Measurement of Time
For measurement of any time interval, we need a clock. Any phenomenon which repeats itself after a fixed interval can serve the purpose of a clock.
Atomic 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 cesium clock, sometimes called atomic clock, used in the national standards. It is highly accurate.
In Cesium clock, a second is equal to 9,192,631,770 vibrations of radiation from the transition between two hyperfine levels of cesium-133 atom.
Cesium clock works on the vibration of cesium atom which is similar to vibrations of balance wheel in a regular wristwatch and quartz crystal in a quartz wristwatch.
National standard time and frequency is maintained by 4 atomic clocks. Indian standard time is maintained by a Cesium clock at National Physical Laboratory (NPL), New Delhi.
- Cesium clocks are very accurate and the uncertainty is very low 1 part in 1013 which means not more than 3 μs are lost or gained in a year.
Range and order of time intervals:
Event Time interval(s) Life-span of most unstable particle 10-24 Time required for light to cross a nuclear distance 10-22 Period of X-rays 10-19 Period of atomic vibrations 10-15 Period of light wave 10-15 Life time of an excited state of an atom 10-8 Period of radio wave 10-6 Period of a sound wave 10-3 Wink of eye 10-1 Time between successive human heart beats 100 Travel time for light from moon to the Earth 100 Travel time for light from the Sun to the Earth 102 Time period of a satellite 104 Rotation period of the Earth 105 Rotation and revolution periods of the moon 106 Revolution period of the Earth 107 Travel time for light from the nearest star 108 Average human life-span 109 Age of Egytian pyramids 1011 Time since dinosuars became extinct 1015 Age of the universe 1017