View source for Planck time

{{short description|Very small unit of time}}
{{Use dmy dates|date=May 2011}}

In [[quantum mechanics]], the '''Planck time''' ({{math|<var>t</var><sub>P</sub>}}) is the [[unit of time]] in the system of [[natural units]] known as [[Planck units]]. A Planck time unit 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<sup>−44</sup> s. The unit is named after [[Max Planck]], who was the first to propose it.

The Planck time is defined as:<ref name=":0">[http://physics.nist.gov/cgi-bin/cuu/Value?plkt CODATA Value: Planck Time] – The [[NIST]] Reference on Constants, Units, and Uncertainty.</ref>
:<math>t_\mathrm{P} \equiv \sqrt{\frac{\hbar G}{c^5}}</math>

where:
:{{math|<var>ħ</var>}} = {{frac|<var>h</var>|2<var>{{pi}}</var>}} is the [[reduced Planck constant]] (sometimes {{mvar|h}} is used instead of {{mvar|ħ}} in the definition<ref name="gsu_hbase">{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/astro/planck.html|title=Big Bang models back to Planck time|date=19 June 2005|publisher=[[Georgia State University]]}}</ref>)
:{{mvar|G}} = [[gravitational constant]]
:{{mvar|c}} = [[speed of light]] in [[vacuum]]

Using the known values of the constants, the approximate equivalent value in terms of the [[SI]] unit, the [[second]], is
:<math> 1 \ t_\mathrm{P} \approx 5.391\,247(60) \times 10^{-44}\ \mathrm{s},</math>

where the two digits between parentheses denote the [[Standard error (statistics)|standard error]] of the approximated value.<ref name=":0" />

==History ==
The Planck time (also known as Planck second) was first suggested by [[Max Planck]]<ref>M. Planck. Naturlische Masseinheiten. Der Koniglich Preussischen Akademie Der Wissenschaften, p. 479, 1899</ref> in 1899. He suggested that there existed some fundamental natural units for [[length]], [[mass]], [[time]] and [[energy]]. Planck derived these using [[dimensional analysis]] only using what he considered the most fundamental universal constants: the speed of light, the Newton gravitational constant and the Planck constant.

==Physical significance==
The Planck time is the unique combination of the [[gravitational constant]] {{mvar|G}}, the [[speed of light|special-relativistic constant]] {{mvar|c}}, and the [[Planck constant|quantum constant]] {{mvar|ħ}}, to produce a constant with dimension of [[time]]. Because the Planck time comes from dimensional analysis, which ignores constant factors, there is no reason to believe that exactly one unit of Planck time has any special physical significance. Rather, the Planck time represents a rough time scale at which quantum gravitational effects are likely to become important. This essentially means that while smaller units of time can exist, they are so small their effect on our existence is negligible. The nature of those effects, and the exact time scale at which they would occur, would need to be derived from an actual theory of [[quantum gravity]].

The [[Multiplicative inverse|reciprocal]] of the Planck time, which is [[Planck frequency]], can be interpreted as an upper bound on the frequency of a [[wave]]. This follows from the interpretation of the [[Planck length]] as a minimal length, and hence a lower bound on the wavelength.

All scientific experiments and human experiences occur over time scales that are many orders of magnitude longer than the Planck time,<ref>{{cite serial |title=[[How the Universe Works#Season 3|How The Universe Works 3]]|episode=First Second of the Big Bang |network=Discovery Science |year=2014}}</ref> making any events happening at the Planck scale undetectable with current scientific knowledge. {{As of|2016|11}}, the smallest time interval uncertainty in direct measurements was on the order of 850 [[Orders of magnitude (time)|zeptoseconds]] (8.50 × 10<sup>−19</sup> seconds).<ref>{{cite web|url=http://www.sciencealert.com/scientists-measure-the-smallest-fragment-of-time-ever-witness-an-electron-escaping-an-atom|title=Scientists have measured the smallest fragment of time ever|last=|first=|date=2010-05-12|website=|publisher=|accessdate=2012-04-19}}</ref>

==See also==
{{div col|colwidth=30em}}
* [[Attosecond]]
* [[Chronon]]
* [[Orders of magnitude (time)]]
* [[Planck energy]]
* [[Planck era]]
* [[Quantum clock]]
{{div col end}}

== References==
{{Reflist|colwidth=30em}}

== External links ==
* [http://abyss.uoregon.edu/~js/cosmo/lectures/lec20.html "Planck Era" and "Planck Time"] (up to 10<sup>−43</sup> seconds after [[Big Bang|birth]] of [[Universe]]) ([[University of Oregon]]).

{{Planck's natural units}}
{{Time measurement and standards}}
{{Orders of magnitude seconds}}
{{Time topics}}
{{Scientists whose names are used in physical constants}}
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[[Category:Physical constants]]
[[Category:Units of time]]
[[Category:Planck units]]