“In space, no one can hear you scream.” The tagline from the sci-fi classic Alien is mostly true, but not entirely true. The transmission of sound requires a medium, which on Earth is usually air. Space is mostly an empty void, an icy vacuum that does not have the medium necessary for transmission of sound that humans can hear. However, space is not entirely empty, which allows for some sound transmission, if you know where to look. Read on to learn more about how sound travels in space.
Sound waves are created by an impulse that then propagates through a medium by compressing and pulling apart particles. The transmission of sound on Earth is possible due to the density of the air we breathe. An average cubic centimeter of air contains approximately 2.7×1019 atoms; space by contrast contains approximately 1 atom per cubic centimeter. While this towering disparity doesn’t allow us to converse with our space helmets off, it does allow for the transmission of certain sounds for the astute observer.
In order for sound waves to travel between these few and far particles, they have to be incredibly low in frequency, waaaay less than 1 Hz (humans can hear down to 20 Hz). This is due to what is called the mean free path, or the average distance a molecule can travel after colliding with one molecule before colliding with another. Sound can only travel through a medium if the wave is longer than the distance between particles, making the vacuum of space less than ideal for listening to a Mozart concerto.
Astronomers at NASA discovered that pressure waves sent out from the black hole at the center of the Perseus galaxy caused ripples in the surrounding hot gas cluster that could be translated into a note using a process called sonification (the translation of astronomical data into sound). This process made the sounds audible to humans for the first time, requiring scaling of 57 and 58 octaves above their true pitch. Mathematically, this is approximately 144 & 288 quadrillion (1.4×1015) times higher than their original frequency.
The Perseus galaxy is 250 million light years from Earth, but what about something in our own galactic neighborhood? Our sun is a gigantic nuclear furnace where hydrogen is fused into helium at temperatures of millions of degrees. Turns out, this process is loud. If audible sound were able to propagate through the 92 million miles of space between the Earth and sun, it would register to our ears at 100 decibels, the equivalent of a loud rock concert. Fans of the show Rick and Morty may recall the end of Season 2, The Wedding Squanchers, in which the Smith family gets a taste of what this may be like. So, the next time you’re woken up early on a Saturday by your overly eager landscaping neighbor, be thankful that it is not the sun that you are hearing!