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Revision as of 09:55, 4 February 2002 by 130.94.122.xxx (talk) (*expanded definition to include temperature and charge)(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.