Planck units
In particle physics and physical cosmology, Planck units are a set of units of measurement defined exclusively in terms of four universal physical constants, in such a manner that these physical constants take on the numerical value of 1 when expressed in terms of these units. Originally proposed in 1899 by German physicist Max Planck, these units are a system of natural units because their definition is based on properties of nature, more specifically the properties of free space, rather than a choice of prototype object. They are relevant in research on unified theories such as quantum gravity.
The term Planck scale refers to quantities of space, time, energy and other units that are similar in magnitude to corresponding Planck units. This region may be characterized by energies of around 10^{19} GeV, time intervals of around 10^{−43} s and lengths of around 10^{−35} m (approximately the energyequivalent of the Planck mass, the Planck time and the Planck length, respectively). At the Planck scale, the predictions of the Standard Model, quantum field theory and general relativity are not expected to apply, and quantum effects of gravity are expected to dominate. The bestknown example is represented by the conditions in the first 10^{−43} seconds of our universe after the Big Bang, approximately 13.8 billion years ago.
The four universal constants that, by definition, have a numeric value 1 when expressed in these units are:
 the speed of light in a vacuum, c,
 the gravitational constant, G,
 the reduced Planck constant, ħ,
 the Boltzmann constant, k_{B}.
Planck units do not incorporate an electromagnetic dimension. Some authors choose to extend the system to electromagnetism by, for example, adding either the electric constant ε_{0} or 4πε_{0} to this list. Similarly, authors choose to use variants of the system that give other numeric values to one or more of the four constants above.
Introduction
Any system of measurement may be assigned a mutually independent set of base quantities and associated base units, from which all other quantities and units may be derived. In the International System of Units, for example, the SI base quantities include length with the associated unit of the metre. In the system of Planck units, a similar set of base quantities and associated units may be selected, in terms of which other quantities and coherent units may be expressed.^{[1]}^{[2]}^{: 1215 } The Planck unit of length has become known as the Planck length, and the Planck unit of time is known as the Planck time, but this nomenclature has not been established as extending to all quantities.
All Planck units are derived from the dimensional universal physical constants that define the system, and in a convention in which these units are omitted (i.e. treated as having the dimensionless value 1), these constants are then eliminated from equations of physics in which they appear. For example, Newton's law of universal gravitation,
can be expressed as:
Both equations are dimensionally consistent and equally valid in any system of quantities, but the second equation, with G absent, is relating only dimensionless quantities since any ratio of two likedimensioned quantities is a dimensionless quantity. If, by a shorthand convention, it is understood that each physical quantity is the corresponding ratio with a coherent Planck unit (or "expressed in Planck units"), the ratios above may be expressed simply with the symbols of physical quantity, without being scaled explicitly by their corresponding unit:
This last equation (without G) is valid with F′, m_{1}′, m_{2}′, and r′ being the dimensionless ratio quantities corresponding to the standard quantities, written e.g. F′ ≘ F or F′ = F/F_{P}, but not as a direct equality of quantities. This may seem to be "setting the constants c, G, etc., to 1" if the correspondence of the quantities is thought of as equality. For this reason, Planck or other natural units should be employed with care. Referring to "G = c = 1", Paul S. Wesson wrote that, "Mathematically it is an acceptable trick which saves labour. Physically it represents a loss of information and can lead to confusion."^{[3]}
History and definition
The concept of natural units was introduced in 1874, when George Johnstone Stoney, noting that electric charge is quantized, derived units of length, time, and mass, now named Stoney units in his honor, by normalizing G, c, and the electron charge, e, to 1.^{[4]} In 1899, one year before the advent of quantum theory, Max Planck introduced what became later known as the Planck constant.^{[5]}^{[6]} At the end of the paper, he proposed the base units later named in his honor. The Planck units are based on the quantum of action, now usually known as the Planck constant, which appeared in the Wien approximation for blackbody radiation. Planck underlined the universality of the new unit system, writing:
... die Möglichkeit gegeben ist, Einheiten für Länge, Masse, Zeit und Temperatur aufzustellen, welche, unabhängig von speciellen Körpern oder Substanzen, ihre Bedeutung für alle Zeiten und für alle, auch außerirdische und außermenschliche Culturen notwendig behalten und welche daher als »natürliche Maßeinheiten« bezeichnet werden können.
... it is possible to set up units for length, mass, time and temperature, which are independent of special bodies or substances, necessarily retaining their meaning for all times and for all civilizations, including extraterrestrial and nonhuman ones, which can be called "natural units of measure".
Planck considered only the units based on the universal constants , , , and to arrive at natural units for length, time, mass, and temperature.^{[6]} His definitions differ from the modern ones by a factor of , because the modern definitions use rather than .^{[5]}^{[6]}
Name  Dimension  Expression  Value (SI units) 

Planck length  length (L)  1.616255(18)×10^{−35} m^{[7]}  
Planck mass  mass (M)  2.176434(24)×10^{−8} kg^{[8]}  
Planck time  time (T)  5.391247(60)×10^{−44} s^{[9]}  
Planck temperature  temperature (Θ)  1.416784(16)×10^{32} K^{[10]} 
Unlike the case with the International System of Units, there is no official entity that establishes a definition of a Planck unit system. Some authors define the base Planck units to be those of mass, length and time, regarding an additional unit for temperature to be redundant.^{[note 1]} Other tabulations add, in addition to a unit for temperature, a unit for electric charge,^{[12]}^{[13]} sometimes also replacing mass with energy when doing so.^{[14]} Depending on the author's choice, this charge unit is given by
or
The Planck charge, as well as other electromagnetic units that can be defined like resistance and magnetic flux, are more difficult to interpret than Planck's original units and are used less frequently.^{[15]}
In SI units, the values of c, h, e and k_{B} are exact and the values of ε_{0} and G in SI units respectively have relative uncertainties of 1.5×10^{−10}^{[16]} and 2.2×10^{−5}.^{[17]} Hence, the uncertainties in the SI values of the Planck units derive almost entirely from uncertainty in the SI value of G.
Derived units
In any system of measurement, units for many physical quantities can be derived from base units. Table 2 offers a sample of derived Planck units, some of which in fact are seldom used. As with the base units, their use is mostly confined to theoretical physics because most of them are too large or too small for empirical or practical use and there are large uncertainties in their values.
Derived unit of  Expression  Approximate SI equivalent 

area (L^{2})  2.6121×10^{−70} m^{2}  
volume (L^{3})  4.2217×10^{−105} m^{3}  
momentum (LMT^{−1})  6.5249 kg⋅m/s  
energy (L^{2}MT^{−2})  1.9561×10^{9} J  
force (LMT^{−2})  1.2103×10^{44} N  
density (L^{−3}M)  5.1550×10^{96} kg/m^{3}  
acceleration (LT^{−2})  5.5608×10^{51} m/s^{2}  
frequency (T^{−1})  1.8549×10^{43} Hz 
Some Planck units, such as of time and length, are many orders of magnitude too large or too small to be of practical use, so that Planck units as a system are typically only relevant to theoretical physics. In some cases, a Planck unit may suggest a limit to a range of a physical quantity where presentday theories of physics apply.^{[18]} For example, our understanding of the Big Bang does not extend to the Planck epoch, i.e., when the universe was less than one Planck time old. Describing the universe during the Planck epoch requires a theory of quantum gravity that would incorporate quantum effects into general relativity. Such a theory does not yet exist.
Several quantities are not "extreme" in magnitude, such as the Planck mass, which is about 22 micrograms: very large in comparison with subatomic particles, and within the mass range of living organisms.^{[19]}^{: 872 } Similarly, the related units of energy and of momentum are in the range of some everyday phenomena.
Significance
Planck units have little anthropocentric arbitrariness, but do still involve some arbitrary choices in terms of the defining constants. Unlike the metre and second, which exist as base units in the SI system for historical reasons, the Planck length and Planck time are conceptually linked at a fundamental physical level. Consequently, natural units help physicists to reframe questions. Frank Wilczek puts it succinctly:
We see that the question [posed] is not, "Why is gravity so feeble?" but rather, "Why is the proton's mass so small?" For in natural (Planck) units, the strength of gravity simply is what it is, a primary quantity, while the proton's mass is the tiny number [1/(13 quintillion)].^{[20]}
While it is true that the electrostatic repulsive force between two protons (alone in free space) greatly exceeds the gravitational attractive force between the same two protons, this is not about the relative strengths of the two fundamental forces. From the point of view of Planck units, this is comparing apples with oranges, because mass and electric charge are incommensurable quantities. Rather, the disparity of magnitude of force is a manifestation of the fact that the charge on the protons is approximately the unit charge but the mass of the protons is far less than the unit mass.
Planck scale
In particle physics and physical cosmology, the Planck scale is an energy scale around 1.22×10^{19} GeV (the Planck energy, corresponding to the energy equivalent of the Planck mass, 2.17645×10^{−8} kg) at which quantum effects of gravity become strong. At this scale, present descriptions and theories of subatomic particle interactions in terms of quantum field theory break down and become inadequate, due to the impact of the apparent nonrenormalizability of gravity within current theories.
Relationship to gravity
At the Planck length scale, the strength of gravity is expected to become comparable with the other forces, and it is theorized that all the fundamental forces are unified at that scale, but the exact mechanism of this unification remains unknown. The Planck scale is therefore the point where the effects of quantum gravity can no longer be ignored in other fundamental interactions, where current calculations and approaches begin to break down, and a means to take account of its impact is necessary.^{[21]} On these grounds, it has been speculated that it may be an approximate lower limit at which a black hole could be formed by collapse.^{[22]}
While physicists have a fairly good understanding of the other fundamental interactions of forces on the quantum level, gravity is problematic, and cannot be integrated with quantum mechanics at very high energies using the usual framework of quantum field theory. At lesser energy levels it is usually ignored, while for energies approaching or exceeding the Planck scale, a new theory of quantum gravity is necessary. Approaches to this problem include string theory and Mtheory, loop quantum gravity, noncommutative geometry, and causal set theory.
In cosmology
In Big Bang cosmology, the Planck epoch or Planck era is the earliest stage of the Big Bang, before the time passed was equal to the Planck time, t_{P}, or approximately 10^{−43} seconds.^{[23]} There is no currently available physical theory to describe such short times, and it is not clear in what sense the concept of time is meaningful for values smaller than the Planck time. It is generally assumed that quantum effects of gravity dominate physical interactions at this time scale. At this scale, the unified force of the Standard Model is assumed to be unified with gravitation. Immeasurably hot and dense, the state of the Planck epoch was succeeded by the grand unification epoch, where gravitation is separated from the unified force of the Standard Model, in turn followed by the inflationary epoch, which ended after about 10^{−32} seconds (or about 10^{11} t_{P}).^{[24]}
Table 3 lists properties of the observable universe today expressed in Planck units.^{[25]}^{[26]}
Property of presentday observable universe 
Approximate number of Planck units 
Equivalents 

Age  8.08 × 10^{60} t_{P}  4.35 × 10^{17} s or 1.38 × 10^{10} years 
Diameter  5.4 × 10^{61} l_{P}  8.7 × 10^{26} m or 9.2 × 10^{10} lightyears 
Mass  approx. 10^{60} m_{P}  3 × 10^{52} kg or 1.5 × 10^{22} solar masses (only counting stars) 10^{80} protons (sometimes known as the Eddington number) 
Density  1.8 × 10^{−123} m_{P}⋅l_{P}^{−3}  9.9 × 10^{−27} kg⋅m^{−3} 
Temperature  1.9 × 10^{−32} T_{P}  2.725 K temperature of the cosmic microwave background radiation 
Cosmological constant  2.9 × 10^{−122} l^{ −2} _{P} 
1.1 × 10^{−52} m^{−2} 
Hubble constant  1.18 × 10^{−61} t^{ −1} _{P} 
2.2 × 10^{−18} s^{−1} or 67.8 (km/s)/Mpc 
After the measurement of the cosmological constant (Λ) in 1998, estimated at 10^{−122} in Planck units, it was noted that this is suggestively close to the reciprocal of the age of the universe (T) squared. Barrow and Shaw proposed a modified theory in which Λ is a field evolving in such a way that its value remains Λ ~ T^{−2} throughout the history of the universe.^{[27]}
Analysis of the units
Planck length
The Planck length, denoted ℓ_{P}, is a unit of length defined as:
It is equal to 1.616255(18)×10^{−35} m,^{[7]} where the two digits enclosed by parentheses are the estimated standard error associated with the reported numerical value, or about 10^{−20} times the diameter of a proton.^{[28]} It can be motivated in various ways, such as considering a particle whose reduced Compton wavelength is comparable to its Schwarzschild radius,^{[28]}^{[29]}^{[30]} though whether those concepts are in fact simultaneously applicable is open to debate.^{[31]} (The same heuristic argument simultaneously motivates the Planck mass.^{[29]})
The Planck length is a distance scale of interest in speculations about quantum gravity. The Bekenstein–Hawking entropy of a black hole is onefourth the area of its event horizon in units of Planck length squared.^{[11]}^{: 370 } Since the 1950s, it has been conjectured that quantum fluctuations of the spacetime metric might make the familiar notion of distance inapplicable below the Planck length.^{[32]}^{[33]}^{[34]} This is sometimes expressed by saying that "spacetime becomes a foam at the Planck scale".^{[35]} It is possible that the Planck length is the shortest physically measurable distance, since any attempt to investigate the possible existence of shorter distances, by performing higherenergy collisions, would result in black hole production. Higherenergy collisions, rather than splitting matter into finer pieces, would simply produce bigger black holes.^{[36]}
The strings of string theory are modeled to be on the order of the Planck length.^{[37]}^{[38]} In theories with large extra dimensions, the Planck length calculated from the observed value of can be smaller than the true, fundamental Planck length.^{[11]}^{: 61 }^{[39]}
Planck time
The Planck time t_{P} is the time required for light to travel a distance of 1 Planck length in a vacuum, which is a time interval of approximately 5.39×10^{−44} s. No current physical theory can describe timescales shorter than the Planck time, such as the earliest events after the Big Bang,^{[23]} and it is conjectured that the structure of time breaks down on intervals comparable to the Planck time.^{[40]} While there is currently no known way to measure time intervals on the scale of the Planck time, researchers in 2020 found that the accuracy of an atomic clock is constrained by quantum effects on the order of the Planck time, and for the most precise atomic clocks thus far they calculated that such effects have been ruled out to around 10^{−33} s, or 10 orders of magnitude above the Planck scale.^{[41]}^{[42]}^{[40]}
Planck energy
Most Planck units are extremely small, as in the case of Planck length or Planck time, or extremely large, as in the case of Planck temperature or Planck acceleration. For comparison, the Planck energy E_{P} is approximately equal to the energy stored in an automobile gas tank (57.2 L of gasoline at 34.2 MJ/L of chemical energy). The ultrahighenergy cosmic ray observed in 1991 had a measured energy of about 50 J, equivalent to about 2.5×10^{−8} E_{P}.^{[43]}^{[44]}
Proposals for theories of doubly special relativity posit that, in addition to the speed of light, an energy scale is also invariant for all inertial observers. Typically, this energy scale is chosen to be the Planck energy.^{[45]}^{[46]}
Planck unit of force
The Planck unit of force may be thought of as the derived unit of force in the Planck system if the Planck units of time, length, and mass are considered to be base units.
It is the gravitational attractive force of two bodies of 1 Planck mass each that are held 1 Planck length apart. One convention for the Planck charge is to choose it so that the electrostatic repulsion of two objects with Planck charge and mass that are held 1 Planck length apart exactly balances the Newtonian attraction between them.^{[47]}
Various authors have argued that the Planck force is on the order of the maximum force that can be observed in nature.^{[48]}^{[49]} However, the validity of these conjectures has been disputed.^{[50]}^{[51]}
Planck temperature
The Planck temperature T_{P} is 1.416784(16)×10^{32} K.^{[10]} At this temperature, the wavelength of light emitted by thermal radiation reaches the Planck length. There are no known physical models able to describe temperatures greater than T_{P}; a quantum theory of gravity would be required to model the extreme energies attained.^{[52]} Hypothetically, a system in thermal equilibrium at the Planck temperature might contain Planckscale black holes, constantly being formed from thermal radiation and decaying via Hawking evaporation; adding energy to such a system might decrease its temperature by creating larger black holes, whose Hawking temperature is lower.^{[53]}
List of physical equations
Physical quantities that have different dimensions (such as time and length) cannot be equated even if they are numerically equal (1 second is not the same as 1 metre). In theoretical physics, however, this scruple can be set aside, by a process called nondimensionalization. Table 4 shows how the use of Planck units simplifies many fundamental equations of physics, because this gives each of the five fundamental constants, and products of them, a simple numeric value of 1. In the SI form, the units should be accounted for. In the nondimensionalized form, the units, which are now Planck units, need not be written if their use is understood.
SI form  Planck units form  

Newton's law of universal gravitation  
Einstein field equations in general relativity  
Mass–energy equivalence in special relativity  
Energy–momentum relation  
Thermal energy per particle per degree of freedom  
Boltzmann's entropy formula  
Planck–Einstein relation for energy and angular frequency  
Planck's law (surface intensity per unit solid angle per unit angular frequency) for black body at temperature T.  
Stefan–Boltzmann constant σ defined  
Bekenstein–Hawking black hole entropy^{[19]}^{: 714–17 }  
Schrödinger's equation  
Hamiltonian form of Schrödinger's equation  
Covariant form of the Dirac equation  
Unruh temperature  
Coulomb's law  
Maxwell's equations 


Ideal gas law  or 
Alternative choices of normalization
As already stated above, Planck units are derived by "normalizing" the numerical values of certain fundamental constants to 1. These normalizations are neither the only ones possible nor necessarily the best. Moreover, the choice of what factors to normalize, among the factors appearing in the fundamental equations of physics, is not evident, and the values of the Planck units are sensitive to this choice.
The factor 4π is ubiquitous in theoretical physics because in threedimensional space, the surface area of a sphere of radius r is 4πr^{2}. This, along with the concept of flux, are the basis for the inversesquare law, Gauss's law, and the divergence operator applied to flux density. For example, gravitational and electrostatic fields produced by point objects have spherical symmetry, and so the electric flux through a sphere of radius r around a point charge will be distributed uniformly over that sphere. From this, it follows that a factor of 4πr^{2} will appear in the denominator of Coulomb's law in rationalized form.^{[25]}^{: 214–15 } (Both the numerical factor and the power of the dependence on r would change if space were higherdimensional; the correct expressions can be deduced from the geometry of higherdimensional spheres.^{[11]}^{: 51 }) LIkewise for Newton's law of universal gravitation: a factor of 4π naturally appears in Poisson's equation when relating the gravitational potential to the distribution of matter.^{[11]}^{: 56 }
Hence a substantial body of physical theory developed since Planck's 1899 paper suggests normalizing not G but 4πG (or 8πG) to 1. Doing so would introduce a factor of 1/4π (or 1/8π) into the nondimensionalized form of the law of universal gravitation, consistent with the modern rationalized formulation of Coulomb's law in terms of the vacuum permittivity. In fact, alternative normalizations frequently preserve the factor of 1/4π in the nondimensionalized form of Coulomb's law as well, so that the nondimensionalized Maxwell's equations for electromagnetism and gravitoelectromagnetism both take the same form as those for electromagnetism in SI, which do not have any factors of 4π. When this is applied to electromagnetic constants, ε_{0}, this unit system is called "rationalized". When applied additionally to gravitation and Planck units, these are called rationalized Planck units^{[54]} and are seen in highenergy physics.^{[55]}
The rationalized Planck units are defined so that .
There are several possible alternative normalizations.
Gravitational constant
In 1899, Newton's law of universal gravitation was still seen as exact, rather than as a convenient approximation holding for "small" velocities and masses (the approximate nature of Newton's law was shown following the development of general relativity in 1915). Hence Planck normalized to 1 the gravitational constant G in Newton's law. In theories emerging after 1899, G nearly always appears in formulae multiplied by 4π or a small integer multiple thereof. Hence, a choice to be made when designing a system of natural units is which, if any, instances of 4π appearing in the equations of physics are to be eliminated via the normalization.
 Normalizing 4πG to 1 (and therefore setting G = 1/4π):
 Gauss's law for gravity becomes Φ_{g} = −M (rather than Φ_{g} = −4πM in Planck units).
 Eliminates 4πG from the Poisson equation.
 Eliminates 4πG in the gravitoelectromagnetic (GEM) equations, which hold in weak gravitational fields or locally flat spacetime. These equations have the same form as Maxwell's equations (and the Lorentz force equation) of electromagnetism, with mass density replacing charge density, and with 1/4πG replacing ε_{0}.
 Normalizes the characteristic impedance Z_{g} of gravitational radiation in free space to 1 (normally expressed as 4πG/c).^{[note 2]}
 Eliminates 4πG from the Bekenstein–Hawking formula (for the entropy of a black hole in terms of its mass m_{BH} and the area of its event horizon A_{BH}) which is simplified to S_{BH} = πA_{BH} = (m_{BH})^{2}.
 Setting 8πG = 1 (and therefore setting G = 1/8π). This would eliminate 8πG from the Einstein field equations, Einstein–Hilbert action, and the Friedmann equations, for gravitation. Planck units modified so that 8πG = 1 are known as reduced Planck units, because the Planck mass is divided by √8π. Also, the Bekenstein–Hawking formula for the entropy of a black hole simplifies to S_{BH} = (m_{BH})^{2}/2 = 2πA_{BH}.
See also
 cGh physics
 Dimensional analysis
 Doubly special relativity
 TransPlanckian problem
 Zeropoint energy
Notes
 ^ For example, both Frank Wilczek and Barton Zwiebach do so,^{[1]}^{[11]}^{: 54 } as does the textbook Gravitation.^{[2]}^{: 1215 }
 ^ General relativity predicts that gravitational radiation propagates at the same speed as electromagnetic radiation.^{[56]}^{: 60 }^{[57]}^{: 158 }
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However, for most experimental physicists, approaching energies comparable to Planck energy is little more than a distant fantasy. The most powerful accelerators on Earth miss Planck energy of 15 orders of magnitude, while ultra high energy cosmic rays are still 9 orders of magnitude short of M_{p}.
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The point at which our physical theories run into most serious difficulties is that where matter reaches a temperature of approximately 10^{32} degrees, also known as Planck's temperature. The extreme density of radiation emitted at this temperature creates a disproportionately intense field of gravity. To go even farther back, a quantum theory of gravity would be necessary, but such a theory has yet to be written.
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External links
 Value of the fundamental constants, including the Planck units, as reported by the National Institute of Standards and Technology (NIST).
 The Planck scale: relativity meets quantum mechanics meets gravity from 'Einstein Light' at UNSW