Shhhh, shhh. You can stop screaming. That’s because nobody can hear you … in space. But why not? How does sound work here on Earth, and what would it sound like on other planets?
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Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit Astronomy.swin.edu.au for more information.
Fraser Cain: Astronomy Cast episode 311 from Monday, June 17th, 2013: sound. Welcome to Astronomy Cast, our weekly facts based journey through the cosmos where we help you understand not only what we know but how we know what we know. My name is Fraser Cain. I’m the publisher of Universe Today and with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville and the director of CosmoQuest. Hey, Pamela. How you doing?
Pamela Gay: I’m doing well. How are you doing, Fraser?
Fraser Cain: Good. And where are you?
Pamela Gay: I am in Turku, Finland.
Fraser Cain: Turku, Finland. There must be some good reason why you’re there.
Pamela Gay: I am here for the European Week of Astronomy and Space Science where I’m working with the Galileo teacher training program to educate a group of 13 teachers on how to better get astronomy into their classroom and I spent today over at the main part of the conference listening to speakers and mostly, I have to admit, I am enjoying some of the very weird things that I’m noticing here.
Fraser Cain: Like what?
Pamela Gay: Like it’s apparently a thing to have a restaurant that is kabob and pizza.
Fraser Cain: Oh, sure.
Pamela Gay: And this is not a combination I’ve ever seen before so the kabob pizzeria is a thing in Finland.
Fraser Cain: If you like fish, they sure do really good fish there in Finland.
Pamela Gay: I haven’t seen the fish. I’ve just seen the kabob pizzerias.
Fraser Cain: The kabob pizzerias. There’s a bunch of those. Okay. Okay, cool. And then you’ve got a few more things that are happening in Europe. Right? You got –
Pamela Gay: Yes. So I’m going from here to Nucleo in Lisbon, Portugal and I’ll be there for two weeks and from there, I’m going to Velos, Greece where I’m going to spend, I think it’s ten days working with the Global Hands on Universe Folks. So Carl Pennypacker will be there. We’re gonna be working on trying to figure out how to integrate CosmoQuest into the global teaching community.
Fraser Cain: That sounds great. I am officially jealous. Okay. Well, let’s get on with the show then.
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Fraser Cain: Shh, shh. You can stop screaming. That’s because nobody can hear you in space. But why not? How does sound work here on Earth and what would it sound like on other planets? So let’s get on with the sound. So I wanna go straight to that which is that famous saying that in space, no one can hear you scream –
Pamela Gay: It’s true.
Fraser Cain: Which comes from alien. It’s true.
Pamela Gay: It’s true.
Fraser Cain: So then how does sound work?
Pamela Gay: Sound is a compression rate wave when it goes through a solid, a liquid, or a gas most of the time. Sometimes, in solid, it’s also created by a sheer wave, a wave that the wave front is perpendicular to the direction of motion. Most of the time, with air and water, it’s just a compression wave which you can imagine with the slinky, it’s where the air gets compressed and that area that’s compressed is small and narrow and moving through space. So as I speak, I’m sending waves of compressed air out away from my mouth.
Fraser Cain: And they are hitting the microphone and jangling the microphone.
Pamela Gay: Oscillating it.
Fraser Cain: Yeah. Yeah, I believe the technical term is jangling but yeah. Go ahead. Right. So you’ve got the situation where in this case, with you talking in the air, like what is causing just even these oscillations to begin?
Pamela Gay: It’s a physical motion. In this case, it’s my vocal chords are physically moving the air. In the case of musical instruments, you have your breath is compressing things in various ways. If it’s a stringed instrument, it’s the vibrations of the string that are moving the air. So it’s literally a physical motion where something is physically pushing on the air, injecting energy into the air and that energy propagates through waves.
Fraser Cain: Right. And this is where it gets kinda surreal when you really think about it that that when we hear things, we’re just sensing these compression waves moving back and forth against our ears drums and the our brain is converting that into sound.
Pamela Gay: And the neat thing is, poetically, if you think about it, every time you hear someone playing a woodwind instrument or a horn instrument, you’re hearing someone blow in your ear.
Fraser Cain: Right. From far away. I think that’s romantic.
Pamela Gay: Yeah. And mutated through –
Fraser Cain: Kind of unnerving, I think is what that is.
Pamela Gay: Yeah. Creepy.
Fraser Cain: Creepy. Yeah. Yeah, very creepy like a stalker. Okay. So right. Okay. So we’ve got the situation where you’ve got these compression waves that are moving through the medium. And so what effects the sound?
Pamela Gay: It’s a combination of what is the medium made of and what is its density. So, different mediums have different impedance to air – not air. Different mediums have different impedance for sound waves moving through them. So some things, for instance, some solids are much more happy to have compression waves move through them so they have less impedance. But then the density of the material also matters.
So when the density goes up, the speed actually slows down and this kind of makes sense because it’s the energy that it takes to move all of those atoms. The higher the density, the more energy’s needed to move them and so the sound waves, it’s not literally it takes more energy because then the sound speed wouldn’t be constant in the medium but the higher the density, the slower the speed of the waves going through.
Fraser Cain: Now, we have like the speed of sound. When you say the speed of sound, if you’re gonna say that very technically, you say, “The speed of sound in air at –”
Pamela Gay: Air at a certain temperature.
Fraser Cain: At a certain pressure. Yeah. At sea level, at room temperature.
Pamela Gay: 20 degrees Celsius at sea level is the standard way it’s quoted but as you move through the atmosphere, what you experience in terms of sound changes in ways that probably aren’t perceptible. So for instance, just if you look only at temperature, the speed of sound decreases as it gets colder and increases as it gets hotter. If instead of looking at temperature, you look at altitude as you go up in altitude, the speed decreases to a point but then it’s this weird interplay between the density, the composition, and the temperature that causes it to go back up again. So there’s lots of weird factors that all come into play with figuring this out.
Fraser Cain: But in general, lower pressure makes the sound go faster? Is that right?
Pamela Gay: So pressure, if you look at it in terms of density, as the density goes down, the sound speed ends up going up. So it’s, again, it’s one of these things where you have to take care to track all of the different things in what you’re discussing when you’re trying to figure out what the answers are. Now, pressure and density aren’t always the same thing. So when you’re dealing with an ideal gas instead of dealing with a solid or a non ideal gas, then you have pressures at the top of the equation. There’s an impedance involved. In this case, it’s called the adiabatic index, density’s still on the bottom.
So if you hold the density constants to the number of atoms per square centimeter per meter, however you wanna measure the density, if that density stays constant, as the pressure goes up, so you’re increase in temperature or something to increase the pressure, then as the pressure goes up, the speed goes up so.
Fraser Cain: Right. And doing this math in your head is gonna be important because we’re about to talk about –
Pamela Gay: You really don’t like me today.
Fraser Cain: Well, I wanna talk about sound on other worlds. Right? So because I think of – you’ve ever done that experiment where you have helium. You breathe in some helium and then you breathe it out and your voice goes up. So the pitch goes up, so the sound waves are going faster. Is that right?
Pamela Gay: The wavelength of the sounds that are made by – so this is where it gets confusing because it’s how your vocal chords are vibrating. It’s the vibration of your vocal chords that gets changed. So your vocal chords are vibrating in a medium that’s made of completely different ideal gas than the nitrogen it’s normally in. And there’s other gasses that you can inhale that I’m not going to name because there’s been cases of people accidently killing themselves, that are much heavier than normal air and so when you inhale them and exhale them while speaking, it totally lowers the pitch of your voice.
Now, the problem is you have to stand upside down or hang upside down and cough all this gas out or it can suffocate you because it is heavier than normal air.
Fraser Cain: Right. Yeah, well, and people die from helium as well. So okay. So then we were just talking about air but we’ve got other medium that we can deal with. And I know that you can hear sound underwater for quite a distance even.
Pamela Gay: Yes. And the speed of sound underwater is actually much faster. So one of the neat things is if you use a starting sound underwater versus in the air and so you have a dolphin 10’ away from the starting sound and a human 10’ away from the starting sound, there’s a miniscule difference that will give the dolphin the advantage.
Fraser Cain: Oh, like he already needed advantages swimming through the water.
Pamela Gay: It’s just more fun to use that example but it is neat to think about over great distances if you shout across a lake versus sending the sound through the water under the lake. Through the water, the sound will get there faster.
Fraser Cain: Right. Yeah. Okay, cool. So now, then I think let’s just sort of get – to take this to the space context, we need to talk about why nobody can hear you in space.
Pamela Gay: And it comes down to the fact that there is no medium. You have nothing to have a compression wave. So if you’re in a dense enough nebula, if you’re in the outer layers of a star, then you can have a speed of sound. Then, you can have sound waves propagating through the medium. But if there isn’t that stuff to have the compression wave moving through, you won’t get any sound. One of the things that ends up happening a lot in astronomy classes is you find yourself having to calculate the speed of sound because it starts to become relevant for different physics and it’s kinda neat to think about the fact that all these different conditions have different speeds of sound.
Now, one thing you need to be careful not to get confused though is how a human being would sound speaking in these different mediums, because again, that’s our vocal cords. That’s the string on a guitar getting changed, not necessarily the speed changing.
Fraser Cain: Right. So I guess back to your star thing. So if you did fall into the sun and screamed, somebody else falling into the sun a little behind you would be able to hear you.
Pamela Gay: Would be able to hear you as they died violently.
Fraser Cain: Yeah, yeah. Yeah. Hitting 5,800 Calvin is gonna ruin your day.
Pamela Gay: One of the things that comes out of this though is submarines actually have to be very careful when they’re running silent to not vibrate because water does transmit all of the sounds. So if you’re in one submarine and you drop a wrench and the sound goes rattling through your submarine, the outer hall of the submarine, depending on how it’s built, most modern ones are built to isolate sound as much as possible but the old ones, especially, that dropped wrench would send vibrations through the water and that’s the whole principle behind sonar is you’re reflecting sound waves off of other things.
And a good sonar operator, while they’re listening for the pinged return, will also catch in the mix the sound of the blades of the submarine, the sound of the dropped wrench, even the chatter of humans getting propagated through the water.
Fraser Cain: Oh, wow. And so the workaround, if you want to sort of talk in space is let’s say you’re an astronaut and I’m an astronaut and we need to talk, what do we do?
Pamela Gay: If you wanna talk without using radio waves that can be picked up by other people, you use Morse code with a light and that’s kinda lame but that’s what you do is you use lights.
Fraser Cain: Now, what about like touching the helmets together? That work?
Pamela Gay: Well, you can do that but I’m assuming you’re far enough away.
Fraser Cain: Right. But say you were –
Pamela Gay: That is entirely true.
Fraser Cain: And that would work too.
Pamela Gay: You can. Yeah. And that’s just a matter of the two helmets transmitting the waves between the two helmets. Same as the submarine.
Fraser Cain: Right. So you’ve got the air transferring to the glass and then the glass is the medium to the other glass and then the other glass to you. But so then, when we look at all of these science fiction shows with these explosions and fiery bursts and –
Pamela Gay: Yeah, those are all silent. Silent.
Fraser Cain: Completely silent. So let’s provide a scientific version and if there’s any science fiction writer, any science fiction show people watching this right now, this is what it would sound like.
Pamela Gay: Nothing.
Fraser Cain: Well, you wouldn’t even hear the word nothing. Right. So let’s imagine some space battle with ships flying past and they’re shooting each other. What would that really – you wouldn’t hear the explosions on the other ship. You wouldn’t hear – but you would hear things happening to your ship.
Pamela Gay: You would hear things happening to your ship and you would hear if there was some sort of a shock wave of another ship exploding, all of the gas escaping, that sort of a shock wave hitting your own ship. You’d hear your hall responding to it.
Fraser Cain: Right. So that ship would be firing out a compression wave. There’d be gas and that gas would be slamming against your ship and then you would hear those sounds.
Pamela Gay: Yeah. Yeah.
Fraser Cain: Even if someone detonated a nuclear bomb out in space.
Pamela Gay: Silent.
Fraser Cain: Really?
Pamela Gay: Yeah.
Fraser Cain: But would there be – because there wouldn’t be a pressure wave.
Pamela Gay: No. It’s a radiation pressure but that’s a completely different experience. With nuclear bombs in our own atmosphere, it’s the pressure of the expanding gas that does so much damage. You do it without the atmosphere involved and it’s just a silent blast of deadly light.
Fraser Cain: Now, have you ever heard this sort of comment that the sort of lowest sound in the universe is the rumbling made by a super massive black hole?
Pamela Gay: Black hole. Yeah. And I have to admit that’s one of those things that just kind of bothers me because it’s the poetic license that press officers go to in order to try and get journalists like you to read their press release.
Fraser Cain: I sure did and then I reported on it. What did I do wrong?
Pamela Gay: You got suckered in by someone desperately wanting you to write about what they had. And the truth is, there are various waves that propagate around super massive black holes depending on what sort of shock waves are taking place, depending on how things are getting drug in, and those waves moving through the accretion disk. Those are compression waves. They’re shock waves.
It’s traveling at the speed of sound in some cases and as you watch these shock waves propagate through a medium, well, a wave moving through a medium, is the definition of a sound. But we don’t think of them as tones or anything like that. It’s that the more realistic analogy is the ringing of acoustic waves inside of stars. The acoustic waves actually tell you something about the interior of the star and that’s kind of neat.
Fraser Cain: Well, sorry. These acoustic waves, what’s going on here?
Pamela Gay: Just depending on how the star is set up, all sorts of different stars for reasons we’re still working to understand have a variety of different sized mechano-acoustic waves that you can see in the fine resolution light curves. There was a project called Gong that looked at our sun looking for high resolution variations and the models of these things show stars basically flexing in a variety of different longitudinal and radial waves.
Fraser Cain: Right. And I can just imagine the press release now. Gong scientists find sun rings like a bell.
Pamela Gay: Yeah, pretty much. I’m sure that one was written at some point.
Fraser Cain: Yeah, that sounds about right. Right. But the point you were mentioned before that the sun is a gas and it can absolutely propagate sounds and so you can have these sounds. And so I wonder, so if you had, I don’t know, like an X class flare or like a magnetic field disconnect and reconnect, would the sound of that, the snapping travel across the star?
Pamela Gay: Well so, again – yes. But we don’t think of it as sound. We think of it as a compression wave moving through a medium. And so this is sort of like the radio waves that we use for television, the microwaves that heat your food in your microwave. Those are all light but we don’t normally think of them as light. So in these cases, all the different waves going through stars, all the different compression waves moving through a medium. Yeah, that’s the same thing as a sound between. We don’t think of it that way.
Fraser Cain: Now, we did an episode back at Halloween which we called Spooky Sounds from Space but we were kinda doing it too because you can’t hear any sounds from Saturn. You can’t hear any sounds from pulsars and such. So in those situations, what’s going on here? Right? There’s some kinda conversion going on that there’s some propagation.
Pamela Gay: That’s just radio waves getting converted. So what we were talking about was radio waves getting converted into audio waves. So if you record the radio signals coming from the different sources or in the case of objects moving through the atmosphere, the actual radio from radio stations getting reflected off of them as they move through the atmosphere. We are used to AM, FM, television. We change radio waves into audio all the time through amplification processes through modulation. All people are doing is listening to the naturally occurring light that comes out in the radio bands and converting it into audio the instead of seeing it with our eyes, we hear it with our ears.
Fraser Cain: Right. And so we’re transferring from the electromagnetic spectrum. In this case, radio, but you can imagine, I don’t know, converting gamma rays to sound.
Pamela Gay: Oh, yeah. You could. Or you could translate the slow change in color of a Cepheid variable into a very slow slide whistle tune.
Fraser Cain: Right. Right. And I guess energy can be changed and so you can have a situation where –
Pamela Gay: Radiation becomes acoustical energy.
Fraser Cain: Yeah. Yeah, as it explodes something. So it’s not entirely craziness. Thank you very much, Pamela.
Pamela Gay: My pleasure.
Male Speaker: Thanks for listening to Astronomy Cast, a non profit resource provided by Astrosphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at AstronomyCast.com. You can email us at firstname.lastname@example.org. Tweet us @astronomycast. Like us on Facebook or circle us on Google+. We record our show live on Google+ every Monday at 12:00p.m. Pacific, 3:00p.m. Eastern or 2000 Greenwich Mean Time. If you miss the live event, you can always catch up over at CosmoQuest.org.
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Duration: 24 minutes