"In Space, No One Can Hear You Scream." That was the tagline for the movie Alien, Ridley Scott's 1979 sci-fi/horror masterpiece. Released two years earlier, Star Wars allowed us to hear plenty of things in space, like the whine of TIE fighter engines and the explosion of the Death Star.
So which movie is right? How does sound work in space?
Here on Earth, sound travels as mechanical waves transmitted through a solid, liquid or gas medium (like the air in a room, the water in a pool, or the walls in an apartment building). Pluck a guitar string and it vibrates. The vibration of the string pushes against the molecules of air around the string. Those air molecules, in turn, push against other air molecules, which push against still others, creating oscillations of pressure in the air: a sound wave.
Outer space (which, for our purposes here, we will define as the universe beyond Earth's atmosphere and between planets and other stellar bodies) makes a pretty terrible medium for mechanical waves. It's a vacuum, but not a perfect one. Sound can travel through it, but not very effectively. There's plenty of matter in space "“- stars, planets, asteroids, galaxies, cosmic dust, elemental atoms, etc. "“- and it's all separated by vast distances. Even at the densest parts, there's only a few hydrogen and helium atoms in a cubic meter. If you plucked a guitar string in outer space, it would still vibrate and scraps of matter like cosmic dust and gases might be able to propagate sound waves if you got enough of the matter together, but the sound be too weak for our not-that-sensitive ears to hear.
So, Alien has it right; while it's not strictly true that sound waves can't travel through space, it is true that humans would not be able to hear those sounds. Scream all you want, no one is hear you. There are some loopholes in what we'll call "Ridley's Law," though. Among the things you could hear in space are:
"¢ Anyone talking to you via radio. Radio waves can travel through space because they're electromagnetic, not mechanical, and can travel through a vacuum. Once the radio in your spaceship or spacesuit receives the signal, it converts the signal into sound, which travels through the air in your ship or helmet to your ear.
"¢ A bump on the head. If you're floating in space wearing a spacesuit and you hit your head on something (your ship, an asteroid, whatever), the sound waves resulting from the vibration of your helmet and the object you bumped would be able to travel through your helmet and the air inside it to your ear.
If you've got the right tools, you could also see sounds of a black hole. In 2002, NASA's Chandra X-ray Observatory detected a B-flat note coming from a black hole in the Perseus Galaxy Cluster, some 250 million light-years away. The note is 57 octaves below a piano's Middle C. That's far too low for us to hear. NASA didn't hear the note, either -- they saw it as ripples in the cosmic gas surrounding the hole, caused by the squeezing and heating of the gas by the gravitational pressure of the clump of galaxies packed together in the cluste. They determined the pitch by calculating how far apart the ripples were, and how fast they traveled.