Atomic clock

The invention: A clock using the ammonia molecule as its oscillator
that surpasses mechanical clocks in long-term stability, precision,
and accuracy.
The person behind the invention:
Harold Lyons (1913-1984), an American physicist
Time Measurement
The accurate measurement of basic quantities, such as length,
electrical charge, and temperature, is the foundation of science. The
results of such measurements dictate whether a scientific theory is
valid or must be modified or even rejected. Many experimental
quantities change over time, but time cannot be measured directly.
It must be measured by the occurrence of an oscillation or rotation,
such as the twenty-four-hour rotation of the earth. For centuries, the
rising of the Sun was sufficient as a timekeeper, but the need for
more precision and accuracy increased as human knowledge grew.
Progress in science can be measured by how accurately time has
been measured at any given point. In 1713, the British government,
after the disastrous sinking of a British fleet in 1707 because of a miscalculation
of longitude, offered a reward of 20,000 pounds for the
invention of a ship’s chronometer (a very accurate clock). Latitude
is determined by the altitude of the Sun above the southern horizon
at noon local time, but the determination of longitude requires an
accurate clock set at Greenwich, England, time. The difference between
the ship’s clock and the local sun time gives the ship’s longitude.
This permits the accurate charting of new lands, such as those
that were being explored in the eighteenth century. John Harrison,
an English instrument maker, eventually built a chronometer that
was accurate within one minute after five months at sea. He received
his reward from Parliament in 1765.
Atomic Clocks Provide Greater Stability
A clock contains four parts: energy to keep the clock operating,
an oscillator, an oscillation counter, and a display. A grandfather
clock has weights that fall slowly, providing energy that powers the
clock’s gears. The pendulum, a weight on the end of a rod, swings
back and forth (oscillates) with a regular beat. The length of the rod
determines the pendulum’s period of oscillation. The pendulum is
attached to gears that count the oscillations and drive the display
hands.
There are limits to a mechanical clock’s accuracy and stability.
The length of the rod changes as the temperature changes, so the
period of oscillation changes. Friction in the gears changes as they
wear out. Making the clock smaller increases its accuracy, precision,
and stability. Accuracy is how close the clock is to telling the actual
time. Stability indicates how the accuracy changes over time, while
precision is the number of accurate decimal places in the display. A
grandfather clock, for example, might be accurate to ten seconds per
day and precise to a second, while having a stability of minutes per
week.
Applying an electrical signal to a quartz crystal will make the
crystal oscillate at its natural vibration frequency, which depends on
its size, its shape, and the way in which it was cut from the larger
crystal. Since the faster a clock’s oscillator vibrates, the more precise
the clock, a crystal-based clock is more precise than a large pendulum
clock. By keeping the crystal under constant temperature, the
clock is kept accurate, but it eventually loses its stability and slowly
wears out.
In 1948, Harold Lyons and his colleagues at the National Bureau
of Standards (NBS) constructed the first atomic clock, which used
the ammonia molecule as its oscillator. Such a clock is called an
atomic clock because, when it operates, a nitrogen atom vibrates.
The pyramid-shaped ammonia molecule is composed of a triangular
base; there is a hydrogen atom at each corner and a nitrogen
atom at the top of the pyramid. The nitrogen atom does not remain
at the top; if it absorbs radio waves of the right energy and frequency,
it passes through the base to produce an upside-down pyramid
and then moves back to the top. This oscillation frequency occurs
at 23,870 megacycles (1 megacycle equals 1 million cycles) per
second.
Lyons’s clock was actually a quartz-ammonia clock, since the signal
from a quartz crystal produced radio waves of the crystal’s fre-
quency that were fed into an ammonia-filled tube. If the radio
waves were at 23,870 megacycles, the ammonia molecules absorbed
the waves; a detector sensed this, and it sent no correction signal to
the crystal. If radio waves deviated from 23,870 megacycles, the ammonia
did not absorb them, the detector sensed the unabsorbed radio
waves, and a correction signal was sent to the crystal. The
atomic clock’s accuracy and precision were comparable to those of a
quartz-based clock—one part in a hundred million—but the atomic
clock was more stable because molecules do not wear out.
The atomic clock’s accuracy was improved by using cesium
133 atoms as the source of oscillation. These atoms oscillate at
9,192,631,770 plus or minus 20 cycles per second. They are accurate
to a billionth of a second per day and precise to nine decimal places.
A cesium clock is stable for years. Future developments in atomic
clocks may see accuracies of one part in a million billions.
Impact
The development of stable, very accurate atomic clocks has farreaching
implications for many areas of science. Global positioning
satellites send signals to receivers on ships and airplanes. By timing
the signals, the receiver’s position is calculated to within several
meters of its true location.
Chemists are interested in finding the speed of chemical reactions,
and atomic clocks are used for this purpose. The atomic clock
led to the development of the maser (an acronym formicrowave amplification
by stimulated emission of radiation), which is used to
amplify weak radio signals, and the maser led to the development
of the laser, a light-frequency maser that has more uses than can be
listed here.
Atomic clocks have been used to test Einstein’s theories of relativity
that state that time on a moving clock, as observed by a stationary
observer, slows down, and that a clock slows down near a
large mass (because of the effects of gravity). Under normal conditions
of low velocities and low mass, the changes in time are very
small, but atomic clocks are accurate and stable enough to detect
even these small changes. In such experiments, three sets of clocks
were used—one group remained on Earth, one was flown west around the earth on a jet, and the last set was flown east. By comparing
the times of the in-flight sets with the stationary set, the
predicted slowdowns of time were observed and the theories were
verified.