How loud is space?

Hint: it's not.

Aug 25, 2021

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Ever seen a space movie? Me neither. But if I had, I’d know that things usually go wrong. Otherwise, it’s not a very interesting movie. In space there’s not much room for error, so when things go wrong, they go very wrong. Close your eyes and picture this: a few hundred miles above the surface of the Earth, in the blackness of space. No sound, no motion, just an inanimate space station stagnant in the night sky. It doesn’t look like it, but it’s traveling a couple of miles every second. Then, for whatever reason, the writers came up with, boom. An airlock blows and the pressurized space station erupts in an explosion. The sound waves echo out and past the camera like any other explosion. Does this sound familiar? Good. Because it’s completely wrong.

I’ll be honest, this level of inaccuracy has been done away with in recent cinema because we’ve gotten a much better idea of what space is actually like. Regardless, sound itself is a pretty funny thing. It can generate heat, break glass, and make you sad (in the case of the Notebook). But all it really is is vibration. Let’s start with a door. When you knock on a door, it vibrates. If you put your hand on the door, you can feel it shake. Is this sound? In a way, but not actually. A sound is a sound when it vibrates your eardrum, which your brain registers as sound. So, unless that door’s vibration makes it to your ear, that’s what it will stay: vibration. But Matt, you think, I can hear when someone knocks on my door. That’s, like, the whole point. Well valued reader, that’s because we have a little thing called air. Fortunately for our ears, gas can vibrate just like a solid can. And when the door begins to vibrate from knocking, that vibration is transferred to the air. That is then transferred all the way down the hallway, and into your ears.

So, in order for sound to travel, there needs to be atoms and molecules that can bump into each other. Without one air molecule hitting the next, the vibration can’t go anywhere. Here on Earth, there are very few naturally occurring absences of molecules. But think of these spaces as a shock absorber for sound. Just as shock absorbers in your car attempt to isolate you from the bumps and bounciness that the road possesses, without other molecules, a sound is isolated, and therefore can’t be transferred from place to place.

This absence of molecules is called a vacuum. And this actually leads us to a pretty weird question. What if you were surrounded by a vacuum? Not in space in a spacesuit but here on Earth. Imagine that it would be possible to surround yourself in a bubble of a vacuum. You have some air right next to you to talk into but beyond that: nothing. What would happen if you tried to talk? The sound waves from your mouth would exit, travel through the immediate air around you, and just… stop. They wouldn’t bounce back to you because there’s nothing to bounce off of. They wouldn’t really be absorbed because that would imply that the vibration is scattered and reduced. The sound would just go away. No echo, nothing to pass onto, the vibrations would simply cease to exist. Now, what exactly would that sound like? I have no idea, nor would I ever like to know.

With this newfound (or already held) knowledge, we can apply it to space. Space, as you may know from some if not all of my other newsletters, is a vacuum. Not a perfect one, as there are still some particles whizzing around, but for any practical purpose, it is. That means there are no molecules to hit each other, and therefore no reasonable path to vibration, and ultimately sound. If you were to weather space without a spacesuit, talking would prove completely fruitless. You’d probably feel the vibrations from your vocal cords, but there’s no air to carry that sound from your mouth to your ears.

The same happens with explosions, or collisions, or really any other space event that would cause things to shake. That vibration will once again stay vibration. Therefore when you see an explosion on a space station, or two satellites colliding in the movies, all that vibratory energy, as intense as it is, has no medium to get to you. As an outside observer, your ears are luckily spared from the likely extremely loud bang that would come with colliding at 2 miles/second. The rest of your body, however, may have a different experience.

So why can we see things in space but not hear them? That’s a great question, and the answer lies in our molecule discussion. Light is radiation, and radiation comes from atoms getting ~excited~. When an atom gets hit with some energy (think electricity in a lightbulb) its electrons get all excited with additional energy. However, this isn’t a stable state for these little guys, so the electron essentially gets de-excited. But all that energy has to go somewhere—emitted as a photon, light. So sound is the vibration and interaction between particles and light itself is a particle. This allows it to travel in space freely.

This in short is an introduction to the world of acoustics. The key takeaway here is that the ability for sound to travel depends on how dense the medium it travels through is. For example, if you make noise underwater, that noise actually gets to your ears much faster than it would on the surface. This is because water is denser, has more molecules, and because they’re closer together, those molecules will transfer the sound faster and more effectively. As you decrease air density by, say, increasing altitude, the speed of sound changes. This leads to the often weird idea that an airplane can be going Mach 2 (2 times the speed of sound) and exactly how fast that is entirely depends on the plane’s altitude. If it is extremely high up, where the air is very thin, there are fewer molecules to vibrate, and thus the speed of sound is slower, 2 times the speed of sound isn’t really that fast. If it’s near the ground, where the air is denser and the speed of sound is faster, 2 times that number is all the more impressive.

Thank you for reading this week’s edition of It’s Not Rocket Science! I hope you learned something new; keep an eye out for next week’s edition and have a great weekend.

It's Not Rocket Science

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