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Natural units or Planck units are a natural system of units of measurement based on the fundamental constants:
The Planck units are often
semi-humorously referred to by physicists as "God's units". They eliminate all arbitrariness
from the system of units: some physicists believe that an extra-terrestrial intelligence might be expected to use the same
system.
These units have the advantage of simplifying many equations in physics by
removing conversion factors; for example, Einstein's famous equation
becomes simply (or effectively ), i.e. a body with mass = 5000 Planck Mass units will have an
intrinsic energy of 5000 Planck Energy units. For this reason, the units are popular in quantum gravity research.
However, they are too small for practical use, 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, especially of the gravitational constant .
The Planck units are:
At the "Planck scales" in length, time, or temperature, the effects of quantum physics dominate the behavior of the system. The Planck density is extremely high, above which our
ideas of gravity and quantum mechanics break down. The phenomena at distances as short as the Planck length and energies (per
particle) as large as the Planck energy (which is the most usual "Planck" observable used in particle physics) require us to find a correct theory of quantum gravity and quantum geometry.
The Planck mass is credible, indeed many living things (such as some fleas) are smaller than it; the issue is that general relativity predicts that smaller black holes
can exist within the event horizon (with a radius less than the Planck
length), while quantum mechanics predicts that the mass would probably be outside the event horizon.
Max Planck's creation of the natural units
Max 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
black-body radiation (first published December 1900) in which the Planck's Constant
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
Dirac's Constant and motivate by references to
quantum physics before things like and
quantum physics were known. Here's a quote from the 1899 paper that gives an idea of how Planck thought about the set of
units.
- ...ihre Bedeutung für alle Zeiten und für alle, auch ausserirdische und ausser menschliche Culturen nothwendig behalten
und welche daher als 'natürliche Maasseinheiten' bezeichnet werden können...
- ...These necessarily retain their meaning for all times and for all civilizations, even extraterrestrial and non-human
ones, and can therefore be designated as 'natural units'...
Other Natural units
Although not formally part of the system of Planck units, the following SI derived units are defined naturally.
See also
External link
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