Time

Until 1960 the standard of time was based on the mean solar day, the time interval between successive passages of the sun at its highest point across the meridian. It is averaged over an year. In 1967, an atomic standard was adopted for second, the SI unit of time. One standard second is defined as the time taken for 9 192 631 770 periods of the radiation corresponding to unperturbed transition between hyperfine levels of the ground state of cesium - 133 atom. Atomic clocks are based on this. In atomic clocks, an error of one second occurs only in 5000 years.

Time is one of the seven fundamental physical quantities in the International System of Units. Time is used to define other quantities such as velocity so defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. The operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy.

Measurement of time

We need a clock to measure any time interval. Atomic clocks provide better standard for time. Some techniques to measure time interval are given below.

Quartz clocks

The piezo-electric property of a crystal is the principle of quartz clock. These clocks have an accuracy of one second in every 109 seconds.

Atomic clocks

These clocks make use of periodic vibration taking place within the atom. Atomic clocks have an accuracy of 1 part in 1013 seconds.