Second
The second (s) is the duration of 9 192 631 770 periods of the radiation corresponding to the transition beween the two hyperfine levels of the ground state of the caesium 133 atom.
Time divisions in the lives of humans are based on the rotation of the earth on its own axis (day) and the orbit of the earth around the sun (year). Ever more precise measurements have shown, however, that these planetary movement are not sufficiently constant for the demands of accuracy. As a result, the unit of time is nowadays defined by an atomic process.
The starting point for this is Planck's Law E = h·n, where difference E between two energy levels of well selected atomic electrons corresponds to electromagnetic radiation at frequency n . Two levels of the caesium nuclide of atomic mass number 133 have proven to be particularly suitable, the energy difference of which corresponds to a frequency in the microwave range. After comparison of the earlier-utilised and astronomically defined second with a caesium atomic clock, the duration of the second was redefined in 1967 as 9 192 631 770 periods of the caesium radiative transition. With a caesium clock built on this principle, today an accuracy of better than 0.1 ns per day can be achieved, whereas the rotation of the earth is uncertain by up to a few ms per day.
In co-operation with 50 other time laboratories around the world, METAS contributes with its commercial atomic clocks to the so-called Universal Time Coordinated (UTC) which is calculated at the Bureau International des Poids et Mesures (BIPM) in Sèvres near Paris. The Official Swiss Time is derived from the UTC and disseminated by the HBG Time Signal Transmitter in Prangins (canton Vaud).
Moreover, METAS operates a primary frequency standard in a laboratory insulated from environmental influences: the second is according to its definition being determined meticulously by using a continuous beam of cesium atoms cooled down to -273 °C by means of laser radiation.