Go further · Chapter 12 · 12a · Real-world case
How does an atomic clock work, and why does GPS need one?
The short answer (tap to open)
An atomic clock tunes a radio or laser signal until it matches the rate at which atoms switch between two energy levels, then counts the cycles. GPS satellites carry them because your position comes from signal timing: a billionth of a second off means about 30 cm off. Optical clocks are so good that scientists plan to redefine the second.
How does an atomic clock keep time?
A caesium atom has two energy levels a fixed gap apart (chapter 1). Microwaves at exactly the matching frequency, 9,192,631,770 cycles a second, make the atoms switch. The clock shines microwaves at a cloud of atoms and counts how many switch. Too high or too low and fewer switch. The electronics keep nudging the frequency towards the peak, then count its cycles. Every 9,192,631,770 cycles, one second passes.
Atoms that switched: 8%
The steps, in words
- Shine microwaves at a cloud of caesium atoms and count how many switch levels.
- Turn the frequency up or down: away from 9,192,631,770 cycles a second, fewer atoms switch.
- At the peak, the most atoms switch. Lock there.
- A feedback loop keeps nudging the frequency back to the peak as noise pushes it away.
- Count the cycles: every 9,192,631,770 of them is one second.
That feedback loop is the heart of every atomic clock.
Why does GPS need atomic clocks?
Your phone works out where you are from how long signals take to arrive from several satellites. Light travels about 30 cm in a billionth of a second. So the satellite clocks must agree to within billionths of a second, or your position drifts by metres. Each GPS satellite carries atomic clocks for exactly this reason.
They also need Einstein. A clock high above the Earth runs a little faster because gravity is weaker there, and a little slower because it's moving fast. The net effect is about 38 millionths of a second a day. Left uncorrected, GPS would drift by kilometres within a day.
What's an optical clock?
An optical clock uses atoms that switch at the frequency of visible light, about 40,000 to 120,000 times faster than caesium's microwaves, depending on the atom. More ticks per second means finer slices of time. The best optical clocks now beat the caesium standard by up to a hundred times:
- NIST aluminium ion (2025, peer reviewed): uncertainty 5.5 parts in 10¹⁹.
- JILA strontium (2024): 8.1 parts in 10¹⁹.
- Two lutetium ion clocks (Nature, 2026): about 1 part in 10¹⁹.
They're so sensitive that raising one by a few centimetres changes its rate measurably, because gravity is slightly weaker higher up. A clock becomes a height sensor.
When will the second change?
The world's measurement conference, the CGPM, aims to agree a new definition at its 2030 meeting. Scientists haven't yet chosen which atom to use, and a 2025 review found some criteria still unmet. The October 2026 meeting won't change the second; it plans the route to 2030. Meanwhile, the UK's defence laboratory Dstl and companies are taking optical clocks out of the lab and onto ships and submarines.
Check
Two quick ones
Collect
Your aha card
Count the atom's ticks, find your place on Earth.
Better clocks mean better navigation, and soon a new second.