Measurement Of Time
To measure any time interval we need a clock. We now use an atomic standard of time, which is based on the periodic vibrations produced in a cesium atom. This is the basis of the caesium clock, sometimes called atomic clock, used in the national standards. Such standards are available in many laboratories. In the caesium atomic clock, the second is taken as the time needed for \(9,192,631,770\) vibrations of the radiation corresponding to the transition between the two hyperfine levels of the ground state of caesium-133 atom. The vibrations of the caesium atom regulate the rate of this caesium atomic clock just as the vibrations of a balance wheel regulate an ordinary wristwatch or the vibrations of a small quartz crystal regulate a quartz wristwatch.
\(\begin{aligned}
&\text { Table } 2.5 \text { Range and order of time intervals }\\
&\begin{array}{|l|l|}
\hline \text { Event } & \text { time interval(s) } \\
\hline \text { Life-span of most unstable particle } & 10^{-24} \\
\hline \text { Period of x-rays } & 10^{-22} \\
\hline \text { Period of atomic vibrations } & 10^{-19} \\
\text { Period of light wave } & 10^{-15} \\
\hline \text { Life time of an excited state of an atom } & 10^{-15} \\
\text { Period of radio wave } & 10^{-8} \\
\text { Period of a sound wave } & 10^{-6} \\
\text { Wink of eye } & 10^{-3} \\
\text { Time between successive human heart beats } & 10^{-1} \\
\text { Travel time for light from moon to the Earth } & 10^{\circ} \\
\text { Travel time for light from the Sun to the Earth } & 10^{\circ} \\
\text { Time period of a satellite } & 10^{2} \\
\text { Rotation period of the Earth } & 10^{4} \\
\text { Rotation and revolution periods of the moon } & 10^{5} \\
\text { Revolution period of the Earth } & 10^{6} \\
\text { Travel time for light from nearest star } & 10^{7} \\
\text { Average human life-span } & 10^{8} \\
\text { Age of Egyptian pyramids } & 10^{9} \\
\text { Time since dinosaurs became extinct } & 10^{11} \\
\text { Age of the universe } & 10^{15} \\
\hline
\end{array}
\end{aligned}\)