes used to create new ones does have the potential to carry sound waves - we just aren't able to listen to them. The particles are so spread out, and the resulting sound waves are of such a low frequency, that they're beyond the capabilities of human hearing.

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