What would the Sun sound like if we could hear it? Does any noise at all happen in the universe? What’s the biggest sound of all? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO GENERATED)

We've all heard that old saw that there's no sound in space, that all the fun parts of our favorite science fiction movies are just junk. There are no pew pew of the lasers. There are no kabooms of the big bombs. There's no whoosh of the spaceship going by. And you know me, one of my favorite hobbies is killing all the joy in our lives and replacing it with the cold hard mathematical truth of physics, but not today.

And honestly, as an aside, I actually hate rating or critiquing or judging science fiction movies because, if you haven't noticed, movies take place in universes that are similar to but not exact copies of our own reality. So our knowledge of physics doesn't always apply, so it's not useful to take our knowledge of physics and use it to judge the physics in the movies because it's a different universe. But but that's a story for a different day. Also, I have enough homework to grade, and when I wanna watch a movie, I don't wanna grade homework. Anyway, I digress.

Outer space is almost always a vacuum, and sound waves only work if there's something to weigh. They are pressure waves that require a medium like air or water. It's not like electromagnetic waves that don't require anything. In the pure vacuum of space, you can just have waves of electricity and magnetism, aka light, propagating as far as you want. But not sound waves.

Sound waves are waves of pressure. Me speaking right now, I've got my my little vocal cords and they are vibrating. They're literally pushing on air molecules. And then those air molecules get denser, and then they push on the next, and they push on the next, and they push on the next in this expanding sphere that escapes my mouth. And then that's how you hear it because those pressure waves, those crunched up air molecules strike your eardrum and physically move your eardrum back and forth, which is what you interpret as sound.

So you need something to squish together to make the sound waves. It can be air. It can be water. It can be even be the Earth. You can be objects that are solid as long as they can support pressure waves, as long as they can support waves of squishing things together, even by a little bit, you can have sound waves.

But in vacuum, there's nothing to squeeze. There are no molecules hanging around or atoms hanging around for you to press against to generate a sound wave. Now if sound waves could travel easily through space, then there would be there would be problems. For example, the sun emits 383 yottawatts per second. That's a lot of watts.

In one second, that could power all of our energy consumption for the next six hundred fifty thousand years. That's right. The sun in one second emits enough power to power all of human civilization, everything we do, every toaster, every fridge, every refinery, every every, you know, the stuff, all podcast recordings and listenings for six hundred fifty thousand years. Even if even a small fraction of that were converted into sound, it would be loud. Like, really loud.

For example, fireworks and gunshots at close range are around a 140 decibels. Decibels is a logarithmic scale, which means if you go up in decibels, you're not going up one or two or three. You're going up by, like, factors of two or factors of five or factors of 10. You're you're making big jumps with every increase in decibel. A 150 decibels can cause permanent hearing damage.

At the surface of the sun, if you could hear the sun, if if sound waves could travel through the vacuum of space, then it would be around 280 decibels, which would be loud enough to just turn you into goo because the pressure waves would be so intense they would just crush your body. And here on Earth, 93,000,000 miles away, the sun would still be 120 decibels. That's like an ambulance driving right past you or standing next to a jet as it's taking off, and it would be constant. Constant. Just like we are used to the light of the sun constantly beaming down on us during the day, we would have to get used to this sound that is so loud that it would make everything else unintelligible constantly.

You would even hear it at night because the sound would reverberate through the solid Earth. It would reverberate through our atmosphere. You couldn't escape it. So it's a very very good thing that there's a whole lot of nothing standing between us and the sun because that would be intensely unbearable to say the least. Roughly as unbearable as a surprise Patreon page.

That's patreon.com/pmsutter. It's my name, which is rather handy. You go to that website, and that is how you contribute to keep the show going, and I truly appreciate every every single dollar. I truly do. That's patreon.com/pmsutter.

But space isn't always a vacuum. And where it isn't a vacuum, we can get some pretty intense sounds. One place is in the sun. I know I just talked about imagining sound waves from the sun reaching all the way to Earth and how that's that's totally impossible because the vacuum of space, there's nothing to squish. That's not what I'm talking about here.

I'm talking about inside the sun. Real sound waves happening right now. How do you make a sound wave? You press against a substance. You press against air.

You press against liquid. You can even press against solids. If you do it right, if you hit the floor, some of the sound waves come up into the air that you hear, and some of the sound waves travel through the floor. You can feel the vibration of the floor. Those are pressure waves.

Waves of pressure moving outward, where you have shells of high density squishing on the next shell, on the next shell, on the next shell. And the sun is churning. It's alive. It's active and it's made of stuff. The sun is dense.

It's one of the densest places in the solar system. It's filled with fluid. Now, the fluid is a plasma, but it's still a fluid. It's super hot, but it's still atoms and molecules doing their thing. They just lost their electrons.

The electrons are are swimming around doing their own thing, but all the atomic nuclei are still there. The stuff there's there's there's stuff in the sun. And if there's stuff, you can squish it, and if you can squish it, you can make sound waves. So what's doing the squishing? Well, the fact that the sun is somewhat violent in its interior.

Think about it. You have the nuclear core, which is consuming a Earth's worth of hydrogen every minute or something ridiculous like that is just chewing through planets worth of of hydrogen converting it into helium and releasing energy. It's a non stop nuclear bomb going off in the core of the sun. The outer edges of the sun are exposed to space. So you have it hot on the inside and cold on the outside.

Just like when you put a pot of water on the stove, it's hot on one side and cold on the other. You get these massive plumes of convection where material is heated by the core of the sun, expands buoyant buoyantly rises up to the surface, cools off, and then slinks back down, so you get this huge churning motion like these conveyor belts of plasma moving up and down. The whole thing is spinning really really quickly, and it's spinning faster at the equator than it is at the pulse. You get this mixing action. There's also sorts of random chaos because you have, radiation pouring out of the core of the sun that's just slamming into atoms as it tries to escape.

It takes like a hundred thousand years for a single photon to escape the core of the sun and reach the surface. The sun is ringing like a bell because it's constantly squishing on itself. And this ringing actually sets up standing wave patterns. Standing waves, you know, when you like pluck a guitar string, there's a standing wave that just sits there and existing until the whole thing runs out of energy. But if you keep plucking at the exact same rate, you can keep the note going.

You can keep that standing wave alive. It's almost like a wave that's frozen in place that just keeps that doesn't travel from one place to another. It's just it's just always there, like, that plucked string is always there. And this is like the sun. You have all these competing forces.

You have all these competing sound waves that are constantly sloshing around, running around, But some of them reinforce each other where they combine and where they're where they peak is at the same place and where they cancel out is at the same place. And this sets up like a a a permanent vibration of the sun. And these waves, just like a pluck string, are on top of each other. You know, if you pluck a string, it's actually a combination of many many different kinds of waves. There's one wave that starts at one end of the string and then goes up to a maximum and then attaches at the other end, and it just goes like, like, up and down, up and down, up and down.

And then you have another wave on top of that that is that's exactly half the wavelength where there's one extreme on one end of the string and then another extreme at the other end, and they trade places like, and then there's another wave on that like beep beep beep beep, and then they all add together to make something that doesn't sound as weird as the noises I just made and something somewhat pleasing like a plucked guitar string. If you've ever learned about music and fundamental modes and overtones and all that, it's the exact same math. It's just on a sphere instead of a spring. So the sun has a fundamental mode of vibration, like a heartbeat, like a pulsation on its surface, like, foom foom foom foom. And then on top of that, there's another overtone that's also ringing.

And then on top of that, and on top of that, on top of that, and they they all meld together. So so on the outside, it just looks like chaos. It looks like total chaos, like the the sun has no idea what what it's doing. But then you can look at these vibrations just so you like, you can look at a at a plucked string or a struck drum, and you can tease out all of the fundamental mode and then all of the overtones, and we we map this. We can actually watch the sun vibrating because little patches of the sun are physically moving in and out.

And so if you look at one little patch, you'll see that patch, you know, sink in a little bit and then rise up a little bit, sink in a little bit, rise up a little bit. And it will be following the waves as they go by. It's like looking at one piece of a drum and seeing that one little patch of the drum rising up and down after a drummer has struck it, and these sound waves are are riding through it. And we map this. It's called helioseismology.

It's and it's one of our best ways of studying the interior of the sun. Because these sound waves that are constantly crashing through the sun's body, they're constantly interfering with each other that set up the fundamental mode and the overtones and all this, they travel at different speeds throughout different parts of the sun. So some of them will be able to make it through the core just fine. Some of them get reflected off the core. Some of them travel through the mantle or the convective zone of the sun in a certain way.

Some of them will get turned or bent in a certain way. Some of them will respond differently to their different rotation rates at different latitudes. It's exactly like how we use seismology to study the interior of the Earth because when there's an earthquake, a big compression of of a rock, it sets up a sound wave that reverberates through the solid Earth, reverberates through the mantle, reverberates through the core. We can create sound waves in the Earth. We call them earthquakes.

And we can map how efficiently different kinds of sound waves travel through the earth and reach their destinations and when and use that to to work out. Okay. If if if this wave, traveled this way and then that wave got deflected by this much, you you just put it all in the blender and you figure out what the heck the Earth is made of. It's one of our key insights to studying the interior of the Earth. It's the exact same mathematics, slightly different technology, but same mathematics for studying the sun.

Those sound waves don't make it outside the sun, but it's not for lack of trying. If we could hear the sound waves, like I said, it wouldn't be all that great. But can we get bigger and even more intense sound waves in the universe? Oh, yes. And not just a little bit big, but really big.

The largest gravitationally bound structures in the universe. The greatest objects that will remain forever as objects despite the accelerated expansion of the universe. The glistening metroloposistises in the night. I'm talking about clusters of galaxies. Clusters of galaxies are gigantic.

They're a million light years across on the small side. They're home to thousands of individual member galaxies. They're home to some of the largest galaxies in the universe in their cores in the centers. They'll have the giant galaxies called brightest central galaxies. It's not the most evocative name, but you you get the idea.

There's a whole lot of dark matter. Heck of a lot of dark matter. Inside of galaxy cluster, it's one of our finest probes of the existence of dark matter, and there's lots of gas. We call it the intracluster medium or ICM. It's the stuff in between within the cluster between all the galaxies.

But between you and me, we can just call it gas. For those of you keeping score at home, about 15% of all the regular matter in a cluster is in this form of gas. Now this gas is thin. It's super thin. It's thin enough that it would register as a vacuum in any laboratory on the Earth.

The typical density of this gas is around one one thousandth of a particle per cubic centimeter, which, to give you a comparison, that's about 10,000 times less dense than air at sea level. So we're talking about nearly a total vacuum. It's also kinda toasty with a typical temperature of around 10 to a 100,000,000 Kelvin, which is so hot it would roast your skin off except that there are basically no atoms around anywhere, so you never actually get to feel it. You you would swim through the intercluster medium. You would swim through this hot, thin gas, and then, ow, you know, one atom would hit you, and it'd be mildly uncomfortable.

Actually, you wouldn't even notice it because it's just a single atom. And then you'd wait a really, really, really long time, and then another atom would hit you. And then a really, really, really long time, and another atom would hit you. So even though all these atoms have this incredibly high temperature and so much energy, they don't actually get to deposit that energy to you, and so you would swim through it just fine. You would think you were in a vacuum.

But, hey, the gas may be gigantic, extremely hot, and unfathomably thin, but it's not nothing, which means it's something, which means it can support sound waves just like anything else that is not nothing and is something. Because it's not exactly a vacuum. And if it's not exactly a vacuum, you can press on it. Sure. There aren't a lot of atoms around, so you, like, waving your hand or trying to talk isn't going to do much.

But what if you're like, I don't know, a super massive black hole? If we go in the centers of these brightest central galaxies, they host the on the on the large end of super massive black holes. And already super massive black holes as a category are somewhat big, and now we're talking about the big kinds of those. These are the supermassive black holes that other supermassive black holes, like, go like, dang. That's a big black hole.

Giant ones. We're talking, billions of times more massive than the sun. Gigantic black holes, gigantic accretion disks, material falling in, getting torn apart, you know, all the usual quasary goodness happening here inside of these galaxies, and they make jets. These jets of high energy particles racing out, screaming out at nearly the speed of light, powered by the gravity of the black hole. These are particles that don't make it inside the event horizon.

They get twisted up and wound around these electric and magnetic field lines near the event horizon, get flung out up and down the spin axis of the black holes. These jets are enormous. They punch through their host galaxies. They punch out into the gas, into the intercluster medium. An individual jet can travel for tens of thousands of light years before petering out.

And while it's doing that, you know, there's this this hot thing gas that surrounds every galaxy inside of a cluster, but then this jet comes piercing in, streaming in these these high energy particles that are even hotter, but they're denser because they're in this jet. And the jet will actually literally inflate bubbles that appear in the intercluster medium. These things can be hundreds of thousands of light years across. And then the bubbles detach and rise up into the intercluster medium where they eventually disperse. But these jets are only active when the black hole is feeding.

When material is actively falling into the black hole, that's the only time you can get these jets to power up and punch through and create these bubbles. But every time a jet fires up, you have this stream of material, thousand light years wide, five, ten thousand light years wide extending for tens of thousands of light years, even though the inter cluster medium is super super thin, this is enough to make a difference. It pushes on atoms. It literally pushes on the atoms of the inter cluster medium. It increases their density, increases their pressure, which then sends off shock waves ripples that go out throughout the entire cluster.

It's like a heartbeat at the center of every cluster. Just wham, wham, wham, cycling on and off every few million years, but hitting the intra cluster medium on a regular basis, sending out these ripples, these ripples, these sound waves in this hot thing gas take millions of years to travel. They have wavelengths of thousands of light years. We can actually map them. We can see them because the this intracluster medium, this gas is so hot, it's emitting x-ray radiation.

And where there's more of the gas, there's more of the radiation. So we can take pictures with, like, the Chandra x-ray telescope. We can take pictures of the inter cluster medium in a bunch of clusters, and we can see these rings. We can see the sound waves. The regions of high density, the regions where the sound wave is currently reaching its crescendo, and then the valleys in between, we can map it out.

And these sound waves are ridiculous. There's something like 57 octaves below middle c. Good luck trying to find that on a piano. Sometimes clusters of galaxies even crash into each other. A famous example of that is the bullet cluster.

Two imagine this, like two objects, each one weighing a thousand quadrillion times the mass of the sun. That's the typical mass of a galaxy cluster. Home to thousands of galaxies crashing into each other. And that means the intracluster medium of each one slams into the other one headlong. And even though it's thin, even though there aren't a lot of atoms, even though the densities are incredibly low, you can make up for that by sheer volume and by trying really really hard.

And the universe is really good at crashing stuff into each other. We'll see shock waves. We'll see bow shocks. That's right. Like like, you imagine a boat traveling through the water and it's traveling faster than the speed of sound in the water and creates a bow shock or it creates, or an aircraft traveling faster than the speed of sound in air, creates a a a shock wave like that.

There can be shock waves that we can literally see in pictures because of the x rays emitted by the hot thin gas. So space is empty now. Intra cluster medium is actually kinda dense compared to the cosmic average. The cosmic average density, if you smooth out all the stuff in the universe, so perfectly equal density is about one atom per cubic meter, which is not a lot at all. But that's on average.

Most of the stuff in the universe is compressed into very small volumes. Sound waves aren't going to have a good time of it. Because even if you have things like super massive black holes, even that's not powerful enough or big enough or have the right scales to to move this stuff at even larger scales outside of galaxy clusters. But it hasn't always been that way. Billions of years ago, our universe was smaller and hotter and crucially denser.

There was a time when our universe was just a few 100,000 years old where the universe was so hot and so small and so dense that it was a plasma just like the interior of the sun. At that time, our universe was about a million times smaller than it is today. And that's when you get the densities and temperatures needed to make a plasma. Which means if you could travel back in time to these early days of the big bang, you could be heard in space. There would be sounds in space.

There would be pupus of lasers. There would be kabooms of bombs. There would be swooshes of spaceships because space was filled with stuff. It hadn't emptied out yet. And man, it was having a time back then.

The thing is, the universe at this epoch really, really, really wanted to be in equilibrium. It just wanted to chill out and expand and take a nap. It just wanted to have the same temperature throughout the universe, no big deal, with everything the same everywhere. But that's impossible, so it was having a tough time. And there were these competing forces at work.

There'd be random pockets of the universe that were ever so slightly more dense than average. And gravity would want to pull material onto those pockets. Because once you have a little bit higher density than average, you have a little bit more gravitational pull than everybody else. So you're like, okay. You if you're in this little high density pocket, you start reaching out your tendrils of gravity to your neighbors and you start pulling stuff together.

But, as you pull that stuff together, the stuff gets hot, it heats up, and then radiation, it it emits light and the heat of that light, the pressure from the light itself pushes the material back out. And then it cools off. And then gravity tries again to pull it out till it's pull it back in. And then radiation pushes it back out, and then gravity pulls it in, and radiation pushes it out. Boom.

Boom. Boom. Boom. These pockets and regions would try to collapse on themselves, but then radiation kept pushing it out, but then it would overshoot and then collapse again. And this was happening all throughout the universe, all the time at all sorts of different scales.

These were sound waves. These were oscillations. These were reverberations in the plasma due to these cyclic regular rhythmic processes of contraction and expansion around all of these random pockets that were distributed all throughout the universe. And this kept going until the cosmic microwave background appeared. Because when the cosmic microwave background appeared, the universe had cooled to the point that it was no longer a plasma.

It cooled to the point where atoms could form, where electrons could finally bind onto atomic nuclei. And with that, radiation became separate from matter. The universe became transparent at this time when the universe was about 380,000 years old. Before this time in the plasma epoch, atoms and radiation talk to each other. The densities are high enough where if you try to move, you're an atom, you try to move, you get you get bombarded by radiation from all directions.

And so these sound waves could reverberate back and forth, back and forth, back and forth because radiation can has enough. There's enough density and there's enough radiation that it can push on matter. But once the universe neutralizes, the party stops. You know, it's that record scratch moment at the teenage house party when the lights come on and the cops are there. Like, it is just frozen in time.

So what we happens is that once the CMB is is released, once the universe neutralizes, once atoms form, these sound waves, wherever they are, they get frozen. You know, if there's like a shell of matter that's nearly collapsing on one of these dense, regions, these dense pockets, and it's about to start getting pushed out by the radiation, well, it gets locked in place. And if there's another shell of material that has rebounded and radiation has has put pushed it away from the dense pocket and it's overshot and it's starting to cool off and it's starting to collapse back in again, it gets locked in place. So you end up with these shells, these overlapping shells of ever so slightly higher than average density in the gas, and it's not much. We're talking one part in a million denser than average, but the universe was young then.

And these shells act as an additional source of gravitational attraction because you have these shells of slightly higher density than average. These are the sound waves that get frozen in place once the universe neutralizes. The gas, the atoms that make up our universe, there's a few more atoms in those shells than there are in other shells. There are few more atoms in those shells than there are in other regions. And so slowly over time, they start gravitationally attracting material of their own just like those dense random pockets do.

They go on to collapse, but then the shells also act as an additional source of attraction. And now, there's no radiation to push back. So it's just gravitational attraction, just growth and growth and growth. This is how we get the large scale structure of the universe, but on top of the pattern of the large scale structure of the universe are these shells. And when you fast forward billions of years and you look out in the universe today, we can calculate how big these shells ought to be today And these shells have a radius of around nearly 600,000,000 light years today.

And that means when you look out at all the galaxies in the universe and you do a big enough galaxy survey, galaxy there are these shells that appear of slightly more galaxies than average. It's nothing you can pick out by eye. It's not like you can visualize this and just look at a map of the universe and say, ah, there it is. But when you do statistics of galaxies in the universe and you map, like, how the distances between galaxies or how many galaxies if you take a random galaxy and you measure, how many galaxies are in shells surrounding it at ever larger radii, and you you just do some statistics for the nerds out there. It's the two point correlation function.

You're welcome. You see a little bump. You see this excess. Like, if I'm a random galaxy and I look at greater and greater distances from me, I'll see it more and more galaxies but then at a at a distance of around 600 light years away, I'll see an extra bump, extra galaxies more than I expect, and then it will go back down to the trend that I do expect. We call these baryon acoustic oscillations.

They are locked in sound waves from the very early universe that we can hear. We can't hear it, but it's it's an echo. It's an echo that was frozen in to the largest scales in the universe left over from when the universe could support sound waves. So you can't technically hear it, but you can see the echo left by it. And to me, that's good enough for counting as a sound in space.

Thank you at doug buchanan junior one and Bert b for the questions that led to today's episode. Please keep those questions coming. That's ask a spaceman dot com or askaspaceman@gmail.com. Please keep the support coming. I truly do appreciate.

That's patreon.com/pmsutter. And please drop a review on your favorite podcasting platform that really keeps this show going. I'd like to thank my top Patreon contributors this month. They're Justin g, Chris l, Alberto m, Duncan m, Corey d, Michael p, Nyla, Sam r, John s, Joshua, Scott m, Rob h, Scott m, Louis m, John w, Alexis, Gilbert m, Rob w, Jessica m, Jim l, David s, Scott r, Heather, Mike s, Pete h, Steve s, Wat Wat Bird, Lisa R. Kuze, Kevin b, Michael b, Eileen g, Dante, Steven w, Brian o, Deborah a, and Michael j.

That's patreon.com/pmsutter, and I will see you next time for more complete knowledge of time and space.

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