It sounds beautiful: Fountains of
atoms. Wouldn’t it be neat to watch the Cesium Atomic
Clock better known as NIST-F1, at work? Located at the National
Institute of Standards and Technology (NIST) in Boulder Colorado,
the atomic clock helps define Coordinated Universal Time (UTC),
the official world time.
So where’s the fountain? As the atoms move through the
cesium oscillator they are gently tossed in the air and gravity
brings them back down in a free fall “fountain”
of atoms. The movement of atoms through the oscillator can
be measured, much like a pendulum measures the time in a Grandfather
clock. The process involves moving the atoms into a ball,
tossing them, and letting gravity carry them back down through
microwaves. The microwaves affect the atoms and cause them
to glow. This process is repeated until most of the atoms
are affected then the fluorescence is measured. The cesium
oscillator is what makes ultra-precise time keeping possible.
The first atomic clock was developed in 1949. The NIST-F1
was built in less than four years by Jefferts and Meekhof
of the Time and Frequency Division of NIST’s Physics
Laboratory in Colorado. Our future is on time. Almost 20 million
years from now the atomic clock will not be off, even by one
second.
Measuring atomic time began in 1949. What did we do before
that? The Egyptian’s used sundials and they had equations
and time standards, while other cultures developed different
time measurement systems. Around 300-100 BCE, the Babylonians
used division to measure time. This is the source that we
use to measure our “modern” minutes and seconds.
In an atomic clock, the steady "tick" of an electronic oscillator is kept steady by comparing it to the natural frequency of an atom -- usually cesium-133. When a cesium atom drops from one particular energy level to another, a microwave photon emerges. The wave-like photon oscillates like a pendulum in an old-style clock. When it has oscillated precisely 9,192,631,770 times -- by
decree of the Thirteenth General Conference on Weights and Measures in 1967 -- we know that one "atomic second" has elapsed.
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