The sounds of the universe

Everyone knows that in space no-one can hear you scream.

We all remember Alien, after all – and more recently, watched Sandra Bullock float through a terrifyingly blank silence in Gravity. Even SpaceX’s Starman, currently orbiting somewhere above us, wouldn’t be able to hear the voice of David Bowie blasting from his Tesla Roadster. (The soundtrack to the footage was overlaid by engineers on the ground, alas.)

But in fact, although we can’t hear anything the traditional way in space, it’s not actually as silent as we think. Pretty mind-blowingly, it turns out that the universe has its own soundtrack – if only you have the right instruments to listen.

So buckle yourself in for a jet-fuelled journey through the weird and wonderful ways of sound in space. It’s just as trippy as you’d imagine.

The silence of space
Let’s start with the science. It’s pretty simple: sound is transmitted – at least on Earth – as a mechanical, physical wave through a medium (like air, or water). Hit the membrane of a drum, for example, and the mechanical vibrations push against the molecules of the air, which jostle other molecules of the air, and others beyond them, and so on. It’s like an atomic Mexican wave, essentially, if only a little more co-ordinated.

But in space, those atoms are r e a l l y far apart. As you might remember from high school, space is, quite literally, space – a vacuum with just the occasional hydrogen or helium atom, and very occasionally, solids like cosmic dust, asteroids or planets. That means that the same vibrating drum would still jostle the atoms nearby, but only very weakly, and not enough to create a soundwave that human ears could hear. Basically, it’d be a Mexican wave in a nearly-empty stadium. Or in other words, silence.

Sounds that you can’t hear
That all sounds pretty logical, right? But what about the kinds of sound you can’t hear?

Nope, that’s not a typo. For centuries, philosophers have debated the idea that certain sounds only exist in a heavenly realm, inaudible to the human ear.

According to ancient Greek philosopher Pythagoras (who you may also remember from high school), the sun, moon and planets all emit their own ‘celestial sounds’ based on their unique orbits. These sounds – or ‘harmonies’ – were in perfect mathematic proportion, just like musical intervals – hence the Latin name for the theory, musica universalis. Though they existed in a purely theoretical realm, apparently Pythagoras could literally hear them, thanks to the blessings of the Egyptian god Thoth (yes, really) – he was a pretty mystical guy.

Later on, Pythagoras’ theories were taken up by Renaissance astronomers like Johannes Kepler, who discovered that the velocity of a planet at certain points in its orbit could be expressed as a musical interval – a major third, minor fifth or semitone, for example. The effect was that of a ‘celestial choir’ made up of bass (Saturn and Jupiter), tenor (Mars), alto (Venus and Earth) and soprano (Mercury) voices. Pretty impressively, even though Kepler was working without modern telescopes or calculators he figured out something pretty incredible: the dynamics of the solar system mirror, almost perfectly, the laws of musical harmony.

Hearing the inaudible
Of course, astrophysics has moved on quite a bit from mystical resonances and celestial choirs – most Renaissance philosophers still believed the Earth was the centre of the universe, and of course now we all know that Hollywood celebrities are.

But amazingly, some of NASA’s recent research would have sounded (literally) pretty familiar to Kepler. In 2013, when the Voyager spacecraft reached the edge of our solar system, it detected a burst of mysterious electromagnetic waves. These waves could then be played back through a loudspeaker – and (at least on Earth), are most definitely audible, though more than a little creepy. Voyager then proceeded to record sounds from Mars, Jupiter and even the Sun. As soundtracks go, they’re pretty minimal and sparse – but incredibly eerie and beautiful.

So really, hearing the harmonies of the universe is just a matter of having the right, ultra-sensitive instruments pointed at the right part of the sky. Easy, right? With equipment like that on board Voyager, scientists can understand space in terms of sound – the same way that traditional telescopes allow astronomers to understand it in terms of visible light.

Or as physicist Janna Levin puts it ‘the universe has a soundtrack, and that soundtrack is played on space itself, because space can wobble like a drum.’

Sounds beyond the ear, music beyond the solar system, and spacecraft beyond Pythagoras’ wildest dreams. It’s paradoxical and perfect all at once. Bowie’s Starman would no doubt be impressed.

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