In Space, No One Can Hear You Scream … And Other Myths
The essential criterion for sound to exist is the presence of air.
Or to put it more accurately… Sound is a wave with a measurable pressure and particle displacement traveling through an elastic medium.
An elastic medium is one where the particles of the medium (atoms and molecules) can move around, transfer energy to each other and the medium itself can compress and expand. While we cannot see it, air, liquids and solids have particles that do exactly that.
When we broaden this horizon of an elastic medium, we realize that sound can exist in water (that’s how whales and dolphins communicate) and in solids as well (that’s how we hear the sound of a guitar, when the string is plucked).
Sound that we describe above is from the point of view of a Stimulus.
But to study sound in its entirety, we also have to understand sound as a Sensation. That beautiful sensation of music, for example. It happens when this pressure wave in air enters through our ears, excites the eardrum, which in turn makes tiny bones vibrate inside a liquid, stimulates neurons which then transfer this sensation of sound to the brain. Wow!
“I told you to do the dishes!!” Yup, that’s stimulus and sensation.
The interesting part of an elastic medium is the concept of a continuum. If we were to take a particular frequency of sound as an example, let’s say 100Hz, the (wave) length of this sound is 3.4 meters. Now, molecules of air are packed together at a spacing of around 3 nano-meters, which is really, really, infinitesimally small compared to this wavelength. In other words, the wavelength is so large compared to the particle density of the medium, it sees the air as a continuum — a continuous, connected and unending elastic medium.
And even though the molecules of air are only 3 nano-meters (0.000000003 meters) apart, almost 99% of air is empty space!
Now let’s come to our favorite final frontier — Space.
We think it is a vacuum, and for most practical purposes, it is. But even this vacuum has got particles. Dark energy, Baryons and a few other Quantum particles. They are not packed so densely around each other as in air, water and metal, but they do have a density. In fact, a cubic meter of space may contain anything between 2 and 10 particles. Now that’s not really a lot, as far as the sound we know is considered, so the sound does not see this space as a continuum.
But let’s think purely in terms of wavelength and density of a medium. For an average of 5 particles per cubic meter of space to be a continuum to some wavelength, that wavelength has to be unnaturally long — think 100s of light years long!
Oh, this is not just theoretical nonsense.
Deep in space, far outside of our own galaxy, lies the Perseus cluster, a massive grouping of galaxies, and one of the largest objects yet observed. While this cluster is often described as a gas cloud, the gas itself is very rarefied. In 2003, observations found waves emanating from the supermassive black hole at the cluster’s center! Their wavelength was around 32,000 light years long, making even the seeming vacuum of space and its incredibly low particle density acceptably continuous and elastic.
As a result, even in space, this black hole can still “scream”.
And if we had ears like elephants’, maybe we could hear this intergalactic scream…
So there’s hope — in space someone may hear you scream.
And when you are falling down that black hole, doomed by St Peter to the fires of hell, scream for forgiveness all you want. Perhaps the collective sound of all you sinners might be heard by the Almighty and trigger some forgiveness.
Essential reading: https://acousticstoday.org/
Watch this beautiful simulation below of the Perseus Cluster from NASA
