This is an old revision of this page, as edited by 130.94.122.xxx (talk) at 16:35, 9 February 2002 (*adding a link to page Practical Planck units, to be submitted). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.
Revision as of 16:35, 9 February 2002 by 130.94.122.xxx (talk) (*adding a link to page Practical Planck units, to be submitted)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)Planck units are a system of units of measurement based on the gravitational constant, Planck's constant, and the speed of light in a vacuum. These units have the advantage of eliminating most of the fundamental physical constants; measured in them, all three constants on which they are based have the value unity. However, they are too small for use for practical purposes, unless prefixed with large powers of ten. They also suffer from uncertainties in the measurement of some of the constants on which they are based.
The SI units are increasingly defined in terms of fundamental constants also, but unlike the Planck units their definition includes arbitrary numbers which are not powers of ten, which are present only for historical reasons.
The above is a partial description. The full set of Planck units includes the Planck temperature (used by cosmologists in describing conditions in the early universe) and the elementary charge. So the complete set is based on five (not three) fundamental physical constants: G, c, h-bar, k, and e. Here k stands for the Boltzmann temperature coefficient. The Planck units are defined by stipulating that all five constants have value one.
The NIST (National Institute of Standards and Technology) website is a convenient source of data on the commonly-recognized constants, including Planck units.
There is a description of human-scale versions of the Planck units at the Misplaced Pages article on practical Planck units.