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KF5JRV > TECH 06.04.16 13:25l 63 Lines 3433 Bytes #999 (0) @ WW
BID : 1086_KF5JRV
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Subj: Atomic Time Standards
Path: IW8PGT<IZ3LSV<F1OYP<F4DUR<CX2SA<GB7CIP<N0KFQ<KF5JRV
Sent: 160406/1120Z 1086@KF5JRV.#NWAR.AR.USA.NA BPQ1.4.65
The "Atomic Age" of Time Standards
Scientists had long realized that atoms (and molecules) have resonances;
each chemical element and compound absorbs and emits electromagnetic
radiation at its own characteristic frequencies.
These resonances are inherently stable over time and space. An atom of
hydrogen or cesium here today is (so far as we know) exactly like one a
million years ago or in another galaxy. Thus atoms constitute a
potential "pendulum" with a reproducible rate that can form the basis
for more accurate
clocks.
The development of radar and extremely high frequency radio
communications in the 1930s and 1940s made possible the generation of
the kind of electromagnetic waves (microwaves) needed to interact with
atoms. Research aimed at developing an atomic clock focused first on
microwave resonances in the ammonia molecule. In 1949, NIST built the
first atomic clock, which was based on ammonia. However, its
performance wasn't much better than the existing standards, and
attention shifted almost immediately to more promising atomic-beam
devices based on cesium.
Laboratory cesium frequency standard The first practical cesium atomic
frequency standard was built at the National Physical Laboratory in
England in 1955, and in collaboration with the U.S. Naval Observatory
(USNO), the frequency of the cesium reference was established or measured
relative to astronomical time. While NIST was the first to start working
on a cesium standard, it wasn't until several years later that NIST
completed its first cesium atomic beam device, and soon after a second
NIST unit was built for comparison testing. By 1960, cesium standards
had been refined enough to be incorporated into the official
timekeeping system of NIST. Standards of this sort were also developed
at a number of other national standards laboratories, leading to wide
acceptance of this new timekeeping technology.
The cesium atom's natural frequency was formally recognized as the new
international unit of time in 1967: the second was defined as exactly
9,192,631,770 oscillations or cycles of the cesium atom's resonant
frequency, replacing the old second that was defined in terms of the
Earth's motions. The second quickly became the physical quantity most
accurately measured by scientists. As of January, 2002, NIST's latest
primary cesium standard was capable of keeping time to about 30
billionths of a second per year. Called NIST-F1, it is the 8th of a
series of cesium clocks built by NIST and NIST's first to operate on
the "fountain" principle.
Other kinds of atomic clocks have also been developed for various
applications; those based on hydrogen offer exceptional stability,
for example, and those based on microwave absorption in rubidium
vapor are more compact, lower in cost, and require less power.
Much of modern life has come to depend on precise time. The day is
long past when we could get by with a timepiece accurate to the
nearest quarter-hour. Transportation, communication, financial
transactions, manufacturing, electric power and many other
technologies have become dependent on accurate clocks. Scientific
research and the demands of modern technology continue to drive
the search for ever more accurate clocks. The next generation
of time standards is presently under development at NIST, USNO,
in France, in Germany, and other laboratories around the world.
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