Natural units
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
E=m·c2 becomes simply
E=m·12 (or effectively
E=m), 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 or too large 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 G.
Planck units
Discussion
At the "Planck scales" in length, time, density, or temperature, one must consider both the effects of quantum mechanics and
general relativity. Unfortunately this requires a theory of
quantum gravity which does not yet exist.
The Planck mass is credible, indeed many
living thingss (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.\nMax Planck: 'Über irreversible Strahlungsvorgänge'.
Sitzungsberichte der Preußischen Akademie der Wissenschaften, vol. 5, p. 479 (
1899)\n \nAt 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
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 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'...
See also
\n* dimensional analysis\n*
physical constants
External link
\n*The NIST website(
National Institute of Standards and Technology) is a convenient source of data on the commonly recognized constants, including Planck units.\n*
Planck's original paper
Category:Systems of units\nCategory:Natural units
\n \n
