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Panoramic view of the clock hall at PTB with the four caesium clocks CS1, CS2, CSF1 and CSF2.

Is the second a special unit of measurement?

The SI base unit "the second" has indeed a special position among the units: It is the SI unit which has been realized by far with the highest precision - and therefore some other base units are defined or realized with reference to it. The metre, for example, has been a derived unit since 1983, defined as the length of the path travelled by light in vacuum during a time interval of 1/ 299 792 458 of a second. The realization of the volt - the unit of the electrical voltage - makes use of the Josephson effect, which links the volt with a frequency via the ratio of two fundamental constants, h/(2e) (e: elementary charge, h: Planck's constant).
Time and frequency measurements play a special role when it comes to answering fundamental questions of physics. Let us mention the development of quantum mechanics as an example from the past: This development progressed when the theoretical explanation of subtle characteristics of atomic spectra succeeded. In most cases, the experimental data were the result of frequency or wavelength measurements. Quantitative tests of the general and special theory of relativity are based on time and frequency comparisons. The earliest - although still indirect - proof of the radiation of gravitational waves in a rotating double-star system was provided by the analysis of the arrival time of the pulsar signals on Earth with respect to atomic time scales. Better clocks and means to compare them will in the future support research in many fields, and PTB is decided to play a vital role in this activity.