# Planck time

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Planck time

In physics, the Planck time ("tP"), is the unit of time in the system of natural units known as Planck units. It is the time it would take a photon travelling at the speed of light in a vacuum to cross a distance equal to the Planck 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 after Max Planck.

It is defined as:$t_P = sqrt\left\{frac\left\{hbar G\right\}\left\{c^5 approx 5.39124\left(27\right) imes 10^\left\{-44\right\} mbox\left\{ s\right\}$ [ [http://physics.nist.gov/cgi-bin/cuu/Value?plkt|search_for=planck+time CODATA Value: Planck Time] – The NIST 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 the gravitational constant:$c$ is the speed of light in a vacuum:$t_P$ is in seconds.: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 the attosecond (10−18 s) time scale, or around 1026 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 the Big Bang, statements can be made about the universe displaying properties equal to some of the other Planck units. (Some hypothesize that gravity must have separated first due to its homogeneity to the others. Some propose that the strong 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 the universe. [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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