Can You Hear Sound in Space? The Surprising Answer

Short answer: no, you cannot hear sound in space, at least not the way you hear it on Earth. Sound is a vibration that travels by making particles bump into each other, and space is so empty that there are almost no particles to carry it. An explosion a few metres away from you in space would be completely silent. But there are exceptions: astronauts can hear sounds through solid objects they touch, and in giant gas clouds around galaxies, real sound waves do exist, with notes far too low for any ear to hear.

"In space, no one can hear you scream," warned the poster of the 1979 film Alien. Physically, it was right. Yet most science fiction films still fill space battles with roaring engines and booming explosions. Here is what the physics really says.

What is sound, exactly?

Sound is a pressure wave. When you clap your hands, you push the air molecules next to them. Those molecules push their neighbours, which push theirs, and the disturbance spreads outward as a series of compressions and expansions. When this wave reaches your ear, it makes your eardrum vibrate, and your brain interprets the vibration as sound.

The important point is that sound needs a medium: a gas, a liquid or a solid whose particles can pass the vibration along. In air at room temperature, sound travels at about 343 metres per second. In water, it goes about four times faster, and in steel, about fifteen times faster, because the particles are packed more tightly and pass the push along more quickly.

Light is different. It is an electromagnetic wave and needs no medium at all. That is why we can see the Sun and the stars across empty space, but we cannot hear them.

Why space is silent

Space is not a perfect vacuum, but it comes very close. The air you are breathing contains about 25 billion billion molecules in every cubic centimetre. In the space between the planets, there are only a few particles per cubic centimetre. Between the stars of our galaxy, about one.

With so few particles, a pressure wave cannot form in any way your ears could detect. A particle pushed by an explosion would travel a long way before meeting another one. There is no chain of collisions, so there is no sound. We describe what this emptiness means for heat in How Cold Is Space?, and the result is the same: no particles, no transfer.

A simple experiment shows this on Earth. Place a ringing alarm clock inside a glass jar and pump the air out. You can still see the hammer striking the bell, but the sound fades until it disappears completely. This is the famous "bell in a vacuum" experiment, first performed by Robert Boyle in the 1660s.

So what would an explosion in space look like?

You would see a flash of light, then a cloud of glowing gas and debris expanding in total silence. No boom, no rumble, no shock wave through the "air". If a piece of debris hit your spacecraft, however, you would hear it, because the impact would vibrate the hull and the air inside the cabin.

A few films have shown this accurately. In 2001: A Space Odyssey (1968), scenes outside the spacecraft are silent, and the only sound is the astronaut's breathing inside his helmet.

Can astronauts hear anything during a spacewalk?

Yes, quite a lot, but not through the vacuum. An astronaut on a spacewalk hears:

• Sounds inside the suit: their own breathing, the fans of the life-support system and voices on the radio, all travelling through the air inside the helmet.
• Sounds through solid contact: if they tap a tool against the station's hull while holding it, the vibration travels through the tool, their glove and the suit, and they can hear a muffled clank. This is sound conduction through solids, the same reason you can hear a train coming by putting your ear on a rail.

Two astronauts floating side by side without touching, however, could not talk to each other directly. Even if they shouted, nothing would reach the other helmet. They must use the radio, which carries their voices as electromagnetic waves. If their helmets touched, they could hear each other faintly through the contact. Learn why astronauts float in the first place in Why Do Astronauts Float in Space?

The exception: real sound waves in space

In some places, space is not quite empty enough to be silent. Clusters of galaxies are filled with enormous amounts of hot, thin gas. It is still extremely sparse, but these clouds are so vast, millions of light-years across, that pressure waves can travel through them.

In 2003, NASA's Chandra X-ray Observatory spotted ripples in the gas around the giant black hole at the centre of the Perseus galaxy cluster, about 240 million light-years away. These ripples are genuine sound waves, created by jets of energy from the black hole. Astronomers calculated their pitch: a B-flat, about 57 octaves below middle C. One vibration takes roughly 10 million years. No ear could ever hear it, but it is real sound, travelling through a real medium.

In 2022, NASA released a "sonification" of these waves, raising their pitch by 57 to 58 octaves so that humans could hear them. The result is an eerie, haunting hum. Keep in mind that most "sounds of space" you find online are sonifications: radio, light or particle data converted into audio by scientists. They are beautiful and useful, but they are translations, not recordings of actual sound.

The sound of the early universe

There is another place where sound once filled space: the early universe itself. For the first 380,000 years after the Big Bang, the universe was a hot, dense mix of matter and light, dense enough for sound waves to ripple through it. When the universe cooled and became transparent, these waves were "frozen" into the pattern of the cosmic microwave background, the oldest light we can observe.

Astronomers study these patterns, called baryon acoustic oscillations, much as you could study a musical instrument by its notes. Their measurement tells us the age, shape and composition of the universe. In a sense, the cosmos began with a sound.

Frequently asked questions

Can you hear the Sun?
No. The Sun's surface is constantly vibrating and churning, and scientists study these vibrations (a field called helioseismology), but the sound cannot cross the near-vacuum between the Sun and the Earth.

Is space completely silent?
For human ears, yes. But very low-frequency pressure waves exist in dense gas clouds, such as around the black hole in the Perseus cluster, and they are technically sound.

How do astronauts talk in space?
By radio. Their voices travel through the air inside their helmets to a microphone, then as radio waves to the other astronauts' receivers.

Can sound travel on the Moon?
Not through the airless surface. The Apollo astronauts could only hear sounds that travelled through their suits, such as vibrations from tools they were holding or from the ground under their boots.

Are the "sounds of planets" on YouTube real?
Most are sonifications: radio or plasma waves recorded by spacecraft and converted into audible sound. A few exceptions exist, such as the real wind and sounds recorded on Mars by microphones on NASA's Perseverance rover, because Mars has a thin atmosphere.

The bottom line

Sound needs matter to travel, and space has almost none. That is why the vast majority of the universe is silent to us, and why every explosion in space happens without a sound. But physics always has a surprise in store: in the gas around distant black holes, and in the hot young universe, sound waves did and do exist, with notes so deep that they last millions of years. Space is not without music; it is simply playing it far too slowly for us to hear.

Curious about what fills "empty" space? Read The Physics of Nothing: Why Empty Space is Teeming with Energy.

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Written by Imad El Bojaddaini, university professor of physics. Physics Voice explains the strangest ideas of physics in plain language.