Physics Notes - Herong's Tutorial Notes - v3.24, by Herong Yang
What Is Duration
This section introduces duration between two events. The standard unit of duration is called 'second', which is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.
What Is Duration? Duration is a measurement we use to describe how much time is there between two events.
In order to measure duration, we have to define a standard unit. Currently (as of 1967), the standard unit to measure duration is called "second" defined by CIPM (International Committee for Weights and Measures) as: "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom".
This definition sounds very precise. But it is based on the following assumption:
Before using radiation periods of an atom, the solar system was used to define "second" in 1956 as: "the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time".
The standard unit defined, we can create clocks and timers to measure durations between two events as shown in the picture below:
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