| Revision as of 02:12, 10 February 2002 view source205.219.85.134 (talk)mNo edit summary← Previous edit | Revision as of 02:24, 10 February 2002 view source 205.219.85.134 (talk)mNo edit summaryNext edit → | ||
| Line 11: | Line 11: | ||
| Planck first listed his set of units (and gave values for them remarkably close to those used today) in May of 1899 in a paper presented to the Prussian Academy of Sciences. | Planck first listed his set of units (and gave values for them remarkably close to those used today) in May of 1899 in a paper presented to the Prussian Academy of Sciences. | ||
| Max Planck: 'Über irreversible Strahlungsvorgänge'. ''Sitzungsberichte der Preußischen Akademie der Wissenschaften'', vol. 5, p. 479 (1899). | Max Planck: 'Über irreversible Strahlungsvorgänge'. ''Sitzungsberichte der Preußischen Akademie der Wissenschaften'', vol. 5, p. 479 (1899). | ||
| At the time he presented the units, quantum mechanics had not been invented. He himself had not yet discovered the theory of blackbody radiation (first published December 1900) the constant h made its first appearance and for which Planck was later awarded the Nobel prize. The relevant parts of Planck's 1899 paper leave some confusion as to how he managed to come up with the units of time, length, mass, temperature etc. which today we define using h-bar and motivate by references to quantum physics ''before'' things like h-bar and quantum physics existed. | At the time he presented the units, quantum mechanics had not been invented. He himself had not yet discovered the theory of blackbody radiation (first published December 1900) in which the constant h made its first appearance and for which Planck was later awarded the Nobel prize. The relevant parts of Planck's 1899 paper leave some confusion as to how he managed to come up with the units of time, length, mass, temperature etc. which today we define using h-bar and motivate by references to quantum physics ''before'' things like h-bar and quantum physics existed. | ||
Revision as of 02:24, 10 February 2002
Planck units are a system of units of measurement based on the gravitational constant, Planck's constant, and the speed of light in 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.
Planck first listed his set of units (and gave values for them remarkably close to those used today) in May of 1899 in a paper presented to the Prussian Academy of Sciences. Max Planck: 'Über irreversible Strahlungsvorgänge'. Sitzungsberichte der Preußischen Akademie der Wissenschaften, vol. 5, p. 479 (1899). At the time he presented the units, quantum mechanics had not been invented. He himself had not yet discovered the theory of blackbody radiation (first published December 1900) in which the constant h made its first appearance and for which Planck was later awarded the Nobel prize. The relevant parts of Planck's 1899 paper leave some confusion as to how he managed to come up with the units of time, length, mass, temperature etc. which today we define using h-bar and motivate by references to quantum physics before things like h-bar and quantum physics existed.