Planck length

Short description: A natural unit of length

Planck length

Unit system| Planck units
Unit of| length
Symbol| ℓP
Conversions
1 ℓP in ...| ... is equal to ...
SI units|  1.616255(18)×10−35 m
natural units|  11.706 ℓS
3.0542×10−25 a0
imperial/US units|  6.3631×10−34 in

In physics, the Planck length, denoted ℓP, is a unit of length in the system of Planck units that was originally proposed by physicist Max Planck, equal to 1.616255(18)×10−35 m.[1][note 1] The Planck length can be defined from three fundamental physical constants: the speed of light, the Planck constant, and the gravitational constant. It is also the reduced Compton wavelength of a particle with Planck mass. Regardless of whether it represents some fundamental limit to the universe, it is a useful unit in theoretical physics.

## Contents

* 1 Value
* 2 History
* 3 Theoretical significance
* 4 Planck length and Euclidean geometry
* 5 See also
* 6 Note
* 7 References
* 7.1 Citations
* 7.2 Bibliography
* 8 External links

## Value

The Planck length ℓP is defined as:

[math]\displaystyle{ \ell_\mathrm{P} =\sqrt\frac{\hbar G}{c^3} \approx 1.616\;255 (18) \times 10^{-35}\ \mathrm{m} , }[/math]

where [math]\displaystyle{ c }[/math] is the speed of light, G is the gravitational constant, and ħ is the reduced Planck constant.[2][3]

The two digits enclosed by parentheses are the standard uncertainty of the reported numerical value.

The Planck length is about 10−20 times the diameter of a proton.[4] It can be defined as the reduced Compton wavelength of a black hole for which this equals its Schwarzschild radius.[5]

## History

In 1899, Max Planck suggested that there existed some fundamental natural units for length, mass, time and energy.[6][7] He derived these using dimensional analysis, using only the Newton gravitational constant, the speed of light and the Planck constant (though it was not yet called this). The modern convention is to use the reduced Planck constant in place of the Planck constant in the definition of the resulting units. The derived natural units became known as the "Planck length", the "Planck mass", the "Planck time" and the "Planck energy".

## Theoretical significance

The Planck length is approximately the size of a black hole where quantum and gravitational effects are at the same scale: where its Compton wavelength and Schwarzschild radius are approximately the same.[2] Some proposals for a theory of quantum gravity predict quantum foam appearing at the Planck scale due to fluctuations in the spacetime metric.[8]

The Planck length is expected to be the shortest measurable distance, since any attempt to investigate the possible existence of shorter distances, by performing higher-energy collisions, would inevitably result in black hole production. Higher-energy collisions, rather than splitting matter into finer pieces, would simply produce bigger black holes.[9]

The strings of string theory are modeled to be on the order of the Planck length.[10] In theories of large extra dimensions, the Planck length has no fundamental, physical significance, and quantum gravitational effects appear at other scales.[11]

## Planck length and Euclidean geometry

The Planck length is the length at which quantum zero oscillations of the gravitational field completely distort Euclidean geometry. The gravitational field performs zero-point oscillations, and the geometry associated with it also oscillates. The ratio of the circumference to the radius varies near the Euclidean value. The smaller the scale, the greater the deviations from the Euclidean geometry. Let us estimate the order of the wavelength of zero gravitational oscillations, at which the geometry becomes completely unlike the Euclidean geometry. The degree of deviation [math]\displaystyle{ \zeta }[/math] of geometry from Euclidean geometry in the gravitational field is determined by the ratio of the gravitational potential [math]\displaystyle{ \varphi }[/math] and the square of the speed of light [math]\displaystyle{ c }[/math]: [math]\displaystyle{ \zeta=\varphi/c^2 }[/math]. When [math]\displaystyle{ \zeta\ll 1 }[/math], the geometry is close to Euclidean geometry; for [math]\displaystyle{ \zeta\sim 1 }[/math], all similarities disappear. The energy of the oscillation of scale [math]\displaystyle{ l }[/math] is equal to [math]\displaystyle{ E=h\nu \sim \hbar c/l }[/math] (where [math]\displaystyle{ c/l }[/math] is the order of the oscillation frequency). The gravitational potential created by the mass [math]\displaystyle{ m }[/math], at this length is [math]\displaystyle{ \varphi=Gm/l }[/math], where [math]\displaystyle{ G }[/math] is the constant of universal gravitation. Instead of [math]\displaystyle{ m }[/math], we must substitute a mass, which, according to Einstein's formula, corresponds to the energy [math]\displaystyle{ E }[/math] (where [math]\displaystyle{ m=E/c^2 }[/math]). We get [math]\displaystyle{ \varphi=GE/l\,c^2=G\hbar/l^2c }[/math]. Dividing this expression by [math]\displaystyle{ c^2 }[/math], we obtain the value of the deviation [math]\displaystyle{ \zeta=G\hbar/c^3l^2=\ell^2_P/l^2 }[/math]. Equating [math]\displaystyle{ \zeta=1 }[/math], we find the length at which the Euclidean geometry is completely distorted. It is equal to Planck length [math]\displaystyle{ \ell_P=\sqrt{G\hbar/c^3}\approx 10^{-35}\mathrm{m} }[/math].[12]

As noted in Regge (1958) "for the space-time region with dimensions [math]\displaystyle{ l }[/math] the uncertainty of the Christoffel symbols [math]\displaystyle{ \Delta\Gamma }[/math] be of the order of [math]\displaystyle{ \ell^2_P/l^3 }[/math], and the uncertainty of the metric tensor [math]\displaystyle{ \Delta g }[/math] is of the order of [math]\displaystyle{ \ell^2_P/l^2 }[/math]. If [math]\displaystyle{ l }[/math] is a macroscopic length, the quantum constraints are fantastically small and can be neglected even on atomic scales. If the value [math]\displaystyle{ l }[/math] is comparable to [math]\displaystyle{ \ell_P }[/math], then the maintenance of the former (usual) concept of space becomes more and more difficult and the influence of micro curvature becomes obvious".[13] Conjecturally, this could imply that space-time becomes a quantum foam at the Planck scale.[14]

## See also

## Note

1. ↑ The two digits enclosed by parentheses are the estimated standard uncertainty.

## References

### Citations

1. ↑ "2018 CODATA Value: Planck length". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. http://physics.nist.gov/cgi-bin/cuu/Value?plkl. Retrieved 2019-05-20.
2. ↑ 2.0 2.1 John Baez, The Planck Length
3. ↑ "Planck length". http://physics.nist.gov/cgi-bin/cuu/Value?plkl.
4. ↑ "The Planck Length". http://www.math.ucr.edu/home/baez/planck/node2.html.
5. ↑ "What Is The Smallest Possible Distance In The Universe?". Forbes. https://www.forbes.com/sites/startswithabang/2019/06/26/what-is-the-smallest-possible-distance-in-the-universe/?sh=2b99cbc848a1.
6. ↑ M. Planck. Naturlische Masseinheiten. Der Koniglich Preussischen Akademie Der Wissenschaften, p. 479, 1899
7. ↑ Gorelik, Gennady (1992). "First Steps of Quantum Gravity and the Planck Values". http://people.bu.edu/gorelik/cGh_FirstSteps92_MPB_36/cGh_FirstSteps92_text.htm.
8. ↑ Borzeszkowski, Horst-Heino; Treder, H. J. (6 December 2012). The Meaning of Quantum Gravity. Springer Science & Business Media. ISBN 9789400938939. https://books.google.com/books?id=fPXwCAAAQBAJ&q=Bohr-Rosenfeld+uncertainty+relations&pg=PA33.
9. ↑ Bernard J. Carr and Steven B. Giddings "Quantum Black Holes", Scientific American, Vol. 292, No. 5, MAY 2005, (pp. 48-55)
10. ↑ Cliff Burgess; Fernando Quevedo (November 2007). "The Great Cosmic Roller-Coaster Ride". Scientific American (Scientific American, Inc.): p. 55.
11. ↑ "The Planck scale: relativity meets quantum mechanics meets gravity". newt.phys.unsw.edu.au. https://newt.phys.unsw.edu.au/einsteinlight/jw/module6_Planck.htm.
12. ↑ Migdal A.B., The quantum physics, Nauka, pp. 116-117, (1989)
13. ↑ T. Regge. "Gravitational fields and quantum mechanics". Nuovo Cim. 7, 215 (1958). doi:10.1007/BF02744199.
14. ↑ Wheeler, J. A. (January 1955). "Geons". Physical Review 97 (2): 511–536. doi:10.1103/PhysRev.97.511. Bibcode: 1955PhRv...97..511W.

### Bibliography

* Garay, Luis J. (January 1995). "Quantum gravity and minimum length". International Journal of Modern Physics A 10 (2): 145–165. doi:10.1142/S0217751X95000085. Bibcode: 1995IJMPA..10..145G.

## External links

* Bowley, Roger; Eaves, Laurence (2010). "Planck Length". Sixty Symbols. Brady Haran for the University of Nottingham. http://www.sixtysymbols.com/videos/plancklength.htm.

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Quantum gravity

Central concepts|

* AdS/CFT correspondence
* Causal patch
* Gravitational anomaly
* Graviton
* Holographic principle
* IR/UV mixing
* Planck scale
* Quantum foam
* Trans-Planckian problem
* Weinberg–Witten theorem

Toy models|

* 2+1D topological gravity
* CGHS model
* Jackiw-Teitelboim gravity
* Liouville gravity
* RST model
* Topological quantum field theory

Quantum field theory
in curved spacetime|

* Bunch–Davies vacuum
* Hawking radiation
* Semiclassical gravity
* Unruh effect

Black holes|

* Black hole complementarity
* Black hole information paradox
* Black-hole thermodynamics
* Bousso's holographic bound
* ER=EPR
* Firewall (physics)
* Gravitational singularity

Approaches|

| String theory|

* Bosonic string theory
* M-theory
* Supergravity
* Superstring theory

|

Canonical quantum gravity|

* Loop quantum gravity
* Wheeler-DeWitt equation

Euclidean quantum gravity|

* Hartle-Hawking state

Others|

* Causal dynamical triangulation
* Causal sets
* Noncommutative geometry
* Spin foam
* Group field theory
* Superfluid vacuum theory
* Twistor theory
* Dual graviton

Applications|

* Quantum cosmology
* Eternal inflation
* Multiverse
* FRW/CFT duality

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Scientists whose names are used in physical constants

Physical constants|

* Isaac Newton (gravitational constant)
* Charles-Augustin de Coulomb (Coulomb's constant)
* Amedeo Avogadro (Avogadro constant)
* Michael Faraday (Faraday constant)
* Johann Josef Loschmidt
* Johann Jakob Balmer
* Josef Stefan (Stefan–Boltzmann constant)
* Ludwig Boltzmann (Boltzmann constant, Stefan–Boltzmann constant)
* Johannes Rydberg (Rydberg constant)
* J. J. Thomson
* Max Planck (Planck constant, reduced Planck constant, Planck length, Planck time)
* Wilhelm Wien
* Otto Sackur
* Niels Bohr (Bohr radius)
* Edwin Hubble (Hubble constant)
* Hugo Tetrode
* Douglas Hartree
* Brian Josephson
* Klaus von Klitzing

List of scientists whose names are used as SI units and non SI units

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