Fundamental constants are the quantitative links in the network of physical theories.
An example: The theory of cavity radiation is linked with the quantum theory via Planck's constant h, with electrodynamics via the velocity of light in vacuum, and with static mechanics via the Boltzmann constant k.
The constants are not fixed by the respective theories. They must rather be determined by experiment – and that as exactly as at all possible, as the accuracy of the quantitative statements achievable in theory cannot be greater than the accuracy with which the constants are known. As exact a knowledge of the fundamental constants as possible requires as exact an experimental realization as possible of the physical units defined in the Système International d'Unités (SI) The determination of the values of the fundamental constants is thus closely connected with metrology, the science of accurate measurements, whose most important and noble task is the best possible experimental realization of the defined units.
At the same time, the fundamental constants are of special interest for metrology, as they themselves serve as ideal units or may form the ideal basis for units. Today they are already made use of for the realization of the SI units. Experiments aimed at the determination of a fundamental constant are often directly carried out at metrology institutes such as the PTB, or at least in close cooperation with such institutes.
By 1998, the Task Group of Fundamental Constants of the "Committee on Data for Science and Technology" (CODATA) of the International Council of Scientific Unions (ICSU)had every 13 years established a new set of fundamental constants, whose values are the result of multivariant adjustment calculus, and recommended it for uniform use in science and technology. Then - due to the fast scientific progress in this field and the quick opportunity of publication via the Internet - it decided to, in future, present new values at least every four years.
The current set "The 2006 CODATA Recommended Values of the Fundamental Physical Constants", was published for the first time on March 07, 2007 on the Internet page http://physics.nist.gov/constants
A selection of the most important fundamental constants can be found in a table.
More information about fundamental constants and the SI can be found in our Thematic tour "Guardian of the units".