|
|
| Line 1: |
Line 1: |
| In [[physics]], the '''Planck time''' (''t<sub>P</sub>'') is the [[unit of time]] in the system of [[natural units]] known as [[Planck units]]. It is the time required for [[light]] to travel, in a [[vacuum]], a distance of 1 [[Planck length]].<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 | publisher = [[Georgia State University]] | date = 19 June 2005}}</ref> The unit is named after [[Max Planck]], who was the first to propose it.
| | Hi, everybody! <br>I'm Russian female ;=). <br>I really like The Simpsons!<br><br>my blog; FIFA 15 coin hack [[http://Www.Fifa15-Coingenerator.com/ My Site]] |
| | |
| The Planck time is defined as:<ref>[http://physics.nist.gov/cgi-bin/cuu/Value?plkt CODATA Value: Planck Time] – The [[NIST]] Reference on Constants, Units, and Uncertainty.</ref>
| |
| :<math>t_P \equiv \sqrt{\frac{\hbar G}{c^5}}</math> ≈ 5.39106(32) × 10<sup>−44</sup> s
| |
| | |
| where:
| |
| :<math>\hbar = h / 2 \pi</math> is the [[reduced Planck constant]] (sometimes <math>h</math> is used instead of <math>\hbar</math> in the definition<ref name="gsu_hbase" />)
| |
| :''G'' = [[gravitational constant]]
| |
| :''c'' = [[speed of light]] in a [[vacuum]]
| |
| :s is the [[SI]] unit of time, the [[second]].
| |
| | |
| The two digits between [[parentheses]] denote the [[Standard error (statistics)|standard error]] of the estimated value.
| |
| | |
| ==Physical significance==
| |
| One Planck time is the time it would take a photon traveling at the speed of [[light]] to cross a distance equal to one [[Planck length]]. Theoretically, this is the smallest time measurement that will ever be possible,<ref>{{cite encyclopedia |url=http://astronomy.swin.edu.au/cosmos/P/Planck+Time |title=Planck Time |encyclopedia =COSMOS - The SAO Encyclopedia of Astronomy |publisher= Swinburne University}}</ref> roughly 10<sup>−43</sup> seconds. Within the framework of the laws of physics as we understand them today, for times less than one Planck time apart, we can neither measure nor detect any change.
| |
| {{As of| May 2010}}, the smallest time interval uncertainty in direct measurements is on the order of 12 [[attoseconds]] (1.2 × 10<sup>−17</sup> seconds), about 3.7 × 10<sup>26</sup> Planck times.<ref>
| |
| {{cite web | url=http://www.physorg.com/news192909576.html | title=12 attoseconds is the world record for shortest controllable time | date=2010-05-12 | accessdate=2012-04-19}}</ref>
| |
| | |
| The Planck time comes from a field of [[mathematical physics]] known as [[dimensional analysis]], which studies [[units of measurement]] and [[physical constants]]. The Planck time is the unique combination of the [[gravitational constant]] ''G'', the [[speed of light|relativity constant]] ''c'', and the [[Planck constant|quantum constant]] ''h'', to produce a constant with units of [[time]]. For processes that occur in a time ''t'' less than one Planck time, the [[dimensionless quantity]] ''t<sub>P</sub> / t'' is greater than one. Dimensional analysis suggests that the effects of both [[quantum mechanics]] and [[gravity]] will be important under these circumstances, requiring a theory of [[quantum gravity]]. All scientific experiments and human experiences happen over billions of billions of billions of Planck times, making any events happening at the Planck scale hard to detect.
| |
| | |
| Analysis of [[Hubble Space Telescope]]s [[Hubble Ultra-Deep Field|deep field]] images in 2003 led to a debate about the physical implications of the Planck time as a physical minimum time interval. According to Lieu and Hillman,<ref>{{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 |issue=2 |pages=L77–L80 |doi=10.1086/374350 |bibcode=2003ApJ...585L..77L|arxiv = astro-ph/0301184 |url=http://iopscience.iop.org/1538-4357/585/2/L77/pdf/1538-4357_585_2_L77.pdf }}
| |
| </ref>
| |
| speculative theories of quantum gravity "[[quantum foam|foam]]" where there are space–time fluctuations on the Planck scale predict that images of extremely distant objects should be blurry. However, blurring was not seen in the Hubble images, which was claimed to be problematic for such theories.<ref name="space.com">{{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}}
| |
| </ref>
| |
| Other authors have disputed this, in particular Ng ''et al.'',<ref>{{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|issue=2|pages=L87–L89 |doi=10.1086/377121|bibcode=2003ApJ...591L..87N|arxiv = astro-ph/0302372 |url=http://iopscience.iop.org/1538-4357/591/2/L87/pdf/1538-4357_591_2_L87.pdf}}
| |
| </ref>
| |
| who stated that the blurring effect was overestimated by Lieu and Hillman by factors of between 10<sup>15</sup> and 10<sup>30</sup>, and thus the observations are very much less effective in constraining theory: "the cumulative effects of spacetime fluctuations on the phase coherence of light [in certain theories of 'foamy' spacetime] are too small to be observable".
| |
| | |
| ==See also==
| |
| * [[Chronon]]
| |
| * [[Orders of magnitude (time)]]
| |
| * [[Planck energy]]
| |
| * [[Planck length]]
| |
| * [[Planck units]]
| |
| * [[Quantum clock]]
| |
| | |
| == Notes and references==
| |
| {{Reflist|2}}
| |
| | |
| {{Planck's natural units}}
| |
| {{Time measurement and standards}}
| |
| {{Time topics}}
| |
| {{Orders of magnitude seconds}}
| |
| {{Portal bar|Physics}}
| |
| | |
| {{Use dmy dates|date=May 2011}}
| |
| | |
| [[Category:Natural units|Time]]
| |
| [[Category:Units of time]]
| |
| [[Category:Physical constants]]
| |
Hi, everybody!
I'm Russian female ;=).
I really like The Simpsons!
my blog; FIFA 15 coin hack [My Site]