do atoms keep time? In some ways, NIST's-F1 is like
an early Chinese water clock. The Chinese clock consisted
of a wheel with a series of evenly spaced cups. As each
cup was filled with water, it became heavy enough to
trip a lever that allowed the next cup to move into
place. The wheel revolved in steps, turning gears that
moved the clock's hands and kept track of time.
Atoms are Created Equal
uses atoms instead of cups. Atoms make better timekeepers
than cups because they are all exactly the same and
behave exactly the same way.
atoms are filled with energy instead of water.
Inside the NIST-F1, a cloud of cesium atoms is tossed
up into a vertical chamber and exposed to microwave
energy (like the energy in a microwave oven). The microwaves
are tuned to a specific frequency, just as a
radio can be tuned to a specific frequency to pick up
a particular station. The atoms absorb energy best at
this resonance frequency.
Atoms: Ready to Shine
the atoms absorb the microwave energy, they change to
a higher energy, excited state. Just as the water clock
cups can hold only a certain amount of water, the atoms
can absorb only a certain amount of energy. When a laser
beam hits the atoms, only those in the excited state
respond by absorbing and re-emitting the laser light.
They shine. If the microwave energy in NIST-F1 is at
exactly the right frequency, the light given off by
the atoms when the laser is shined on them is maximized.
of Ticks per Second
our water clock, each filled cup advances the clock
by one tick. In the NIST-F1 clock, each wave peak of
the microwaves at the correct frequency -the resonance
frequency of cesium-equals one tick.
second equals the time it takes for exactly 9,192,631,770
ticks at the resonance frequency of cesium. In order
for our water clock to keep up with NIST-F1, it would
have to fill more than 9 billion cups in a second!
by William Welsh
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last updated: Apr. 05, 2010