Planck length
McGraw-Hill Science & Technology Dictionary:
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Planck length
(′pläŋk ′leŋkth)
(physics) The length √(Gh/2πc3) (where G is the gravitational constant, h is Planck's constant, and c is the speed of light) at which quantum fluctuations are believed to dominate the geometry of space-time; it is equal to 1.6162 × 10-35 m.
Oxford Dictionary of Units & Measures:
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Planck length
etc.
sub-atomic physics Units of length, mass, and time chosen to give normalized values for the Newtonian constant of gravitation (G), the speed of light (c) and the Planck constant over 2π(ħ). Similar to those for atomic units but seemingly very disparate in size (though perhaps understandably so
[Wilczek F. Phys. Today Vol. 52:8, 11-16 (1999)]), they are, based on the latest valuations of the underlying constants:
each value having relative standard uncertainty 7.5 × 10-4.
[Mohr P. J., Taylor B. N. CODATA Recommended Values of the Fundamental Physical Constants: 2002 (to be published)]
[Mohr P. J., Taylor B. N. Rev. Mod. Phys. Vol. 72:351-495 (2000)]
[Mohr P. Phys. Today Vol. 53:7, 11-16 (2000)]
[For latest recommended values, see
[Gross D. J. Phys. Today Vol. 42:6, 9-10 (1989)] The units are hopelessly sized for base units, but can readily be scaled - decimally for human use.
Wikipedia on Answers.com:
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Planck length
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This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2008) |
| SI units | |
|---|---|
| 16.162×10−36 m | 16.162×10−27 nm |
| Natural units | |
| 11.706 ℓS | 305.42×10−27 a0 |
| US customary / Imperial units | |
| 53.025×10−36 ft | 636.30×10−36 in |
In physics, the Planck length, denoted ℓP, is a unit of length, equal to 1.616199(97)×10−35 metres. It is a base unit in the system of Planck units. The Planck length can be defined from three fundamental physical constants: the speed of light in a vacuum, Planck's constant, and the gravitational constant.
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Contents
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Value
The Planck length 
is defined as
where 
is the speed of light in a vacuum, 
is the gravitational constant, and 
is the reduced Planck constant. The two digits enclosed by parentheses are the estimated standard error associated with the reported numerical value.[1][2]
The Planck length is about 10−20 of the diameter of a proton, and thus is an extremely small length. Measurements of electron radius showed that it is smaller than 10−20 m,[3] which is a value still 1015 larger than Planck length.
Physical significance
The physical significance of the Planck length is a topic of research. Because the Planck length is so many orders of magnitude smaller than any currently possible measurement, there is currently no way of probing this length scale directly. Research on the Planck length is therefore mostly theoretical.
In some forms of quantum gravity, the Planck length is the length scale at which the structure of spacetime becomes dominated by quantum effects, and it would become impossible to determine the difference between two locations less than one Planck length apart. The precise effects of quantum gravity are unknown; often it is suggested that spacetime might have a discrete or foamy structure at Planck length scale.
The Planck area, equal to the square of the Planck length, plays a role in black hole entropy. The value of this entropy, in units of the Boltzmann constant, is known to be given by 
, where 
is the area of the event horizon.
If large extra dimensions exist, the measured strength of gravity may be much smaller than its true (small-scale) value. In this case the Planck length would have no fundamental physical significance, and quantum gravitational effects would appear at other scales.
In string theory, the Planck length is the order of magnitude of the oscillating strings that form elementary particles, and shorter lengths do not make physical sense.[4]
In loop quantum gravity, area is quantized, and the Planck area is, within a factor of order unity, the smallest possible area value.
In doubly special relativity, the Planck length is observer-invariant.
According to the generalized uncertainty principle, the Planck length is, within a factor of order unity, the shortest measurable length.
The search of the laws of physics valid at the Planck length are a part of the search for the theory of everything.
See also
- Doubly special relativity
- Fock–Lorentz symmetry
- Orders of magnitude (length)
- Planck energy
- Planck epoch
- Planck scale
- Planck time
- Planck unit
- Max Planck
Notes
- ^ John Baez, The Planck Length
- ^ NIST, "Planck length", NIST's published CODATA constants
- ^ Dehmelt, Hans (1988). "Single Atomic Particle Forever Floating at Rest in Free Space: New Value for Electron Radius". Physica Scripta T22: 102–110. Bibcode 1988PhST...22..102D. doi:10.1088/0031-8949/1988/T22/016.
- ^ Cliff Burgess; Fernando Quevedo (November 2007). "The Great Cosmic Roller-Coaster Ride" (print). Scientific American (Scientific American, Inc.): p. 55.
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McGraw-Hill Science & Technology Dictionary
McGraw-Hill Dictionary of Scientific and Technical Terms. Copyright © 2003, 1994, 1989, 1984, 1978, 1976, 1974 by McGraw-Hill Companies, Inc. All rights reserved.
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Oxford Dictionary of Units & Measures
A Dictionary of Weights, Measures, and Units. Copyright © Donald Fenna 2002, 2004. All rights reserved.
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Mentioned in
superstring theory
(particle physics)
Planck mass
(physics)
normalized
(mathematics, physics)
» More