Kid Asked

How does light travel faster than sound?

Tap the yellow button to begin.

For the grown-up

The story uses a thunderstorm, because lightning and thunder are where most children first notice that seeing comes before hearing. Here is what is going on underneath, where the story cuts corners, what to ask, and what to try.

What is really happening

  1. Together. A lightning flash heats a thin channel of air to about 30,000 °C in a fraction of a second. The air bursts outward, and that shove is the thunder. Flash and boom are made in the same instant.
  2. Sound is handed along. Air is a crowd of molecules flying about at roughly 500 metres a second and colliding all the time. A shove on one patch of air reaches the next patch through those collisions. The molecules end up about where they started; only the shove travels.
  3. So sound has a speed limit. A shove carried by molecules cannot outrun the molecules. In air at room temperature sound moves at 343 metres a second, about two thirds of the molecules’ own speed. A loud sound goes no faster than a quiet one.
  4. Light carries itself. Light is a wave of electric and magnetic fields. A changing electric field makes a changing magnetic field, and the other way round, so the wave keeps itself going. It needs no material, which is how sunlight crosses empty space. In empty space it moves at 299,792,458 metres a second, and nothing can go faster.

The numbers

Light is about 874,000 times faster than sound in air. In one second sound covers 343 metres, about a third of a kilometre. In the same second light could circle the Earth seven and a half times.

Counting a storm. Sound takes about three seconds for each kilometre, or five seconds for each mile. Count the seconds from flash to boom and divide by three for kilometres, by five for miles. Nine seconds means about three kilometres.

From that storm the flash reaches you in ten millionths of a second. Thunder can usually be heard up to about 16 km (10 miles) away.

Where the story stops being true

“The light is here right away.”
It takes time, far too little to notice. From the Sun it takes about eight minutes.
“Light is the fastest thing there is.”
True in empty space. Water slows light by a quarter and glass by a third. It is still hundreds of thousands of times faster than sound.
“Sound takes its time.”
That is sound in air. It goes about four times faster through water (1,493 m/s) and fifteen times faster through iron (5,130 m/s).
“Light carries itself.”
This leaves out what light is (step 4 above) and why its speed is what it is. Physics takes that speed as a given: since 1983 the metre has been defined from it.
The row of bits
Real molecules do not line up or take turns. They fly in every direction, and the bump is a slight crowding that passes through the whole crowd. Most are pairs of atoms: about four in five are nitrogen (blue in the pictures) and one in five is oxygen (red).
The jar
The story’s pump takes out every last bit of air. No real pump does. In the classroom version the bell goes quiet mainly because thin air and thick glass pass sound to each other badly; the air left inside could still carry it. The conclusion holds all the same: with no air at all there is no sound, which is why space is silent.
One bump
Thunder is millions of bumps. Every part of a long bolt makes its own, and together they give the long rumble.

Questions to ask

  • Before you start: “Which comes first, the flash or the boom? Why do you think?” Let the guess stand, then find out together.
  • “If the storm comes closer, will the wait get longer or shorter?”
  • “Could you hear a bell out in space? Could you see it?” Space has no air.
  • “What else do you see before you hear?” Fireworks, someone hammering far away, a ball kicked at the other end of a field.
  • “When you shout to me across the park, what carries your voice?”

Try it

  • Count the storm. From indoors, count from the flash to the boom. Do it again a few minutes later. Is the storm coming or going?
  • The far-off clap. In a big open space, one of you walks about a hundred big steps away and bangs two blocks of wood together overhead. The other watches the blocks meet and listens. The clack comes about a third of a second late.
  • A row of marbles. Line up six marbles touching in a groove, such as the spine of an open book. Flick one more into the end. Only the last one rolls off. The middle ones stay home.
  • Feel a sound. Hold a blown-up balloon between your hands while someone talks close to it. Your fingers feel the air bumping.

Storm safety. If you can hear thunder, lightning is close enough to strike. The US National Weather Service says to go indoors and stay there for 30 minutes after the last thunder.

Where this comes from

The comparisons (874,000 times, seven and a half times round the Earth, ten millionths of a second) are worked out from the figures above. The lightning on these pages is one soft flash at a time, never a flicker. The voice is a computer voice. With the sound off, the words stay on the page for you to read aloud.

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