Why can't you hear anything in space?
Sound is not a thing that travels — it is a thing that happens to matter. Take the matter away and there is nothing left to do the sounding. Except, in one place, there is.
5 min read
In space, nobody can hear you scream. It is a good tagline and it is also correct, but the usual explanation of it is slightly off. Sound is not blocked by space. There is simply nothing there for sound to be made of.
Sound is something air does
When you speak, your vocal cords push the air next to them. That squeezed air pushes the air next to it, which pushes the air next to that, and a pattern of squeezes travels outward until some of them reach an eardrum and push on that too.
Every part of that needs molecules. Sound is not an object moving through air — it is a pattern in the air itself, in the same way a wave is a pattern in water rather than a thing sitting on top of it. Remove the water and you cannot have a wave. Remove the air and you cannot have a sound.
So how do astronauts talk?
By radio. Radio is an electromagnetic wave, and electromagnetic waves need no medium at all — the same reason sunlight crosses 150 million km of vacuum to reach us while sound cannot cross a metre of it.
Two astronauts on a spacewalk, floating a foot apart, cannot hear each other speak directly. Their voices go into a microphone inside the helmet, become radio, cross the gap at the speed of light, and become sound again inside the other helmet, in the air trapped there.
Touch two helmets together, though, and you can hear each other faintly through the shells. The sound travels through solid material rather than air. It works because solids carry sound too, and rather well.
Inside the station it is loud
The space station is pressurised, so sound behaves entirely normally inside it. In fact it is a noisy place — fans, pumps and air handling run constantly, and crews have used earplugs and worked on reducing the noise for years.
There is no quiet in space. There is quiet in vacuum, and the inside of a spacecraft is deliberately not vacuum.
The exception nobody expects
Space is nearly empty, but nearly is not entirely — and in a few places there is enough gas for a pressure wave to travel after all.
The Perseus galaxy cluster is one. The hot gas filling it is thin by any earthly standard, but it envelops hundreds of galaxies, and the black hole at the centre pushes pressure waves out through it. Astronomers identified them in 2003.
They found a note. It is about 57 octaves below middle C — so far below human hearing that no ear could ever detect it. In 2022 NASA extracted those waves and shifted them up by 57 octaves to make them audible for the first time.
So the honest version of the rule is this: sound needs matter, most of space has almost none, and in the rare places that do, sound exists exactly as you would expect — just unimaginably deeper than anything a person could hear.
Common questions
Why can't you hear anything in space?
Sound is a pattern of pressure travelling through matter. Space is very close to empty, so there are almost no molecules to carry that pattern. It is not that sound is blocked — there is nothing for it to be made of.
How do astronauts talk to each other in space?
By radio. Radio is an electromagnetic wave and needs no medium, so it crosses vacuum freely. Voices become radio inside the helmet and turn back into sound in the air inside the other astronaut's helmet.
Is there really no sound in space at all?
Almost none, but not quite. Where enough gas exists, pressure waves travel normally. The black hole in the Perseus galaxy cluster produces a note about 57 octaves below middle C, which NASA made audible in 2022 by shifting it up.
Can astronauts hear inside the space station?
Yes, completely normally. The station is pressurised with air, so sound behaves as it does on Earth. It is actually a noisy environment, with fans and pumps running constantly.