View source for Physics:Planck temperature

'''Planck temperature''', denoted by ''T''<sub>P</sub>, is the unit of [[Physics:Temperature|temperature]] in the system of [[Natural units|natural units]] known as [[Planck units]].
It serves as the defining unit of the [[Physics:Planck temperature scale|Planck temperature scale]].  In this scale the magnitude of the Planck temperature is equal to 1, while that of [[Physics:Absolute zero|absolute zero]] is 0.
Other temperatures can be converted to Planck temperature units. For example, {{val|0|u=°C}} = {{val|273.15|u=K}} = {{val|1.9279|e=-30|u=''T''<sub>P</sub>}}.

In SI units, the Planck temperature is about {{val|1.417|e=32}} kelvin (equivalently, degrees Celsius, since the difference is trivially small at this scale), or {{val|2.55|e=32}} degrees Fahrenheit or Rankine.<ref>[https://www.translatorscafe.com/cafe/EN/units-converter/temperature/6-1/Planck_temperature-kelvin/ TranslatorsCafe.com] - Unit Converter</ref>

==History==
What today is known as the Planck temperature was first introduced in 1899 by [[Biography:Max Planck|Max Planck]] together with his introduction of what today is known as the Planck length, the Planck mass and Planck time.<ref>Tomilin, K. A., 1999, "[http://www.ihst.ru/personal/tomilin/papers/tomil.pdf Natural Systems of Units: To the Centenary Anniversary of the Planck System]", 287–296.</ref><ref>M. Planck. "Natürliche Einheiten." ''Königlich-Preußische Akademie der Wissenschaften'', pp. 479–480, 1899.</ref>

==Definition==
The Planck temperature is defined as:

<math>T_\text{P} = \frac{m_\text{P} c^2}{k} = \sqrt{\frac{\hbar c^5}{G k^2}}\approx</math>  
{{val|1.416808|(33)|e=32|u=[[Kelvin|K]]}}
where:
*''m''<sub>P</sub> is the [[Physics:Planck mass|Planck mass]],
*''c'' is [[Physics:Speed of light|speed of light]] in a vacuum,
*<math>\hbar</math> is the [[Physics:Reduced Planck constant|reduced Planck constant]] defined as <math>\hbar\ = \frac{h}{2\pi}</math>,
*''k'' is the [[Physics:Boltzmann constant|Boltzmann constant]],
*''G'' is the [[Physics:Gravitational constant|gravitational constant]].

The two digits between the [[Bracket|parentheses]] are used to denote the standard error of the last two digits of the estimated value.<ref>{{cite web |url= http://physics.nist.gov/cgi-bin/cuu/Value?plktmp |title= CODATA Value: Planck temperature |publisher= NIST | work = The NIST Reference on Constants, Units, and Uncertainty |accessdate=2011-10-12 }}</ref>

==Significance==
As with most of the Planck units, a Planck temperature of 1 (unity) is a fundamental limit of [[Physics:Quantum mechanics|quantum theory]], in combination with gravitation, as presently understood. In other words, the wavelength of light emitted by an object can be calculated by its temperature. If an object were to reach the temperature of {{val|1.42|e=32|u=kelvin}} (''T''<sub>P</sub>), the radiation it would emit would have a wavelength of {{val|1.616|e=−35|u=meters}} ([[Planck length]]), at which point quantum gravitational effects become relevant. At temperatures greater than or equal to ''T''<sub>P</sub>, current physical theory breaks down because we lack a theory of [[Physics:Quantum gravity|quantum gravity]].<ref>Nova: ''[https://www.pbs.org/wgbh/nova/zero/hot.html Absolute Hot]''</ref>

== See also==
* [[Physics:Absolute hot|Absolute hot]]
* [[Physics:Hagedorn temperature|Hagedorn temperature]]
* [[Orders of magnitude (temperature)]]
* [[Natural units]]
* [[Physics:Planck constant|Planck constant]]
* Planck units

== Notes and references==
{{Reflist}}

==External links==
* [http://physics.nist.gov/cgi-bin/cuu/Value?plktmp NIST reference: Planck temperature]
* [http://www.straightdope.com/columns/read/807/what-is-the-opposite-of-absolute-zero What is the opposite of absolute zero?]

{{Scales of temperature}}

[[Category:Units of temperature]]
[[Category:Physical constants]]
[[Category:Planck units]]

{{Sourceattribution|Planck temperature|1}}