Planck length

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Planck length
Unit systemPlanck units
Unit oflength
1 P in ...... is equal to ...
   SI units   1.616229(38)×10−35 m
   natural units   11.706 S
3.0542×10−25 a0
   imperial/US units   6.3631×10−34 in

In physics, the Planck length, denoted P, is a unit of length, equal to 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.


The Planck length P is defined as:

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

where 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.


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[edit]

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.[citation needed]

Planck length and Euclidean geometry[edit]

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 of geometry from Euclidean geometry in the gravitational field is determined by the ratio of the gravitational potential and the square of the speed of light : . When , the geometry is close to Euclidean geometry; for , all similarities disappear. The energy of the oscillation of scale is equal to (where is the order of the oscillation frequency). The gravitational potential created by the mass , at this length is , where is the constant of universal gravitation. Instead of , we must substitute a mass, which, according to Einstein's formula, corresponds to the energy (where ). We get . Dividing this expression by , we obtain the value of the deviation . Equating , we find the length at which the Euclidean geometry is completely distorted. It is equal to Planck length .

As noted in,[7] "for the spacetime region with dimensions the uncertainty of the Christoffel symbols be of the order of , and the uncertainty of the metric tensor is of the order of . If is a macroscopic length, the quantum constraints are fantastically small and can be neglected even on atomic scales. If the value is comparable to , 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]


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)[citation needed] 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.

See also[edit]

Notes and references[edit]

  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
  4. ^ a b c d 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.
  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.


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