A world that runs on atomic time

PHYSICS · 4 MIN

How can a clock stay right for 300 million years?

No physics needed. We'll zoom from the whole planet into a time lab, inside a fountain of frozen atoms, and down to the one electron that defines every second.

8 steps · 10 quick challenges

Step 1 of 8 · A world on time

Your phone's map, your bank and the lights in your room all depend on a clock you've never seen.

Maps, phones, power grids and banks all get their time from atomic clocks, the most exact clocks ever made.

Maps need perfect clocks. GPS satellites carry atomic clocks. Each one keeps sending a simple message: the exact time right now. Your phone hears a few satellites and checks how long each message took to arrive. Radio waves move at the speed of light, so that delay tells it how far away each satellite is. If a clock were off by just a millionth of a second, your dot on the map would be 300 metres out. Like timing the gap between lightning and thunder to tell how far away a storm is.
Your phone has no atomic clock of its own. It listens to at least four satellites, and the extra one lets it fix its own cheap clock while it works out where you are.
Light travels about 30 cm in a billionth of a second, so even a tiny clock error becomes a big map error.
Hidden timekeepers. Phone masts hand your call from one to the next in tiny time slots, so they must all agree on the time. Power stations push current back and forth 50 or 60 times a second, and the whole grid has to keep in step. Banks stamp every trade with the exact time, so nobody can argue about who was first. All of them get their time from atomic clocks. Like an orchestra: hundreds of players only sound right if they follow the same conductor.
If two phone masts disagreed by even a few millionths of a second, your call could drop as you moved between them.
Power grids use the same shared time to line up readings taken far apart, so engineers can spot trouble before a blackout spreads.
One time for the whole planet. No single clock decides what time it is. About 450 atomic clocks in labs around the world send their readings to one office near Paris. It averages them into one official time, called UTC (Coordinated Universal Time). Every country sets its clocks from it. Your time zone is just UTC plus or minus a few hours. Like a class timing a race with many stopwatches and taking the average.
Averaging helps because each clock makes its own small, random mistakes. Put hundreds together and the mistakes mostly cancel out.
The steadiest clocks in the mix are a few 'fountain' clocks, like the one we're about to zoom into.

Light in a billionth of a second: ≈ 30 cm
Clocks in world time: ≈ 450

Did you know? Speed and gravity change how fast time runs (Einstein's relativity). If GPS clocks weren't corrected for it, your map would drift about 10 km a day.

For most of history, time came from the sky: a day was one turn of the Earth. But the Earth's spin wobbles a little, so it isn't a perfect clock.

Since 1967, the official second has come from atoms instead. Time labs pass it on to GPS satellites and internet servers, and your phone quietly sets itself from them.

If atomic time vanished, GPS would drift by kilometres within hours, and phone networks and power grids would slowly fall out of step.

The time on your phone came from atomic clocks through a chain of labs, satellites and networks, and arrived accurate to a tiny fraction of a second.

Step 2 of 8 · The best clock on Earth

What makes a clock tick

All of that leans on one kind of clock. To see why it's special, step into a time lab and ask: what is a clock, really?

Every clock is just something that swings at a steady rate, plus a counter. The steadier the swing, the better the clock.

A swing and a counter. Every clock has two parts: something that repeats at a steady rate, and a counter that adds up the repeats. In a grandfather clock, a 1-metre pendulum swings once a second, and gears count the swings to move the hands. The problem: on a warm day the metal rod stretches a little, and a longer pendulum swings more slowly. So the clock runs slow. Like timing a walk by counting footsteps: fine, as long as every step is the same length.
A pendulum's swing time depends mostly on its length. Galileo noticed this in the 1600s while watching a lamp sway in a church.
Even the best pendulum clocks drifted by about a second every few days, because heat, cold and wear kept changing them.
Quartz: a faster swing. Most watches use a tiny sliver of quartz, a crystal found in sand. Give quartz a small electric push and it bends; let go and it springs back. A tiny circuit keeps it vibrating 32,768 times a second and counts the vibrations. That's far steadier than a pendulum. But no two crystals are cut exactly alike, so each one ticks at its own, slightly wrong rate. Like a tuning fork humming one note, except this one hums 32,768 times a second.
A simple circuit halves 32,768 fifteen times in a row to get exactly one pulse a second, which moves the second hand.
A cheap quartz watch can drift by about 15 seconds a month, because warmth, cold and age change the crystal a little.
Atoms never vary. Here's the big idea. Everything is made of atoms, tiny building blocks far too small to see. Every atom of one kind is exactly the same as every other: a caesium atom here is identical to one on the far side of the galaxy. And each kind of atom 'rings' at one exact note. Use that note as the swing, and every clock built this way agrees. Like every copy of a song having exactly the same beat, no matter who presses play.
No factory can make two crystals perfectly alike. Nature makes atoms alike for free, and they never wear out.
The first caesium atomic clock was built in 1955 at the National Physical Laboratory in Britain.

Pendulum (1 m): 1 swing / s
Watch quartz: 32,768 / s

Did you know? The Earth is a wobbly clock too: its spin is slowly slowing, so a day today is a couple of thousandths of a second longer than a day 100 years ago.

Anything that repeats can be a clock: the Sun crossing the sky, a dripping tap, your heartbeat. A good clock repeats very steadily, and very fast.

A pendulum ticks once a second; quartz 32,768 times; caesium, as we'll see, about 9.2 billion times.

Pendulums and crystals change with heat, cold and wear, so no two ever quite agree. Atoms don't change, which is why the second itself is now defined by an atom.

The best pendulum clocks of the 1920s drifted about a second a year. Today's best atomic clocks wouldn't drift a second in billions of years.

Step 3 of 8 · The best clock on Earth

The fountain clock

So the best ticker is an atom. But you can't glue an atom to a clock face. Here's the machine that listens to them.

A fountain clock catches caesium atoms with lasers inside an empty, shielded tower, and uses them to keep an ordinary clock exactly right.

A table full of lasers. To listen to atoms, you first have to catch them, and the tool for that is light. On this table, mirrors and lenses split a laser into several beams and set their colour very exactly. Glass fibres carry the beams to the tower. Laser light is very pure: every wave is the same colour and moves in step, so it can push atoms gently and precisely. Like a pool cue for atoms: a tiny, precise nudge exactly where you aim it.
The lasers are tuned to the exact colour caesium atoms soak up, so they affect caesium and nothing else.
The light is really infrared, just beyond what our eyes can see. We draw it red so you can see it.
Empty, quiet and shielded. The tower is a tall tube with almost all the air pumped out, so the atoms don't bump into anything. Around it are layers of special metal that block magnetism, even the Earth's own. That matters because magnets slightly change the note an atom rings at. Inside, nothing touches, warms or pushes the atoms unless the scientists want it to. Like a soundproof room for recording a singer who only whispers.
Any leftover air atom that bumped into a caesium atom would knock it off its note, so the tube is pumped almost completely empty.
The metal shields soak up magnetism and steer it around the inside, like a bypass taking traffic around a town.
A clock the atoms steer. The atoms don't move any clock hands themselves. In this rack, an ordinary but very steady clock ticks all the time. Over and over, the fountain checks that clock against the atoms and nudges it a little faster or slower. This corrected clock is the one that sends out official time: to the world time office, to GPS and to the internet. Like a sat-nav and a driver: the car does the moving, the sat-nav keeps it on course.
The nudges are tiny and smooth, so the official time never jumps.
If the fountain stopped for a day, the steady clock would keep going and stay almost perfectly right while scientists fixed it.

NIST-F2 drift: 1 s / 300 million years
Atom toss: ≈ 1 m

Did you know? NIST-F2, a fountain clock in Colorado, would neither gain nor lose a second in about 300 million years.

Only a few dozen fountain clocks exist, in national labs around the world. They measure the second straight from the caesium atom.

A fountain works in rounds. Every second or so it catches fresh atoms, tosses them, and asks: were the microwaves exactly in tune? It averages thousands of answers.

Its job isn't to tick. Its job is to keep an ordinary ticking clock honest.

A fountain clock spends weeks collecting rounds before its best reading is ready: patience is part of being the most accurate clock on Earth.

Step 4 of 8 · Inside the fountain

Stopping atoms with light

Let's step inside the tower. At room temperature the caesium atoms fly faster than a jet, far too fast to study. First, they have to be stopped.

Six laser beams slow a cloud of caesium atoms from jet speed to almost a standstill.

Heat is speed. Temperature is really speed. The atoms in anything warm are always jiggling or flying about, and the hotter it is, the faster they go. Caesium metal is warmed into a gas of single atoms. At room temperature each one zooms along at about 200 metres a second, faster than a passenger jet. That's far too fast: they'd cross the clock before you could study them. Like trying to read the label on a ball while it pings around a pinball machine.
Your skin feels heat because fast-moving air atoms hit it harder. 'Cold' just means the atoms are moving slowly.
The coldest anything can be is absolute zero, about −273 °C. There, atoms would have no movement left to lose.
Light that pushes. Light gives a tiny push. Each time an atom soaks up a bit of light, it gets nudged the way the light was going. The lasers are set to a colour that only atoms flying towards a beam can soak up. So every atom gets pushed back against its motion, and slows down. Six beams from every side slow all of them, whichever way they're going. Like running into a hail of ping-pong balls: whichever way you run, they hit your front.
Why only atoms flying towards a beam? Like a siren sounding higher as it rushes at you, light looks slightly bluer to an atom flying into it. The laser is set a touch too red, so only those atoms see the right colour.
Each push is tiny, but an atom can take thousands of pushes in a thousandth of a second.
Colder than space. In under a second, millions of atoms are herded into a ball about the size of a pea. They now crawl along at about a centimetre a second. That's around a millionth of a degree above absolute zero, the coldest anything can be, and far colder than outer space. Slow atoms can be studied for much longer, and that's exactly the trick the fountain uses next. Like turning a swarm of angry bees into a sleepy cloud hanging in the air.
Outer space is about 3 degrees above absolute zero. These atoms are millions of times colder.
The ball is still a very thin gas, not a solid. The atoms are far apart; they're just barely moving.

Atom speed, warm: ≈ 200 m/s
Atom speed, cooled: ≈ 1 cm/s

Did you know? Three scientists won the 1997 Nobel Prize in Physics for working out how to cool and trap atoms with laser light.

Laser cooling sounds backwards: light usually warms things. The trick is the colour. Tuned just right, the light only pushes against an atom's motion, never with it.

The six beams come in three pairs: left–right, front–back and up–down. Whichever way an atom goes, a beam pushes back, so it feels like wading through syrup.

The whole thing takes under a second, and repeats for every round of the clock.

For a split second, the ball of atoms in a fountain clock is one of the coldest places in the solar system.

Step 5 of 8 · Inside the fountain

Throwing atoms up

Now there's a still ball of cold atoms. Next, the fountain throws it upward through a little copper box full of microwaves.

The ball of atoms is tossed up through a microwave box and falls back through it. Then a laser counts how many atoms changed.

A gentle toss. The lasers give the ball a gentle push upwards, then switch off. Now nothing touches the atoms. They fly up about a metre, slow down, stop and fall back, just like a ball thrown in the air. The whole trip takes about a second. That's why it's called a fountain: the atoms rise and fall like the water in a garden fountain. Like throwing a ball straight up: it slows, stops at the top, and falls back down.
The push comes from making the beams below a little stronger than those above, so the ball drifts upwards.
Gravity does the rest, just like with any ball. Our animation is slowed down so you can follow it.
Two passes through microwaves. On the way up, and again on the way down, the atoms pass through this copper box of microwaves. These are the same kind of waves that heat food, but very weak. If the microwaves are at exactly the right pitch, many atoms change (we'll zoom in to see how). The long gap between the two passes makes this test extremely picky about the pitch. Like checking a guitar string against a tuner twice, a second apart: any mistuning shows.
The longer the gap between the two passes, the sharper the test. That's why the atoms are cooled and tossed slowly.
In older caesium clocks, warm atoms zipped through in a hundredth of a second. In a fountain they take most of a second.
Count, then correct. As the ball falls back, it crosses one more laser beam. Only the atoms that changed glow in this light, and a detector measures the glow. Lots of glow means the microwaves were in tune; less means they were slightly off. A computer then nudges the clock that makes the microwaves, and the next toss checks again, round after round, all day. Like a singer listening to a tuning fork and adjusting until both sound exactly alike.
The computer tests a pitch a tiny bit too high, then a tiny bit too low. If both glow the same, it's right in the middle of caesium's note.
One toss gives a rough answer. Thousands of tosses, averaged over days, make it incredibly exact.

Toss height: ≈ 1 m
Up and back: ≈ 1 s

Did you know? Caesium is a soft, gold-coloured metal that melts at 28.5 °C, so it would melt in your warm hand (don't try: it bursts into flame in water).

Each toss asks the atoms one yes-or-no question: 'Was that the right pitch?' Each atom answers by changing or not.

One atom's answer is a bit random, like a coin flip. But millions of atoms and thousands of tosses give a very reliable average.

The loop never stops: count, compare, nudge, toss again. It's like a thermostat keeping a room warm, just far more precise.

The atoms in a fountain clock travel slower than a lazy snail's pace when they reach the top of the toss.

Step 6 of 8 · The ticking atom

Inside a caesium atom

Every toss asks the atoms one question. To see what they're answering, zoom into a single caesium atom from the ball.

A caesium atom has a heavy centre, 54 electrons in tidy layers, and one lone electron on the outside.

A heavy centre. At the centre of every atom is a tiny, heavy core called the nucleus. Caesium's holds 55 protons, which carry a positive electric charge, and 78 neutrons, which carry none. Charge is what makes tiny things pull or push: opposites attract, and the same kinds push apart. Almost all of the atom's weight is in this core, drawn here much bigger than real. Like a pea in the middle of a football stadium: tiny, but almost all the weight.
Any atom with 55 protons is caesium. The number of protons decides what kind of atom it is.
An atom is about 100,000 times wider than its nucleus, so drawn to scale, the nucleus would be far too small to see.
Electrons in layers. Around the nucleus move electrons: tiny, light particles with a negative charge. The positive nucleus pulls them in. Caesium has 55, one for each proton. Fifty-four sit in neat, full layers called shells. Full shells are calm: their electrons pair up and cancel each other out. We draw them as faint bubbles, because electrons are more like a blur than tiny planets. Like seats in a cinema filling up row by row from the front.
Each shell holds only so many electrons. When one is full, the next electron must go further out.
Electrons are about 1,800 times lighter than a proton, so we draw them as tiny, fast dots.
One lonely electron. The 55th electron sits alone in the outer shell, far from the nucleus, with no partner. That's what makes caesium perfect for clocks. Everything inside is calm, so this one electron decides how the atom behaves. It's easy to reach, and it can change in one very exact way. Keep scrolling: the clock's tick happens between this electron and the nucleus. Like the one kid left standing in musical chairs: easy to spot, easy to reach.
Rubidium, a close cousin of caesium, also has one lone outer electron. It's used in the smaller atomic clocks on GPS satellites.
With the inner electrons all paired up, nothing blurs the lone electron's behaviour. It's like a soloist on a silent stage.

Protons: 55
Neutrons: 78

Did you know? Caesium-133 is the only stable type of caesium. Every caesium atom in every fountain clock on Earth is this very same type.

An atom is mostly empty space: a tiny nucleus in the middle, and a cloud of electrons around it.

Electric pull holds it together. The positive nucleus pulls on the negative electrons, the same kind of pull that sticks a rubbed balloon to a wall.

Caesium's lone outer electron is held so loosely that the metal reacts fiercely with air and water, so it's kept in sealed glass tubes.

If you could stand all the atoms in a single grain of sand in a line, they'd stretch hundreds of thousands of kilometres.

Step 7 of 8 · The ticking atom

The flip that defines a second

That lone electron holds the secret. It and the nucleus both act like tiny magnets, and microwaves of one exact pitch can flip one of them.

The electron and the nucleus act like two tiny magnets. Microwaves of one exact pitch flip one of them, and counting those waves gives us the second.

Two tiny magnets. Both the nucleus and the lone electron behave like tiny bar magnets, each with a north end (red) and a south end (blue). The electron's magnet can point the same way as the nucleus's, or the opposite way, and nothing in between. These two set-ups hold very slightly different amounts of energy. That tiny difference is what the whole clock is built on. Like two fridge magnets: flip one over and they go from pushing apart to snapping together.
Scientists call this property 'spin'. The particles aren't really spinning balls, but they act a bit like it, and that makes them magnetic.
Before the toss, the lasers set every atom up the same way, with the two magnets pointing opposite ways (blue).
Microwaves flip it. To flip the electron's magnet, the atom must take in exactly the energy difference between the two set-ups, no more and no less. Microwaves carry energy in packets, and the packet size depends on how many waves pass each second. Only microwaves at 9,192,631,770 waves a second have exactly the right size. At any other pitch, the atom ignores them. Like a vending machine that only takes one exact coin: anything else falls straight through.
Faster waves carry bigger energy packets. That's why ultraviolet light can burn your skin but radio waves can't.
After the flip, both magnets point the same way (orange). The counting laser makes only these flipped atoms glow.
Counting to one second. This is the tick of an atomic clock. The fountain tunes its microwaves until the most atoms flip, which means they're exactly at caesium's pitch. Then electronics count the waves. When 9,192,631,770 have passed, that's exactly one second, everywhere on Earth. Since 1967 the official second hasn't come from the Earth's spin, but from this flip.
Fast electronics divide those billions of waves down into one steady pulse each second.
The number wasn't picked to be neat. It was measured in the 1950s so the new atomic second matched the old one.

Caesium's pitch: 9,192,631,770 / s
Microwave length: ≈ 3.3 cm

Did you know? Before 1967, a second was a tiny slice of the year 1900. Now it's a count of 9,192,631,770 caesium waves.

Atoms can only hold energy in exact amounts, like a staircase with fixed steps. To climb a step, an atom must take in exactly that step's energy.

For caesium's lone electron, the two magnet set-ups are two steps very close together. The gap matches microwaves at 9,192,631,770 waves a second.

Every caesium atom is identical, so every one has the same gap. That's why caesium clocks everywhere measure exactly the same second.

Every second of your life is officially 9,192,631,770 flips' worth of caesium microwaves.

Step 8 of 8 · What's next

Clocks that feel gravity

So a second is 9,192,631,770 waves. Now pull back out to the whole Earth: clocks this good can do things nobody expected.

Atomic clocks are now good enough to feel a step up the stairs, could help steer spacecraft, and will soon get even better.

Time runs faster upstairs. Einstein worked out that gravity slows time. The closer you are to something heavy, like the Earth, the slower your clock ticks. The difference is tiny, but today's best clocks can see it. In 2010, scientists lifted one clock just 33 cm above another and measured it ticking faster. One day, clocks like this could help map hills, ice and hidden rock. Like a ball rolling slower up a hill: a clock deeper in Earth's pull runs a touch slower.
The effect is incredibly small: over a whole lifetime, your head ages only about half a millionth of a second more than your feet.
Because a clock's tick changes with height, comparing clocks could show magma rising under a volcano or sea levels changing.
Navigating deep space. Spacecraft far from Earth are steered by sending them radio signals and timing the replies, so every course check has to wait for Earth. From 2019 to 2021, NASA tested a small, tough atomic clock in space. With its own atomic clock on board, a spacecraft could time signals itself and work out where it is, without waiting for Earth. Like a hiker with their own map and watch, instead of phoning home to ask where they are.
Signals to Mars take up to 22 minutes each way, so asking Earth where you are can take most of an hour.
Clocks like this could one day let astronauts find their way on the Moon or Mars, the way your phone uses GPS.
Optical clocks: the next second. New 'optical' clocks use atoms like strontium that tick as fast as visible light waves, about 50,000 times faster than caesium. More ticks per second means finer timing. The best wouldn't be off by a second in 15 billion years, longer than the universe has existed. Scientists plan to use them to redefine the second, perhaps around 2030. Like swapping a ruler marked in centimetres for one marked in thousandths of a millimetre.
Light ticks too fast for ordinary electronics to count, so optical clocks use a special laser that steps the count down, like gears.
The atoms are held in a grid of laser light, like eggs in an egg box, so they can be watched for longer.

Smallest height seen: ≈ 1 mm
Strontium ticks: 429 trillion / s

Did you know? Your head ages slightly faster than your feet, because it sits a little higher in Earth's gravity. Over 80 years, the difference adds up to about half a millionth of a second.

Atomic clocks have got about ten times better every decade since 1955. Optical clocks are already about 100 times better than caesium fountains.

Scientists are now comparing optical clocks around the world to choose which atom should define the next second.

Better clocks mean sharper GPS, faster networks, steadier power grids and new science, all resting on atoms that never change.

The best optical clocks wouldn't lose a second if they'd started ticking at the Big Bang.

How Does an Atomic Clock Work?: How an atomic clock works