Atomic Clocks

Scientists are building atomic clocks that keep time with mind-boggling precision. Such devices will help farmers, physicists, and interstellar travellers alike.

In John Masefield's poem, "Sea Fever," all he needs to roam the seas is "a tall ship and a star to steer her by." In fact, a good clock was just as important. Mariners who navigated by the stars needed to know when they were looking at the sky. Otherwise their charts and tables would be useless.

At the time Masefield wrote his poem, at the beginning of the 20th century, navigating by the stars had been made much more accurate by the invention of maritime chronometres. They were spring-driven clocks that, once set, kept the time within a fraction of a second each day.

In the 21st century, our ships travel much greater distances - not only from London to New York, but from Earth to Mars and beyond. As a result, the accuracy of our time pieces must be greater as well.

Modern navigators rely on atomic clocks. Instead of old-style springs or pendulums, the natural resonances of atoms - usually cesium or rubidium - provide the steady "tick" of an atomic clock. The best ones on Earth lose no more than one second in millions of years.

That's impressive, but scientists working in NASA's Fundamental Physics Program would like to do better. For those of us who mutter "just a minute" when we mean "half-an-hour," improved precision might seem overboard. Yet there are many uses for it: to test theories of gravity, for example, to guide spaceships, and to solve a surprising variety of down-to-earth problems.

Sailers, truck drivers, soldiers, hikers, and pilots ... they all rely on atomic clocks, even if they don't know it. Anyone who uses the Global Positioning System (GPS) benefits from atomic time. Each of the 24 GPS satellites carries 4 atomic clocks on board. By triangulating time signals broadcast from orbit, GPS receivers on the ground can pinpoint their own location.

Tiny instabilities in those orbiting clocks contribute at least a few metres of error to single-receiver GPS measurements. Making the clocks smaller (so that more of them can fit on each satellite) and increasing their stability could reduce such errors to fractions of a metre.

Pilots landing on narrow airstrips at night would appreciate the improvement. So would surveyors, prospectors, search and rescue teams, and farmers. "Precision farmers" already use GPS-guided tractors to dispense custom-doses of water, fertiliser and pesticides over garden-sized plots. Better GPS data could guide those tractors to individual rows or perhaps even to individual plants for special care.