Have you ever wondered how time is
regulated, I mean how time in its absolute sense is measured ??
In the United States, the standard of
time is regulated by the US Naval Observatory's Master Clock (USNO) and in
other countries of course there are different authorities. The effects of these
mechanisms are felt by all of us in the f¬orm of alarm clocks, computers,
answering machines and meeting schedules. In this article, we will learn a
stuff or two about atomic clocks and how they keep the world on a tick-tock
mode..
ARE THEY RADIOACTIVE ??
Atomic clocks keep time better than
any other clock. They even keep time better than the rotation of the Earth and
the movement of the stars. Without atomic clocks, GPS navigation would be
impossible, the Internet would not synchronize leaving all of us in complete
information darkness, and the position of the planets would not be known with
enough accuracy for space probes and explorer crafts to be launched and
monitored.
Atomic clocks are not radioactive.
They do not rely on atomic decay. Rather, they have an oscillating mass and a
spring, just like ordinary clocks.
The big difference between a standard
clock in your home and an atomic clock is that the oscillation in an atomic
clock is between the nucleus of an atom and the surrounding electrons. This
oscillation is not exactly a parallel to the balance wheel and hairspring of a
clockwork watch, but the fact is that both use oscillations to keep track of
passing time. The oscillation frequencies within the atom are determined by the
mass of the nucleus and the gravity and electrostatic "spring"
between the positive charge on the nucleus and the electron cloud surrounding
it..and I know all of this can knock both of ours brains off..
SHAPE, SIZE AND VARIETIES
Today, though there are different
types of atomic clocks, the principle behind all of them remains the same. The
major difference is associated with the element used and the means of detecting
when the energy level changes. The various types of atomic clocks include:
• CESIUM AC
: Employ a beam of cesium atoms. The clock separates cesium atoms of
different energy levels by magnetic field.
• HYDROGEN AC : Maintain hydrogen atoms at the
required energy level in a container with walls of a special material so that
the atoms don't lose their higher energy state too quickly.
• RUBIDIUM AC
: The simplest and most compact of all, use a glass cell of rubidium gas
that changes its absorption of light at the optical rubidium frequency when the
surrounding microwave frequency is just right.
A CESIUM AC ON WORK
Atoms have characteristic oscillation
frequencies. Perhaps the most familiar frequency is the orange glow from the
sodium in table salt if it is sprinkled on a flame. An atom will have many
frequencies, some at radio wavelength, some in the visible spectrum, and some in
between the two. Cesium 133 is the element most commonly chosen for atomic
clocks.
To turn the cesium atomic resonance
into an atomic clock, it is necessary to measure one of its transition or
resonant frequencies accurately. This is normally done by locking a crystal
oscillator to the principal microwave resonance of the cesium atom. This signal
is in the microwave range of the radio spectrum, and just happens to be at the
same sort of frequency as direct broadcast satellite signals. Engineers
understand how to build equipment in this area of the spectrum in great detail.
To create a clock, cesium is first
heated so that atoms boil off and pass down a tube maintained at a high vacuum.
First they pass through a magnetic field that selects atoms of the right energy
state; then they pass through an intense microwave field. The frequency of the
microwave energy sweeps backward and forward within a narrow range of
frequencies, so that at some point in each cycle it crosses the frequency of
exactly 9,192,631,770 Hertz (Hz, or cycles per second). The range of the
microwave generator is already close to this exact frequency, as it comes from
an accurate crystal oscillator. When a cesium atom receives microwave energy at
exactly the right frequency, it changes its energy state.
At the far end of the tube, another
magnetic field separates out the atoms that have changed their energy state if
the microwave field was at exactly the correct frequency. A detector at the end
of the tube gives an output proportional to the number of cesium atoms striking
it, and therefore peaks in output when the microwave frequency is exactly
correct. This peak is then used to make the slight correction necessary to
bring the crystal oscillator and hence the microwave field exactly on
frequency. This locked frequency is then divided by 9,192,631,770 to give the
familiar one pulse per second required by the real world.
SO WHEN WAS IT
INVENTED ??
In 1945, Columbia University physics
professor Isidor Rabi suggested that a clock could be made from a technique he
developed in the 1930s called atomic beam magnetic resonance. By 1949, the
National Bureau of Standards (NBS, now the National Institute of Standards and
Technology, NIST) announced the world’s first atomic clock using the ammonia
molecule as the source of vibrations, and by 1952 it announced the first atomic
clock using cesium atoms as the vibration source, NBS-1.
In 1999, NIST-F1 began operation with
an uncertainty of 1.7 parts in 10 to the 15th power, or accuracy to about one
second in 20 million years, making it the most accurate clock ever made (a distinction
shared with a similar standard in Paris).
MEASURING THE TIME
The correct frequency for the
particular cesium resonance is now defined by international agreement as
9,192,631,770 Hz so that when divided by this number the output is exactly 1
Hz, or 1 cycle per second.
The long-term accuracy achievable by
modern cesium atomic clocks (the most common type) is better than one second
per one million years. Hydrogen atomic clocks show a better short-term (one
week) accuracy, approximately 10 times the accuracy of cesium atomic clocks.
Therefore, the atomic clocks have increased the accuracy of time measurement
about one million times in comparison with the measurements carried out by
means of astronomical techniques.
The National Company in Massachusetts produced the first
commercial atomic clocks using cesium. Today, they are produced by various
manufacturers, including Hewlett Packard, Frequency Electronics, and FTS. New
technology continues to improve performance. The most accurate laboratory
cesium atomic clocks are thousands of times better than commercially produced
units.
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