Planck length

Planck length
Unit system	Planck units
Unit of	length
Symbol	ℓP
Unit conversions
1 ℓP in ...	... is equal to ...
SI units	1.616229(38)×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 $ℓ P$ , is a unit of length, equal to 6965161622900000000♠1.616229(38)×10⁻³⁵ meters. It is a base unit in the system of Planck units, developed by physicist Max Planck. The Planck length can be defined from three fundamental physical constants: the speed of light in a vacuum, the Planck constant, and the gravitational constant.

Value

The Planck length $ℓ P$ is defined as:

\ell _{\mathrm {P} }={\sqrt {\frac {\hbar G}{c^{3}}}}

Solving the above will show the approximate equivalent value of this unit with respect to the meter:

$1\ \ell _{\mathrm {P} }\approx 1.616\;229(38)\times 10^{-35}\ \mathrm {m}$

where $c$ is the speed of light in a vacuum, $G$ is the gravitational constant, and $ħ$ 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 times the diameter of a proton. It can be defined using the radius of the hypothesized Planck particle.

History

In 1899 Max Planck^[3] suggested that there existed some fundamental natural units for length, mass, time and energy. These he derived using dimensional analysis, using only the Newton gravitational constant, the speed of light and the Planck constant. The natural units he derived later became known as "the Planck length", "the Planck mass", "the Planck time" and "the Planck energy".

Theoretical significance

The Planck length is the scale at which quantum gravitational effects are believed to begin to be apparent, where interactions require a working theory of quantum gravity to be analyzed.^[4] The Planck area is the area by which the surface of a spherical black hole increases when the black hole swallows one bit of information.^[5] To measure anything the size of Planck length, the photon momentum needs to be very large due to Heisenberg's uncertainty principle and so much energy in such a small space would create a tiny Black hole with the diameter of its event horizon equal to a Planck length.

The Planck length is sometimes misconceived as the minimum length of spacetime, but this is not accepted by conventional physics, as this would require violation or modification of Lorentz symmetry.^[4] However, certain theories of loop quantum gravity do attempt to establish a minimum length on the scale of the Planck length, though not necessarily the Planck length itself,^[4] or attempt to establish the Planck length as observer-invariant, known as doubly special relativity.

The strings of string theory are modelled to be on the order of the Planck length.^[4]^[6] In theories of large extra dimensions, the Planck length has no fundamental physical significance, and quantum gravitational effects appear at other scales.

Planck length and 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 $\zeta$ of geometry from Euclidean geometry in the gravitational field is determined by the ratio of the gravitational potential $\varphi$ and the square of the speed of light $c$ : $\zeta =\varphi /c^{2}$ . When $\zeta \ll 1$ , the geometry is close to Euclidean geometry; for $\zeta \sim 1$ , all similarities disappear. The energy of the oscillation of scale $l$ is equal to $E=\hbar \nu \sim \hbar c/l$ (where $c/l$ is the order of the oscillation frequency). The gravitational potential created by the mass $m$ , at this length is $\varphi =Gm/l$ , where $G$ is the constant of universal gravitation. Instead of $m$ , we must substitute a mass, which, according to Einstein's formula, corresponds to the energy $E$ (where $m=E/c^{2}$ ). We get $\varphi =GE/l\,c^{2}=G\hbar /l^{2}c$ . Dividing this expression by $c^{2}$ , we obtain the value of the deviation $\zeta =G\hbar /c^{3}l^{2}=\ell _{P}^{2}/l^{2}$ . Equating $\zeta =1$ , we find the length at which the Euclidean geometry is completely distorted. It is equal to Planck length ${\textstyle \ell _{P}={\sqrt {G\hbar /c^{3}}}\approx 10^{-35}\mathrm {m} }$ .

As noted in,^[7] "for the spacetime region with dimensions $l$ the uncertainty of the Christoffel symbols $\Delta \Gamma$ be of the order of $\ell _{P}^{2}/l^{3}$ , and the uncertainty of the metric tensor $\Delta g$ is of the order of $\ell _{P}^{2}/l^{2}$ . If $l$ is a macroscopic length, the quantum constraints are fantastically small and can be neglected even on atomic scales. If the value $l$ is comparable to $\ell _{P}$ , then the maintenance of the former (usual) concept of space becomes more and more difficult and the influence of micro curvature becomes obvious". Conjecturally, this could imply that spacetime becomes a quantum foam at the Planck scale.^[8]

Visualization

The size of the Planck length can be visualized as follows: if a particle or dot about 0.1 mm in size (which is approximately the smallest the unaided human eye can see) were magnified in size to be as large as the observable universe, then inside that universe-sized "dot", the Planck length would be roughly the diameter of an actual 0.1 mm dot. In other words, a 0.1 mm dot is halfway between the Planck length and the size of the observable universe on a logarithmic scale.

Notes and references

↑ John Baez, The Planck Length
↑ NIST, "Planck length", NIST's published CODATA constants
↑ M. Planck. Naturlische Masseinheiten. Der Koniglich Preussischen Akademie Der Wissenschaften, p. 479, 1899
1 2 3 4 Klotz, Alex (2015-09-09). "A Hand-Wavy Discussion of the Planck Length". Physics Forums Insights. Retrieved 2018-03-23.
↑ Bekenstein, Jacob D (1973). "Black Holes and Entropy". Physical Review D. 7 (8): 2333. Bibcode:1973PhRvD...7.2333B. doi:10.1103/PhysRevD.7.2333.
↑ Cliff Burgess; Fernando Quevedo (November 2007). "The Great Cosmic Roller-Coaster Ride". Scientific American (print). Scientific American, Inc. p. 55.
↑ T. Regge, Nuovo Cim. 7, 215 (1958). Gravitational fields and quantum mechanics
↑ Wheeler, J. A. (January 1955). "Geons". Physical Review. 97 (2): 511. Bibcode:1955PhRv...97..511W. doi:10.1103/PhysRev.97.511.

Bibliography

Garay, Luis J. (January 1995). "Quantum gravity and minimum length". International Journal of Modern Physics A. 10 (2): 145 ff. arXiv:gr-qc/9403008v2. Bibcode:1995IJMPA..10..145G. doi:10.1142/S0217751X95000085.

External links

Bowley, Roger; Eaves, Laurence (2010). "Planck Length". Sixty Symbols. Brady Haran for the University of Nottingham.

This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.

[1] John Baez, The Planck Length

[2] NIST, "Planck length", NIST's published CODATA constants

[3] M. Planck. Naturlische Masseinheiten. Der Koniglich Preussischen Akademie Der Wissenschaften, p. 479, 1899

[physicsforums-4] 1 2 3 4 Klotz, Alex (2015-09-09). "A Hand-Wavy Discussion of the Planck Length". Physics Forums Insights. Retrieved 2018-03-23.

[5] Bekenstein, Jacob D (1973). "Black Holes and Entropy". Physical Review D. 7 (8): 2333. Bibcode:1973PhRvD...7.2333B. doi:10.1103/PhysRevD.7.2333.

[Burgess_and_Quevedo-6] Cliff Burgess; Fernando Quevedo (November 2007). "The Great Cosmic Roller-Coaster Ride". Scientific American (print). Scientific American, Inc. p. 55.

[7] T. Regge, Nuovo Cim. 7, 215 (1958). Gravitational fields and quantum mechanics

[8] Wheeler, J. A. (January 1955). "Geons". Physical Review. 97 (2): 511. Bibcode:1955PhRv...97..511W. doi:10.1103/PhysRev.97.511.

Planck's natural units
Planck constant Planck units
Base Planck units	Planck time Planck length Planck mass Planck charge Planck temperature
Derived Planck units	Planck energy Planck force Planck power Planck density Planck angular frequency Planck pressure Planck current Planck voltage Planck impedance Planck momentum Planck area Planck volume Planck acceleration Planck angle