Sunday, January 18, 2009

Naval Observatory Soon Will Track Time to 100 Trillionths of Second

By John Ohab, Special to American Forces Press Service

The ultra-precise timing technology that enables NAVSTAR Global Positioning Systems and high-speed Internet communication soon may resolve the measure of time to 100 trillionths of a second, according to the world's authority in time-keeping and celestial observation.

"To know when an event occurred, you need a clock. We are that clock," said Geoff Chester, public affairs officer at the U.S. Naval Observatory, the majority contributor to the international determination of time. He explained the development of this new timing technology during yesterday's premier of the Defense Department's "Armed with Science" radio program on

For centuries, clocks have measured seconds through regular, rhythmic oscillations of a pendulum, a swinging weight susceptible to influence by factors such as gravity, temperature, and air viscosity. In the 1950s, scientists began investigating the oscillations of particular atoms as a more precise way to define the second.

"Atomic time is independent of what Earth does," Chester said. "Atomic clocks define time scales in terms of a certain number of oscillations of a certain type of atom that take place in the course of one second. The master clock at the Naval Observatory is an ensemble of dozens of these devices, and we take a weighted average of all of them to determine our base-reference time scale."

Standard atomic clocks measure microwave signals emitted from atoms as they change energy levels. Since 1967, the one-second time interval has been defined as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two energy levels of the cesium-133 atom.

"We guarantee that no two seconds that come out of here over the course of a year will differ by more than one billionth of a second," Chester said. "Our clock is so precise that it will not gain or lose one second on the order of 3 million years."

To meet the demands of technology and the needs of society, researchers at the U.S. Naval Observatory continue to develop more precise time-keeping systems. By 2010, they hope to release an operational version of their newest clock, known as a "fountain clock," which uses laser beams to induce oscillations of the rubidium atom. This rubidium fountain clock will provide a measure of time accurate to 100 trillionths of a second, about 10 to 100 times more precise than the current master clock.

"Rubidium atoms are smaller and easier to manipulate," Chester explained. "They allow us to keep a much better timescale than what we keep today."

The U.S. Naval Observatory, one of about 50 scientific laboratories concerned with time-keeping, maintains one-third of the operational atomic clocks currently deployed around the world.

In addition to its role in defining and maintaining universal time, the Naval Observatory also acts as a reference point for navigation and communications technologies that affect people's everyday lives. For instance, its ultra-precise time-keeping systems enable computer networks to rapidly and accurately transmit information, and the constellation of satellites used in GPS relies on the master clock to calculate locations on the Earth's surface.

"People ask what time is about," Chester said. "Timing is everything."