| Special
Atomic Clocks Fact File
2661/ / An early pioneering
atomic clock from Hewlett Packard (HP 5060A cesium-beam atomic
clocks) gained worldwide recognition in the 1960s as the 'flying
clocks' after they were flown from Palo Alto to Switzerland
to compare time.
2662/ Atomic Time replaced
Earth Time as the world's official scientific time standard
in 1972.
2663/ The measurement
of time is currently determined by an international consortium
based in France which averages the time from approximately
220 atomic clocks in over two dozen countries.
2664/ Radio-controlled
(atomic) time was invented by scientists at the National Institute
of Standards & Technology, an agency of the U.S Department
of Commerce. Established in 1901, NIST's main objective was
to develop & apply technology, measurments & standards.
After much trial and error through the 1940s and 1950s, Nist
completed it's first cesium atomic beam device in 1957.
2665/ The second according
to atomic time is defined as exactly 9,192,631,770 oscillations
or cycles of the cesium atom's frequency. This replaced the
old second that was defined in terms of the earth's motions.
2666/ Here’s how
an Atomic Clock gets to send its signals:
The Atomic clock sends
out a signal to a unit called a time code generator which
produces the time code.
The signal is then amplified
by powerful radio transmitters at several radio stations.
The high power signal
is now sent to an antenna using a transmission line. This
thick cable connects the transmitters to the antenna towers.
The antenna radiates
the time signal through a network of wires connected to several
antenna towers.
Finally, the radio signal
travels through the atmosphere and is received by millions
of people every day using equipment ranging from low cost
clocks & watches to sophisticated weather stations.
2667/ NIST-F1, the cesium
atomic clock at NIST's Boulder, Colorado., laboratories, is
referred to as a fountain clock because it uses a fountain-like
movement of atoms to obtain its improved reckoning of time.
Introduced in 1993 it is reckoned to be three times more accurate
then the model it replaced (the NIST-7).
2668/ The NIST-F1 will
neither gain nor lose a second in nearly 20 million years.
2669/ High-accuracy timekeeping
is critical to a number of important systems, including telecommunications
systems that require synchronization to better than 100 billionths
of a second and satellite navigation systems such as the Defence
Department's Global Positioning System where billionths of
a second are significant. Electrical power companies use synchronized
systems to accurately determine the location of faults (for
example, lightning damage) when they occur and to control
the stability of their distribution systems. In the domain
of space exploration, radio observations of distant objects
in the universe, made by widely separated receivers in a process
called long-baseline interferometry, require exceedingly good
atomic reference clocks. And navigation of probes within our
solar system depends critically on well-synchronized control
stations on earth.
2670/ Steven Chu (Stanford),
Claude Cohen-Tannoudji (College de France), and Bill Phillips
(NIST) shared the 1997 Nobel Prize in Physics for their work
on laser cooling - a key technology for modern atomic clocks.
2671/ The most accurate
clocks in the world are the new atomic fountain clocks, in
which thousands of extremely cold atoms are tossed gently
into a vacuum chamber, where they fall under gravity's pull.
Before atomic fountain clocks, the most accurate timekeepers
were clocks that measured the vibrations of atoms in a beam
flying through a vacuum chamber. Although they are extremely
accurate, such clocks suffer from errors caused by the speed
of the atoms flying through the chamber. Atomic fountain clocks
measure the atoms' vibrations at the top of the fountain,
where they are practically motionless for a fraction of a
second before they fall back down. As a result, the time measured
by such clocks is accurate to within one second in more than
thirty million years.
2672/ The first atomic
clock, invented in 1948, utilized the vibrations of ammonia
molecules. The error between a pair of such clocks, i.e.,
the difference in indicated time if both were started at the
same instant and later compared, was typically about one second
in three thousand years.
2673/ An atomic clock
powered by a hydrogen atom is accurate to 1 part in 2 quadrillion.
2674/ Atomic clocks are
quite complex, but the basic theory is simple. Like all clocks,
they are intended to make the same event happen over and over.
The repetition of this event produces a frequency, which is
intended to be as stable as possible. For example, the pendulum
in a grandfather clock swings back and forth at the same rate,
over and over. The swings of the pendulum are counted to keep
time. In a cesium oscillator, the transitions of the cesium
atom as it moves back and forth between two energy levels
are counted to keep time. The best cesium oscillators (such
as NIST-F1) can produce frequency with an uncertainty of about
1 x 10-15, which translates to a time error of about 0.1 nanoseconds
per day.
2675/ Leap years are
years with 366 days, instead of the usual 365. Leap years
are necessary because the actual length of a year is 365.242
days, not 365 days, as commonly stated. Basically, leap years
occur every 4 years, and years that are evenly divisible by
4 (2004, for example) have 366 days. This extra day is added
to the calendar on February 29th.
2676/ A leap second is
a second added to Coordinated Universal Time (UTC) to make
it agree with astronomical time to within 0.9 second.
2677/ The first leap
second was added on June 30, 1972, and they occur at a rate
of slightly less than one per year, on average. Although it
is possible to have a negative leap second (a second removed
from UTC), so far, all leap seconds have been positive (a
second has been added to UTC). Based on what we know about
the earth's rotation, it is unlikely that we will ever have
a negative leap second.
2678/ Since a millennium
is 1000 years, and the first millennium began at the start
of the year 1, it ended at the end of the year 1000. The second
millennium then began with the year 1001 and concluded at
the end of the year 2000. Therefore, the current millennium
technically began with the year 2001.
2679/ PARCS is an atomic-clock
mission scheduled to fly on the International Space Station
(ISS) in 2008. The mission, funded by NASA, involves a laser-cooled
cesium atomic clock, and a time-transfer system using Global
Positioning System (GPS) satellites. PARCS will fly concurrently
with SUMO(Superconducting Microwave Oscillator), a different
sort of clock that will be compared against the PARCS clock
to test certain theory. The objectives of the mission are
to:
- Test gravitational
theory
- Study laser-cooled atoms in microgravity
- Improve the accuracy of timekeeping on earth
2680/ In 1958 Commercial
cesium clocks became available, and cost $20,000 each.
Click on the links below for more great
facts...
More
next week... |
