Why can't you hear explosions in space?

The full story

The episode's narration, word for word. Numbers link to the sources below; timestamps play that moment.

Welcome to Hollywood Physics 0:00

Welcome to Hollywood Physics, where we take the movie moments we love and ask what would really happen. First up, the biggest bang in cinema. A starship blows up, the screen fills with fire, and the whole theatre shakes with the boom ¹. Now picture yourself floating out there, a few kilometres away. What would you actually hear? Nothing at all, and the reason why runs from a ticking watch in 1660 to a black hole humming a note no one will ever hear ^2,3.

The tagline that got it right 0:34

In Star Wars, in 1977, the rebels blow up the Death Star, and it goes out with a loud explosion ^1,4. So does the second one, in Return of the Jedi ¹. Two years later, Alien arrived with a tagline that got the physics exactly right: in space, no one can hear you scream ^1,5. Even so, by one science writer's count, Alien still lets its own ship, the Nostromo, go up with a loud bang ¹. Hollywood doesn't do that by accident, and we'll come back to why ^1,6.

What sound is 1:10

First, what is a sound? When something vibrates, it shoves the air next to it, squashing the molecules together, then pulls back and lets them spread out ^1,7. Those molecules shove their neighbours, and they shove theirs, so a ripple of squeezes and stretches travels outwards ¹. When it reaches you, it pushes your eardrum in and out, tiny bones pass the wobble inwards, and hair cells in your inner ear turn it into nerve signals ¹. That travelling ripple of pressure is called a longitudinal wave ⁷.

Passing the push along 1:45

So sound is a push handed from molecule to molecule ¹. That's why it travels at the speed it does: about three hundred and forty-three metres a second in air at twenty degrees ⁸. By our calculation, air molecules themselves dash about at roughly four hundred and sixty metres a second on average, so the push can't outrun its own messengers ⁸. Each molecule gets less than a ten-thousandth of a millimetre before it bumps into another, and it does that billions of times a second ^9,10. A push needs neighbours close enough to bump into.

A watch in a jar 2:22

This question is old ². In 1660, Robert Boyle published a book of experiments made with an air pump he'd built with Robert Hooke ¹¹. One of them answered a puzzle ². Athanasius Kircher and others had fixed a bell inside a glass tube, emptied the tube of air, and still heard it ring ². Some concluded that sound must travel through something subtler than air ². So Boyle hung a ticking watch inside his glass vessel by a thread, which he chose as the unlikeliest thing to carry a sound ².

The ticking fades 2:56

At first Boyle and his helpers could plainly hear the watch ². As the pump worked, the ticking grew fainter and fainter, until neither they nor some visitors who happened to be in the room could hear anything, even with their ears against the glass ². Yet they could see the second hand still moving ². Then they let the air back in, and the ticking came back ². Boyle's verdict: air is at least the principal medium of sound ².

Why the bell still rang 3:26

So why did Kircher's bell ring? It was fastened to the tube itself, so its shaking had a solid way out, through the glass. Boyle suspected a little leftover air as well ^2,12. Read today, though, his own next test points to the glass: he propped a bell on a springy stick pressed against the inside of his vessel, pumped out the air, and it hardly got any quieter ^2,12. Solids and liquids carry sound too, and usually faster ^1,13. Sound crosses water at nearly fifteen hundred metres a second, and steel at almost six thousand, about seventeen times faster than through air ¹³. Remember that loophole; we'll need it.

How empty is space? 4:08

Now take that idea to space. Space isn't perfectly empty ^14,15. Near Earth, a single cubic centimetre of it, about a thimbleful, holds just a handful of particles: protons and electrons streaming out from the Sun in the solar wind ^14,15. The same thimbleful of the air around you holds about twenty-five billion billion molecules ⁸. So the air is billions of billions of times more crowded than the space outside ^14,15. But a few particles is still something, so why can't they carry a boom?

Too far apart to pass it on 4:44

Because sound needs those bumps to happen within each wave. The notes you can hear have waves from about seventeen metres long for the deepest to under two centimetres for the highest ¹⁶. In air, more than a hundred thousand of those tiny hops between collisions fit inside even the shortest of those waves ⁹. In the solar wind near Earth, a proton typically travels about three times the distance from Earth to the Sun before collisions knock it off course ¹⁴. That's billions of times longer than the deepest note you can hear ¹⁴. There's nobody close enough to pass the push along, and nothing left to wiggle your eardrum ¹⁶.

The deepest note 5:26

But there's a twist. In September 2003, astronomers using NASA's Chandra X-ray Observatory announced that they had found sound waves in space ³. They ripple through the hot gas around the giant black hole at the centre of the Perseus galaxy cluster, about two hundred and fifty million light-years away ³. Andrew Fabian, of Cambridge's Institute of Astronomy, led the study ³. NASA described the note as a B-flat, fifty-seven octaves below middle C ³. A piano spans only about seven ³.

A wave every nine million years 6:03

How can that work, when the gas there is even thinner than the solar wind ^17,14? Scale. The ripples are about thirty-six thousand light-years apart, and they move at about eleven hundred and seventy kilometres a second, because the gas is incredibly hot ¹⁷. Divide one by the other and a single wave takes roughly nine million years to pass ¹⁷. The gas is thin, but the waves are so enormous that, on their scale, the gas still acts as a medium ¹⁷. No ear in the universe could ever hear it ³.

Why films add the boom 6:37

So why do most space battles roar ¹? One science writer put it simply: it would be unsatisfying to see an explosion and not hear anything ¹. On the original Star Trek, the producers reportedly decided that dramatic licence called for the ship to make noise in its exterior shots ⁶. Sound tells you how big and how scary something is, and films have a lot of feeling to get across.

The film that honoured silence 7:04

In 2013, Alfonso Cuarón's Gravity went the other way ^18,19. When its first trailer came out with explosions in it, he said at Comic-Con: as we know, there is no sound in space; in the film, we don't do that ²⁰. The film opens with titles that say so ¹⁸. Out in space, you hear the score, the astronauts' voices and breathing, and little else ¹⁸. It won Oscars for both sound editing and sound mixing ¹⁹.

Hearing by touch 7:34

There's one loophole, and Gravity uses it ¹⁸. When the astronaut holds a drill, Cuarón explained, she hears it through the vibrations in her hand ^21,18. The shaking travels through the tool, her glove and her suit, then through the air inside her helmet to her ears ^1,18. That's the solid path again, like Boyle's bell on its stick ^2,12. Its sound team recorded with contact microphones at car plants and hospitals ¹⁸. Touch something, and you can hear it; let go, and the silence comes back ¹⁸.

Kubrick's silence 8:10

Gravity wasn't the first ¹. In Stanley Kubrick's 2001: A Space Odyssey, released in 1968, the inside of the ships hums with machinery and alarms, but the moment the camera goes outside, the sound effects stop: there's only music, or silence ^1,22. When the astronaut Bowman blasts himself from a pod into an open airlock without his helmet, it happens in total silence, and the sound only returns as the airlock fills with air ¹. The TV series Firefly and its film, Serenity, kept space silent too ^1,23.

Probably came through the structure 8:48

And we do have ear-witnesses to a real explosion in space ²⁴. In 1970, an oxygen tank blew up in the service module of Apollo 13, on the way to the Moon ²⁴. The crew heard it ²⁴. Commander Jim Lovell later described a dull but definite bang, without much vibration ²⁴. Jack Swigert said it was just a noise, and Fred Haise felt a slight shudder ²⁴. Lovell's explanation: it probably came through the structure ²⁴.

So what would you hear? 9:19

So, back to that exploding starship. If you're a few kilometres away, you'd see the flash and hear nothing ¹. Even the gas from the blast thins out fast. In our rough toy model, a tonne of gas spread evenly is too thin to carry even the deepest note you can hear once it's a few kilometres across ^9,16. If debris actually hits your own ship, you would hear that: thuds and bangs travelling through the hull and into the air of your cabin, just as Apollo 13's crew heard their own tank blow ^13,24. In a spacesuit, it's your breathing, your radio, and whatever you're touching ¹⁸.

Why the boom stays 9:59

Even Cuarón didn't go all the way ^21,18. He said he thought about keeping everything in absolute silence, then realised he was just going to annoy the audience, so music carries the energy instead ^21,18. That's the honest answer to why space battles roar. Films aren't trying to fool you; they're trying to make you feel something, and silence is a very hard thing to feel.

Silence, then 10:24

So, no booms. Sound is a push passed from particle to particle, and space has too few particles, too far apart, to pass it on ^1,14. Far from the blast, you would hear nothing; close enough to be hit, you'd hear your own ship thud ²⁴. Hollywood knows; the boom is a choice ^6,18. But there's another bit of air that film fighters can't seem to do without. Next time: why don't spaceships bank like planes?

Sources

Every factual claim in the episode is tied to one of these. Spotted an error? Tell us.

1. Craig Freudenrich, "How Sci-fi Doesn't Work" (page "Sound in Space"), HowStuffWorks,… — web.archive.org
2. Robert Boyle, New Experiments Physico-Mechanical, Touching the Spring of the Air, and… — archive.org
3. NASA / Chandra X-ray Center, "Chandra 'Hears' A Black Hole For The First Time", press… — chandra.harvard.edu
4. Wikipedia, "Star Wars (film)" (release date; orientation only) — en.wikipedia.org
5. Wikipedia, "Alien (film)" (release date and tagline; orientation) — en.wikipedia.org
6. Wikipedia, "USS Enterprise (NCC-1701)", section on the original series' sound, citing… — en.wikipedia.org
7. R. Nave, "Transverse and Longitudinal Waves" and "Sound Waves in Air", HyperPhysics — hyperphysics.gsu.edu
8. R. Nave, "Sound Speed in Gases", HyperPhysics, Georgia State University — hyperphysics.gsu.edu
9. R. Nave, "Mean Free Path, Molecular Collisions", HyperPhysics — hyperphysics.gsu.edu
10. Wikipedia, "Mean free path", 159, 1988) — en.wikipedia.org
11. John B. West, "Robert Boyle's landmark book of 1660 with the first experiments on… — pubmed.ncbi.nlm.nih.gov
12. Robert Boyle, New Experiments Physico-Mechanicall, Touching the Spring of the Air, and… — archive.org
13. R. Nave, "Wave Speeds" (sound speed in liquids and metals, from the CRC Handbook),… — hyperphysics.gsu.edu
14. Daniel Verscharen, Kristopher G. Klein and Bennett A. Maruca, "The multi-scale nature… — arxiv.org
15. Wikipedia, "Interplanetary medium" (about 5 particles per cubic centimetre near Earth;… — en.wikipedia.org
16. R. Nave, "Sensitivity of Human Ear" and "Threshold of Hearing", HyperPhysics — hyperphysics.gsu.edu
17. A. C. Fabian et al., "A deep Chandra observation of the Perseus cluster: shocks and… — arxiv.org
18. David Bordwell, "GRAVITY, part 2: Thinking inside the box", Observations on film art,… — davidbordwell.net
19. Wikipedia, "Gravity (2013 film)" (release, Academy Awards, trailer note; orientation) — en.wikipedia.org
20. Ben Child, "Comic-Con 2013: five things we learned", The Guardian, 22 July 2013 — theguardian.com
21. Caitlin Roper, "Why Gravity Director Alfonso Cuarón Will Never Make a Space Movie… — web.archive.org
22. Wikipedia, "2001: A Space Odyssey" (release 1968; orientation only) — en.wikipedia.org
23. Wikipedia, "Firefly (TV series)" (first aired 20 September 2002; "portrays space as… — en.wikipedia.org
24. W. David Woods, Johannes Kemppanen et al., Apollo 13 Flight Journal, "Day 3: 'Houston,… — apollojournals.org

Image credits

• Alien (1979) poster · 20th Century Fox · shown for criticism and review

Researched and scripted with AI assistance, fact-checked claim by claim, with synthetic narration and diagrams drawn in code. How we make episodes.