What If? Lab
What if you could run at the speed of sound?
Sound travels super fast, but not as fast as light. What if you could run as fast as sound? You might make a big BOOM! Let's imagine with Olly.
Sound zips through air at about 343 meters per second, more than 30 times faster than the fastest human sprint. Anything that goes faster than sound pushes the air into a shock wave that people hear as a sonic boom. Imagine running that fast, then meet the jets that really do it.
At 20 °C sound travels at 343 m/s. A source moving faster than that outruns its own waves, which pile up into a cone-shaped shock wave heard on the ground as a sonic boom. Explore Mach numbers, why sound arrives only after a supersonic object passes, and NASA's work on quieter supersonic flight.
The big question
When you clap, the sound zooms through the air to someone's ears. Sound is very fast!
What if you could run as fast as sound? What would people hear?
Sound is a wave that travels through air, and it's fast: about 343 meters every second on a mild day.
No person can run anywhere near that fast. But what if you could? What would your friends hear as you raced past, and what would happen to the air around you? Planes have actually done it, so we can find out.
Sound in air is a pressure disturbance carried by molecular collisions; its speed depends on the gas and its temperature (NASA Glenn). An object's speed divided by the local speed of sound is its Mach number.
This thought experiment imagines a runner at Mach 1 and beyond. The physics of shock waves and sonic booms is established fact from OpenStax and NASA; a human runner at those speeds is an idea, used to make the physics vivid.
Explore the idea
Make a prediction
You zoom past your friend faster than sound. When does your friend hear your footsteps?
If you ran past a friend faster than the speed of sound, when would they hear you?
For an observer ahead of a supersonic source, when does the source's sound arrive?
- Before you reach them
- Only after you've passed
- At the same time they see you start
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Established fact
Things faster than sound make a big BOOM called a sonic boom.
Objects faster than sound push the air into a shock wave that we hear as a sonic boom.
Supersonic sources create a conical shock wave heard as a sonic boom as it sweeps the ground (OpenStax).
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Established fact
Sound is much faster than any runner.
Sound travels about 343 meters per second in air at 20 °C, about 33 times the average speed of the 100-meter world record.
343 m/s at 20 °C (OpenStax) versus 10.44 m/s for the 9.58 s 100 m record (Guinness).
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An idea scientists discuss
Running as fast as sound is only pretend.
No person can run at the speed of sound; what it would be like is an idea built on real physics.
A human at Mach 1 is hypothetical; the shock-wave physics is real, the runner is not.
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Established fact
Sound travels about 343 meters every second in air.
In air at 20 °C, sound travels at 343 meters per second; at 0 °C, 331 meters per second.
v_sound = 343 m/s at 20.0 °C and 331 m/s at 0 °C, a less than 4% change (OpenStax).
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Established fact
The fastest 100-meter run ever took less than 10 seconds.
Usain Bolt ran 100 meters in 9.58 seconds in Berlin in 2009, the fastest time on record.
Bolt's 9.58 s (Berlin, 16 August 2009) averages 10.44 m/s (Guinness World Records).
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Established fact
Something going as fast as sound catches up with its own sound waves.
A source moving at the speed of sound keeps up with the waves it makes in front of it, so they pile up.
At the speed of sound, each wave ahead interferes with the previous one, and an observer receives them all at once (OpenStax).
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Established fact
Air pushes back harder the faster you go.
For big objects like cyclists and cars, air's push grows with the square of speed: twice as fast means about four times the push.
For large objects not moving too slowly, drag F_D ∝ v² (OpenStax).
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An idea scientists discuss
A person could not really run that fast. The air would push back too hard.
A runner at the speed of sound might feel more than a thousand times the air push of a sprint, so no human could do it without a vehicle.
Scaling v² from 10 to 343 m/s gives over 1,000 times the drag; a human runner at Mach 1 is not physically plausible (hypothetical).
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Established fact
You don't hear a supersonic plane until it has gone past.
If something moves faster than sound, you hear nothing from it until it has passed you.
For a supersonic source, no sound reaches an observer ahead until the source has passed (OpenStax).
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Established fact
A sonic boom is heard wherever the shock wave sweeps over.
A sonic boom isn't only heard when a plane first 'breaks the sound barrier'; it's heard wherever the shock wave sweeps along the ground.
The boom occurs as the shock wave sweeps the ground, not just at the transition through Mach 1; nose and tail shocks can give two booms (OpenStax).
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An idea scientists discuss
A super-fast runner might leave a trail of booms behind them!
If you could run faster than sound, you might drag a boom along behind you, heard by everyone you passed.
A supersonic runner would trail a shock cone heard by every observer along the path (hypothetical).
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Established fact
The first plane faster than sound flew in 1947.
On October 14, 1947, the Bell X-1 became the first airplane to fly faster than sound, reaching Mach 1.06.
Bell X-1, 14 October 1947: Mach 1.06, 1,127 km/h at 13,000 m, pilot Capt. Charles 'Chuck' Yeager (Smithsonian NASM).
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Established fact
NASA's X-59 is made to go faster than sound quietly.
NASA's X-59 is designed to fly faster than sound without making a loud sonic boom.
Quesst's X-59 demonstrates supersonic flight with quieter 'thumps'; NASA will share community responses with regulators (NASA).
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Established fact
Cameras can take pictures of shock waves!
Using a special camera technique called schlieren photography, NASA photographed the shock waves of two supersonic jets flying together.
NASA captured the first air-to-air schlieren images of interacting shock waves from two T-38s flying about 30 feet apart at supersonic speed.
How fast is sound?
Sound zooms about 343 meters every second. The fastest runner goes about 10 meters every second.
So sound is more than 30 times faster than the fastest runner!
In air at 20 °C, sound travels 343 meters per second, OpenStax says; at 0 °C it's a bit slower, 331 meters per second. That's about 1,235 kilometers per hour.
The fastest 100-meter run on record, Usain Bolt's 9.58 seconds in Berlin in 2009, averaged about 10.4 meters per second. Sound is about 33 times faster than that. So when a friend shouts, the sound easily outruns you.
OpenStax gives 343 m/s at 20.0 °C and 331 m/s at 0 °C; NASA Glenn quotes about 761 mph (1,100 ft/s) at sea level on a standard day. The dependence on temperature means Mach 1 is not one fixed speed: it changes with altitude.
Usain Bolt's 100 m record of 9.58 s (Berlin, 16 August 2009, Guinness World Records) averages 10.44 m/s, about one thirty-third of the speed of sound at 20 °C.
Running at Mach 1
Imagine running as fast as sound. Your footsteps' sound could not get ahead of you!
The sound would pile up in front of you, like a big squished wave.
Now imagine you could run at 343 meters per second. Every footstep sends out a sound wave, but you'd be keeping up with your own sound. The waves in front of you would pile up on top of each other.
Go even faster and you'd outrun your own sound completely. A friend ahead of you wouldn't hear you coming at all. They'd only hear you after you'd zoomed past, and then all at once, as a boom.
At Mach 1, each wave emitted ahead is overtaken by the source as it is made, so successive crests pile up in front; OpenStax notes the observer would receive them all at the same instant. Above Mach 1, no sound reaches an observer ahead until the source has passed.
A supersonic runner is hypothetical: the drag force on large objects grows roughly with the square of speed, so at 343 m/s it would be over a thousand times that of a 10 m/s sprint. Any real human would need a vehicle to go that fast.
Why there's a boom
When something goes faster than sound, its sound waves bunch up into one big wave.
When that big wave reaches your ears, you hear BOOM!
Sound spreads out from anything that makes it in growing circles, like ripples on a pond. A fast object makes new ripples before the old ones get out of its way. Faster than sound, the ripples bunch up along a cone that trails behind the object.
That cone is a shock wave. When it sweeps over you, you hear a sonic boom. Many people think the boom happens only at the moment a plane 'breaks the sound barrier', but OpenStax explains it's heard wherever the cone sweeps along the ground.
Waves from a supersonic source spread spherically from where they were emitted while the source moves ahead; constructive interference along a cone creates a shock wave, with sin θ = 1/M (OpenStax). The faster the source, the narrower the cone.
An aircraft makes two shock waves, from its nose and its tail, which is why space shuttle landings were often heard as two distinct booms. The boom is not a one-time event at Mach 1: it is heard wherever the cone sweeps the ground. Low, close booms can break windows.
Jets that go faster than sound
In 1947, a rocket plane flew faster than sound. It was the first plane ever to do it!
Today, NASA is testing a new fast plane. It makes a quiet 'thump', not a loud boom.
Some scientists once thought nothing could fly faster than sound. On October 14, 1947, pilot Chuck Yeager proved otherwise in the Bell X-1 rocket plane, reaching about 1,127 kilometers per hour, Mach 1.06, high over California.
Today, NASA's X-59, which first flew on October 28, 2025, is test-flying as part of its Quesst mission. It's designed to fly faster than sound while making only a quiet 'thump' instead of a loud boom, and NASA will ask people on the ground what they hear.
OpenStax notes it was once argued that pressure build-up would make exceeding the speed of sound impossible. On 14 October 1947 the Bell X-1, flown by Capt. Chuck Yeager in a joint Air Force–NACA program, reached Mach 1.06 (1,127 km/h) at about 13,000 m (Smithsonian National Air and Space Museum).
NASA's Quesst mission uses the X-59 to demonstrate supersonic flight with a quieter sonic 'thump', then survey community responses to inform regulators on noise limits for supersonic flight over land. Christine Darden earlier led NASA work on predicting and reducing sonic booms (OpenStax).
Amazing facts
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Space shuttles landing used to make two booms.
When space shuttles came in to land, people often heard two booms: one from the nose and one from the tail.
Shuttle landings produced two distinct booms, separated by the time the orbiter took to pass a point (OpenStax).
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The first plane faster than sound was named after the pilot's wife.
Chuck Yeager named the Bell X-1 'Glamorous Glennis' after his wife. It's now in the Smithsonian.
The X-1 #1, 'Glamorous Glennis', flew 78 times and is held by the National Air and Space Museum.
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Space shuttles came home going 25 times faster than sound.
The Space Shuttle re-entered the atmosphere at about 25 times the speed of sound, NASA explains.
At Mach 25 re-entry, heated air becomes an ionized plasma, so the spacecraft must be insulated (NASA Glenn).
Words to know
- speed of sound
- how fast sound travels; about 343 meters per second in air at 20 °C
- Mach number
- an object's speed divided by the speed of sound; Mach 1 is the speed of sound
- supersonic
- faster than the speed of sound
- shock wave
- a sudden, strong pressure wave made when something moves faster than the waves it creates
- sonic boom
- the loud sound heard when a shock wave from a supersonic object sweeps past
Discovery challenge
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How fast does sound travel in air?
About how fast does sound travel in air at 20 °C?
What is the speed of sound in air at 20 °C?
- About 343 meters per second
- About 10 meters per second
- About 300,000 kilometers per second
Show the answer
About 343 meters per second
About 343 meters every second!
343 meters per second. 10 is a sprinter; 300,000 kilometers per second is light.
343 m/s (OpenStax); 300,000 km/s is the speed of light.
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What makes a sonic boom?
What causes a sonic boom?
A sonic boom is…
- Sound waves bunched into a shock wave
- An engine exploding
- Thunder from clouds
Show the answer
Sound waves bunched into a shock wave
Squished-up sound waves from something faster than sound.
A shock wave: sound waves bunched up by something moving faster than sound.
…the shock wave from constructive interference of waves from a supersonic source.
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When do you hear a plane that's faster than sound?
When do you hear a supersonic jet flying toward you?
For an observer ahead of a supersonic aircraft, when does its sound arrive?
- After it has passed
- Before it arrives
- Never at all
Show the answer
After it has passed
After it has zoomed past!
Only after it has passed: it outruns its own sound.
Only once the shock cone reaches the observer, after the aircraft passes.
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When did a plane first fly faster than sound?
In what year did the Bell X-1 first fly faster than sound?
When was the first supersonic flight?
- 1947
- 1903
- 1969
Show the answer
1947
In 1947!
October 14, 1947, with Chuck Yeager at the controls.
14 October 1947, Bell X-1, Mach 1.06.
Experiments to try
- Watch What If? Lab What if there were no springs? A spring stores energy when you squash or stretch it, then gives it back as it springs into shape. Imagine a world with no metal springs at all, and follow what would happen to bouncy beds, clicky pens, old-style clocks and car rides.
- Watch What If? Lab What if heat flowed from cold to hot? Drop an ice cube into hot tea and the tea cools while the ice melts: heat moves from hot to cold, all by itself. It never flows the other way on its own. Fridges and heat pumps can move heat from cold to hot, but only by using energy. Imagine a world where heat flowed backward.
- Watch Try it What If? Lab What if sound couldn’t travel through air? Sound is a push passed from particle to particle. In air it travels about 343 meters every second. Take the air away, as in empty space, and there is nothing to pass the push along, so a clap makes no sound at all. Water and steel still carry it, and much faster.
- Watch What If? Lab What if you could see sound? Sound is a wave of squeezed and stretched air that our ears can feel but our eyes can't see. Imagine seeing it, then meet the real tools, like spectrograms and special cameras, that scientists use to turn sound and moving air into pictures.
Explore next
- Watch What If? Lab What if there were no springs? A spring stores energy when you squash or stretch it, then gives it back as it springs into shape. Imagine a world with no metal springs at all, and follow what would happen to bouncy beds, clicky pens, old-style clocks and car rides.
- Watch What If? Lab What if heat flowed from cold to hot? Drop an ice cube into hot tea and the tea cools while the ice melts: heat moves from hot to cold, all by itself. It never flows the other way on its own. Fridges and heat pumps can move heat from cold to hot, but only by using energy. Imagine a world where heat flowed backward.
- Watch Try it What If? Lab What if sound couldn’t travel through air? Sound is a push passed from particle to particle. In air it travels about 343 meters every second. Take the air away, as in empty space, and there is nothing to pass the push along, so a clap makes no sound at all. Water and steel still carry it, and much faster.
- Watch What If? Lab What if you could see sound? Sound is a wave of squeezed and stretched air that our ears can feel but our eyes can't see. Imagine seeing it, then meet the real tools, like spectrograms and special cameras, that scientists use to turn sound and moving air into pictures.
- Watch Try it What If? Lab What if gravity disappeared? Gravity is the pull that makes a dropped ball fall and keeps your feet on the ground. Switch it off in our model and watch what a ball does instead, then find out why astronauts float.
- Watch Try it What If? Lab What if water were denser? Water pushes up on anything you put in it. A heavier, denser liquid pushes up harder. In our model, a block that sinks in water floats in a pretend liquid twice as dense, with only 60 percent of it under the surface.
- Watch Try it What If? Lab What if wheels were square? A square wheel on a flat road lifts its axle up and down with every corner. In our model it bounces almost 9 centimeters and loses most of its motion at each bump. On a road of curved humps, though, the same square rolls perfectly smoothly.
- Watch What If? Lab What if every surface were slippery? Friction is the grip between surfaces. It lets you walk, hold a pencil and stop a bike. Imagine a world where every surface was as slippery as ice, and follow what would happen to walking, writing and driving.
For parents & teachers
What children will learn
- Sound travels through air at about 343 meters per second (at 20 °C), far faster than any person can run.
- An object faster than sound outruns its own sound waves, which bunch into a shock wave.
- A sonic boom is heard wherever that shock wave sweeps over the ground, not only when a plane first passes Mach 1.
- Engineers have flown faster than sound since 1947 and are now working on quieter supersonic flight.
Recommended ages: 9 to 17 · About 15 minutes
Talk about it
- Why might people living under supersonic flight paths want quieter planes?
- If you saw a jet overhead but heard nothing, what might that tell you?
- Where else do you see something moving faster than the waves it makes, like a boat's V-shaped wake?
Try at home or in class
- During a thunderstorm, from indoors, count the seconds between a lightning flash and its thunder. Sound covers about 343 meters every second, so multiply to estimate how far away it struck.
- Drag a finger slowly through still water in a tub, then quickly. Compare the ripples: when your finger outruns them, you'll see a V-shaped wake, like a shock wave cone.
Key vocabulary: speed of sound, Mach number, supersonic, shock wave, sonic boom
Where these facts come from
- University Physics Volume 1, §17.2 Speed of Sound , OpenStax
- University Physics Volume 1, §17.8 Shock Waves , OpenStax
- University Physics Volume 1, §6.4 Drag Force and Terminal Speed , OpenStax
- Speed of Sound Interactive , NASA Glenn Research Center
- Quesst Mission , NASA
- NASA's X-59 Flies for the First Time , NASA
- First Air-to-Air Images Taken of Shockwave Interaction in Flight , NASA
- Bell X-1, Glamorous Glennis , Smithsonian National Air and Space Museum
- Breaking the Barrier , NASA
- Fastest run 100 metres (male) , Guinness World Records
Checked, not yet human-reviewed. Last updated October 26, 2026. Spotted a mistake? Tell us.