2.5 MEASUREMENT OF TIME

Accessible NCERT Class 11 Physics text book facilitated by Dr T K Bansal.

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 cesium clock, sometimes called atomic clock, used in the national standards. Such standards are available in many laboratories. In the cesium 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 cesium-133 atom. The vibrations of the cesium atom regulates the rate of this cesium atomic clock just as the vibrations of a balance wheel regulates an ordinary wristwatch or the vibrations of a small quartz crystal regulates a quartz wristwatch.

The cesium atomic clocks are very accurate. In principle they provide portable standard. The national standard of time interval ‘second’ as well as the frequency is maintained through four cesium atomic clocks. A cesium atomic clock is used at the National Physical Laboratory (NPL), New Delhi to maintain the Indian standard of time.

In our country, the NPL has the responsibility of maintenance and improvement of physical standards, including that of time, frequency, etc. Note that the Indian Standard Time (I S T) is linked to this set of atomic clocks. The efficient cesium atomic clocks are so accurate that they impart the uncertainty in time realisation as ± 1 × 10^−13, i.e. 1 part in 10^13. This implies that the uncertainty gained over time by such a device is less than 1 part in 10^13; they lose or gain no more than 3 µs in one year. In view of the tremendous accuracy in time measurement, the SI unit of length has been expressed in terms of the path length light travels in certain interval of time (1/ 299, 792, 458 of a second) (Table 2.1). The time interval of events that we come across in the universe vary over a very wide range. Table 2.5 gives the range and order of some typical time intervals.

You may notice that there is an interesting coincidence between the numbers appearing in Tables 2.3 and 2.5. Note that the ratio of the longest and shortest lengths of objects in our universe is about 10^41. Interestingly enough, the ratio of the longest and shortest time intervals associated with the events and objects in our universe is also about 10^41. This number, 10^41 comes up again in Table 2.4, which lists typical masses of objects. The ratio of the largest and smallest masses of the objects in our universe is about (10^41)^2. Is this a curious coincidence between these large numbers purely accidental?

Table 2.5 Range and order of time intervals