Can a scream be heard in space?

13 June 2017

Interview with 

Carolin Crawford

ASTRONAUT-ON-MOON

Apollo astronaut on the Moon

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As the saying goes, "In Space, no one can hear you scream"... But is it true? Chris asks Carolin Crawford to debunk this myth. 

Chris - But before we jump back into your questions, let’s delve into the topic of myths which we touched on earlier. Now we love busting myths here on The Naked Scientists as regular listeners will know, and I’d like to know our panel’s favourites howlers.

For instance, I enjoy the old adage - “a strong drink puts hairs on your chest!” It did not work for me!

Carolin, have you come across any scientific howlers in you neck of the woods?

Carolin - Oh, plenty. But the one I’ve picked up out today is that phrase “in space no-one can hear you scream.” Now this is actually right, but perhaps not for the reasons people expect because sound requires a medium. So the pet answer is there’s no sound in space because space is a vacuum.

Actually, there’s plenty of sound in space because the space between the stars, and even the space between galaxies isn’t a true vacuum.

Chris - There is one atom per cubic metre or something is it?

Carolin - If you think the air in this room, probably the air molecules a thousandth of the volume. They're separated by a distance ten times their size. Once you go out is space they're separated by at least a million, ten million times their size, so there’s still a medium, it can still transmit sounds. But if you think how a sound works, you have to displace a molecule far enough that it hits another molecule, and it’s those vibrations that set up the compressions and the rarefactions that it’s a soundwave.

So in space you can transmit sounds but they’re got to be an incredibly long wavelength.

Chris - Very, very low frequency in other words?

Carolin - Very, very low frequency - yes. So, if you could remove your helmet, there are sounds in space, but you wouldn’t hear them because they’re going to be way out of the 20 to 20,000 hertz that our ears pick up.

Chris - Because our ears are sensitive from about 50 upwards to about 15,000 in an adult aren’t they, so under 50 hertz we're not going to hear it. So you’re saying these frequencies would be so low there’s no chance of hearing them?

Carolin - That’s right. And indeed, one of these sounds I have studied is in an atmosphere that’s in a whole cluster of galaxies where the frequency is one cycle every 10 million years, so that is extreme sound. There are sounds in space but it’s just but it’s just such a very different kind of atmosphere that they’re not sounds you would hear.  So it’s correct but not for the reasons you might think.

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