“In space no one can hear you scream.”
That now classic tagline (from Alien, one of the greatest science-fiction horror movies ever made) hinges on a big assumption that most of us broadly make: space is empty. And it is—mostly. But there is stuff out there between the stars, and in some cases there’s enough of it to make a little noise over.
So maybe we should amend that line. In space no one can hear you scream—unless, that is, you scream loudly enough and in the right place.
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What we think of as “sound” is really just a vibration that travels through some sort of material (what scientists call a medium). The music I’m listening to right now as I write these words is just such a vibration, created by electricity pulsing through magnets inside my computer’s speakers. The magnets drive a membrane that rapidly wiggles back and forth, pushing on the surrounding air. This creates waves—usually called sound waves but more technically known as acoustic waves—of slightly compressed and decompressed air that travel to my ears. And finally, within my inner ear, another membrane vibrates in response and sends signals to my brain, which interprets them as music.
Acoustic waves travel through a medium by causing the atoms or molecules in it to successively bump into each other. For my music, that medium is air, but you can also hear sounds underwater—or through solid matter if you put your ear to it. The waves travel through these materials a bit differently than they do through air because of differences in composition and density, but the principle is the same.
If space were truly empty—an utter vacuum, devoid of any matter—then yes, the Alien slogan would be unquestionably correct. And in general it’s true; by human standards, space pretty much lives up to its reputation.
Human standards are not a great basis for comparison, though. Understanding why requires some basic order-of-magnitude numerical thinking about the varying amounts of sound-sustaining stuff in space. Let’s just use the word “particle” as a generic term for this material; it can refer to any sort of individual unit of matter—an atom, a molecule, a subatomic particle, whatever.
With that in mind, let’s ask: How empty is empty? A laboratory vacuum chamber, for example, might contain a trillion particles per cubic centimeter, or cm3 (a volume of about one fourth of a typical six-sided die). That may seem like a lot, but it’s a particle density tens of millions of times less than that of the air you’re breathing, which has tens of quintillions of molecules per cm3.
Yet as relatively empty as that lab vacuum may be, space makes it look like soup. Interplanetary space is far more rarefied, with just a few dozen particles in each cubic centimeter. That thin gruel can reach up to more than a million particles per cm3 if the sun blasts out a solar storm, but even then it’s less substantial than all but a handful of ultrahigh vacuums achieved on Earth.
And the space between stars—the interstellar medium—is even thinner, with as little as 100 particles per cubic meter (m3), or, on average, 0.0001 per cm3. Intergalactic space, the truly deep space between galaxies, has an average of one—one!—particle per m3. Scream all you want; no one will hear you through that.
By now you probably appreciate that not all space is evacuated equal. In nebulas and other celestial regions, matter is thicker. A typical density for a brilliantly illuminated gas cloud like the Orion Nebula is around 10,000 particles per cm3. The density in other locations can be quite a bit higher, however. Barnard 68, for example, is a small, cold, dense molecular cloud that has roughly a million particles per cm3. That’s much lower than in a lab-grade vacuum, yet across vast expanses of space, even very low particle densities can add up, so Barnard 68’s tenuous material is still enough to absorb essentially all the light that would otherwise just pass through. Some giant molecular clouds can have dense cores that can spike to a billion particles per cm3.
Even then, your shout wouldn’t get far. There just aren’t enough particles to bump into each other to transport the acoustic wave. If you want sound to move through space, you need a much louder source that operates over vast volumes.
An exploding star, for example, blasts out huge quantities of material into space at exceedingly high speed. That ejecta slams into so much of the interstellar medium so hard that sufficient numbers of particles strike each other to make an acoustic wave.
The speed of that wave depends on the density of the medium, but in a typical nebula, it’s about 10 kilometers per second (km/s). That’s much faster than the less than 1 km/s speed of sound in Earth’s air, so it’s speedy for us Earthlings. But the material from an exploding star leaves that in the dust (so to speak)—it plows into the surrounding gas at literally supersonic speeds. This generates shock waves, much like a fighter jet emitting a sonic boom. The ambient material around the exploding star is compressed by the shock waves, creating the lovely filaments and ribbons of gas commonly seen in a supernova’s expanding cloud of debris.
Somewhat surprisingly, the speed of sound in a nebula isn’t just a matter of arcane astrophysics—it turns out to be important to our very existence here on Earth. When a dense clump of gas and dust in a molecular cloud collapses, it flattens and shapes itself into a disk around the newly forming star. A very rough estimate of a typical density for such a disk is tens to hundreds of trillions of particles per cm3, denser than a lab vacuum but extremely diluted compared with, say, air. I’d say that qualifies as “space,” but it’s still enough to sustain sound waves, which is critical. If the material is dense enough, it becomes viscous and even turbulent, allowing clumps of material to gradually grow into planets. Those conditions depend on the speed of sound within the disk, and without them, the particles there would tend to orbit the star without giving rise to planets at all.
In other words, without sound in space, we probably wouldn’t be here to talk about sound in space. That may go against conventional wisdom, but I’m willing to shout loudly enough to make my voice heard about it.