BobbyMike
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Registered: Dec 1999
Location: "Children are a gift from God, they are a reward"
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Sorry Josh.
July 31, 2001
A New Atomic Clock May Push Precision to the Next Level
By KENNETH CHANG
The world's most precise timepieces are accurate to within a few millionths of a billionth of a second, losing or gaining a second every 20 million years or so. A new type of atomic clock has the potential to do far better, to be accurate to within a second every few billion years.
Scientists at the National Institute of Standards and Technology in Boulder, Colo., have built a clock tuned to a frequency where a single atom of mercury absorbs ultraviolet light. That advance could eventually enable scientific investigations into such quixotic questions as "Does the flow of time change over time?"
The mercury clock does not yet match the accuracy of the current atomic clocks, but the institute's researchers say that with additional work, it may prove 100 to 1,000 times as accurate.
"It certainly is a very big advance for atomic clocks," said Dr. Alan A. Madej, a research scientist in the frequency and time group at the National Research Council in Ottawa. "It's quite an achievement."
A paper describing the clock will appear in the journal Science.
Every clock consists of two fundamental parts: one that generates a stream of evenly timed ticks � the swings of a pendulum in a grandfather clock, for instance � and one that counts the ticks and converts them into seconds, minutes and hours. The faster the pendulum, the more accurate the clock.
In atomic clocks, oscillations of laser light serve as the pendulum swings. The current generation of atomic clocks uses lasers that emit microwaves, a lower frequency form of light, tuned to 9,192,631,770 oscillations a second, the precise frequency that microwaves are absorbed by atoms of cesium.
In principle, a clock tuned to the higher frequencies of optical lasers would be even more accurate. "It gives you more precision," said Dr. James C. Bergquist, one of the institute's clock makers. "You would like to divide up time into smaller and smaller increments."
But until now scientists lacked the requisite second part, a counter that could keep up with optical laser "ticks" rushing in at more than 400 trillion a second. "We want to count every one of them," said Dr. Scott A. Diddams, another scientist at the National Institute of Standards and Technology. "This has been the longstanding problem of trying to make an optical clock."
To solve the counting problem, institute scientists pulled off a neat trick. They used a pulsed optical laser, where each pulse consisted of exactly 532,361 oscillations. When they nudged the laser to a lower frequency, the duration of the pulses lengthened proportionally; higher frequencies produced quicker pulses.
While the scientists still could not keep track of individual light oscillations, they could easily count the pulses, each of which bundled together 532,361 oscillations. To tune the laser, a single mercury ion, an atom with one electron removed, was cooled to a few thousandths of a degree above absolute zero, minimizing error-creating heat vibrations. Mercury ions absorb a specific frequency of ultraviolet light.
The pace of ticks coming out of the mercury clock are about 20 times as steady as those coming out of current atomic clocks, the scientists said. That does not automatically mean that the new clock is a better timepiece. The scientists are building a second mercury clock and will perform experiments to determine how stray electric and magnetic fields and other environmental conditions alter the behavior of the mercury ion.
"All of those things could give a little shift that changes the frequency of the clock," Dr. Diddams said. "Those are things you have to quantify before you claim accuracy."
They will also explore whether other atoms like calcium may be more accurate than mercury.
A clock ready for official timekeeping is still years away, but its benefits could include better precision in global positioning satellite systems that aid navigation. It could also enable physicists to test whether some properties describing atomic forces, now believed to be constant, are slowly changing as the universe expands.
More precise clocks could also enable a closer probing of Einstein's theory of general relativity, which dictates that time flows more slowly under the pull of gravity. Current atomic clocks can already detect the speeding of time when they are lifted up about one yard, where gravity is just slightly weaker.
The new clocks may be able to detect the differences in the flow of time resulting from a half-inch change in altitude.
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07-31-2001 07:27 PM
or the Jester. Whichever. 
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