Furious because your watch is running too fast, and the clock at the bank and the voice on the telephone are telling you two different things? Better days--literally--are at hand, courtesy of the U.S. government.
The Commerce Department's National Institutes of Standards and Technology yesterday formally presented the country's newest national timepiece--the cesium fountain atomic clock, "NIST F-1," accurate to within one second every 20 million years.
The fountain clock, in residence at NIST's Boulder, Colo., laboratories, is the latest in a series of atomic clocks to serve as the nation's primary time standard since 1972.
It formally debuted Dec. 20 when it made its first report to the pool of international clocks that is used to define Coordinated Universal Time. It is the most accurate clock in the world, sharing the distinction with a similar fountain clock in France.
Which clock is better? "They're about the same," said NIST scientist Steve Jefferts, who developed the clock along with agency colleague Dawn Meekhof. "Both will get you a more accurate second."
Jefferts said the fountain clock is about three times more accurate than "NIST 7," the clock that had served as the primary standard since 1993, and more accurate than anyone in the world needs today.
"Industry may not be beating down our door right now, but in five years they will be," Jefferts said. "Our experience is that if you build a better clock, people will use it."
Throughout much of history, scientists' notion of time was tied to the somewhat variable movement of celestial bodies. During the 19th century and for a good part of the 20th century a second was defined as one 86,400th of a "mean solar day," the average time between two consecutive noons.
With the advent of the computer age after World War II, and the new requirements of industries such as telecommunications, navigation and aerospace, scientists needed a time measurement more accurate than that afforded by the heavens.
They discovered in the 1940s that the regular vibration, or "resonance," within an atom was much less changeable than the heavens, and almost immediately decided that cesium, a metal that burns in air, had properties that made it particularly useful in timekeeping devices.
In 1972 the international community redefined the second as the time it took an atom of Cesium 133 to tick through exactly 9,192,631,770 resonant cycles after it has been passed through an electromagnetic field.
Since then, the world's "primary standard" clocks have been atomic, and timekeeping accuracy has been upgraded by constructing devices better able to tabulate cesium's resonance. The devices are not "clocks," in the sense that most people understand them, Jefferts explained. "We call them 'frequency standards.' "
NIST's new device is a cylindrical vacuum chamber about 5 feet tall. A gas of cesium atoms is released inside the chamber, and six infrared lasers placed at right angles around the perimeter gently press the atoms together into a ball, slowing their movement and dropping their temperature close to absolute zero.
Next, two vertical lasers toss the ball of cesium gas gently upward through a microwave-filled cavity. Then the lasers are shut off, and gravity drops the ball back through the cavity. Jefferts compared the movement to throwing a basketball gently upward through the hoop and letting it come back down.
As the ball passes through the cavity, the microwave frequency alters some of the atoms in such a way that they will emit light, or "fluoresce," when they return to the starting point.
The "fountain" movement is repeated several times while the microwave frequency is tuned until it can make most of the atoms fluoresce. This frequency is the natural resonance frequency for the cesium atom--the definition of a second.
The NIST laboratory in Boulder is one of a handful of facilities worldwide doing state-of-the-art low-temperature research, and Jefferts explained that the key characteristic of the fountain clock is laser cooling.
By slowing the movement of the atoms, Jefferts said, the device has a much longer time--one second--to "interrogate" the atoms about their resonance. NIST 7, a high-temperature cesium clock, could interrogate the atoms for only 10 one-thousandths of a second.
The new "clock" does not run all the time, Jefferts said, but is being used several times a year to calibrate Boulder's other nine atomic clocks, which in turn are used to calibrate clocks across the country--by radio, over the Internet (for computers) or remotely (by industry, using a special device rented from NIST).
"It takes three of us full time to keep it running," Jefferts said of the new clock, and it takes up to a week to calibrate all the other clocks. The team uses four days to tune the fountain clock's microwave frequency precisely and needs the rest of the time for before and after tests, calibration and emergencies.
CAPTION: The ultra-precise "NIST F-1" cesium fountain atomic clock and its developers Dawn Meekhof, left, and Steve Jefferts.
The Cesium Fountain Clock
The nation's newest atomic clock, "NIST F-1," is accurate to within one second every 20 million years.
1. A gas of cesium atoms enters the clock's vacuum chamber. Six lasers slow the movement of the atoms and force them into a spherical cloud at the intersection of the beams.
2. The cloud is tossed upward by two lasers through a cavity filled with microwaves. All of the lasers are then turned off.
3. Gravity pulls the ball of cesium atoms back through the cavity. The microwaves partially alter the atomic states of the cesium atoms.
4. The altered cesium atoms emit light when hit by a laser beam. This light is measured by a detector. The entire process is repeated until the maximum brightness of the atoms is determined. This defines the natural vibration rate of cesium, which is used to define the second.
SOURCE: National Institute of Standards and Technology