- Planck time
In

physics , the**Planck time**("t_{P}"), is the unit oftime in the system ofnatural units known asPlanck units . It is the time it would take aphoton travelling at the speed oflight in avacuum to cross a distance equal to thePlanck length .cite web | url = http://hyperphysics.phy-astr.gsu.edu/hbase/astro/planck.html | title = Big Bang models back to Planck time | publisher =Georgia State University | date =19 June 2005 ] The unit is named afterMax Planck .It is defined as:$t\_P\; =\; sqrt\{frac\{hbar\; G\}\{c^5\; approx\; 5.39124(27)\; imes\; 10^\{-44\}\; mbox\{\; s\}$ [

*[*]*http://physics.nist.gov/cgi-bin/cuu/Value?plkt|search_for=planck+time CODATA Value: Planck Time*] – TheNIST Reference on Constants, Units, and Uncertainty.where::$hbar\; =\; h\; /\; 2\; pi$ is the reduced Planck constant (sometimes $h$ is used instead of $hbar$ in the definition cite web | url = http://hyperphysics.phy-astr.gsu.edu/hbase/astro/planck.html | title = Big Bang models back to Planck time | publisher =

Georgia State University | date =19 June 2005 ] ):$G$ is thegravitational constant :$c$ is thespeed of light in avacuum :$t\_P$ is insecond s.:The two digits between the parentheses denote the uncertainty in the last two digits of the value.**Physical significance**The Planck time is simply the time it takes a beam of light to travel a

Planck length . See the corresponding section for that unit for details. As of 2006, the smallest unit of time that has been directly measured is on theattosecond (10^{−18}s) time scale, or around 10^{26}Planck times.cite web | url = http://news.bbc.co.uk/2/hi/science/nature/3486160.stm | title = Shortest time interval measured | publisher =BBC News | date =25 February 2004 ] cite web | url = http://news.bbc.co.uk/1/hi/sci/tech/4766842.stm | title = Fastest view of molecular motion | publisher =BBC News | date =4 March 2006 ] There is also speculation that one Planck time after theBig Bang , statements can be made about the universe displaying properties equal to some of the other Planck units. (Some hypothesize thatgravity must have separated first due to its homogeneity to the others. Some propose that thestrong nuclear force is the most likely candidate due to its strength.)cite web|url=http://www.space.com/scienceastronomy/quantum_bits_030402.html|title=Hubble Pictures Too Crisp, Challenging Theories of Time and Space|date=2003-04-02|publisher=Space.com|accessdate=2008-05-30]One Planck time should be the smallest measurable unit of time, according to quantum mechanics. But according to news reports, analyses of

Hubble Space Telescope Deep Field images in 2003 brought up a possible discrepancy. Images should have been blurry at very far distances, but the news articles stated that they were not, challenging the theory that Planck time is indeed the smallest measurable unit of time in theuniverse . [*cite journal|last=Lieu|first=Richard|coauthors=Hillman, Lloyd W.|date=2003-03-10|title=The Phase Coherence of Light from Extragalactic Sources: Direct Evidence against First-Order Planck-Scale Fluctuations in Time and Space|journal=The Astrophysical Journal|volume=585|pages=L77–L80|url=http://www.journals.uchicago.edu/doi/pdf/10.1086/374350|accessdate=2008-05-30|doi=10.1086/374350*] [*cite journal|last=Ng|first=Y. Jack|coauthors=Christiansen, W. A.; van Dam H.|date=2003-07-10|title=Probing Planck-Scale Physics with Extragalactic Sources?|journal=The Astrophysical Journal Letters|publisher=The American Astronomical Society|volume=591|pages=L87–L89|url=http://www.journals.uchicago.edu/doi/abs/10.1086/377121|accessdate=2008-05-30|doi=10.1086/377121*]**References**

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