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In physics, the Planck time (t_P), 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. The unit is named after Max Planck, who was the first to propose it.

The Planck time is defined as:

${\displaystyle t_{P}={\sqrt {\frac {\hbar G}{c^{5}}}}\approx 5.39124(27)\times 10^{-44}{\mbox{ s}}}$

where:

${\displaystyle \hbar =h/2\pi }$ is the reduced Planck constant (sometimes ${\displaystyle h}$ is used instead of ${\displaystyle \hbar }$ in the definition )

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 of the estimated value.

Physical significance

The Planck time is the time a beam of light needs to travel 1 Planck length. As of 2006, the smallest unit of time that has been directly measured is on the order of 1 attosecond (10 s), or about 10 Planck times. There is also speculation that 1 Planck time after the start of 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 been the first fundamental interaction to emerge from the Big Bang, due to its homogeneity to the others. Others have proposed that the strong nuclear force is the most likely first fundamental interaction, because of its strength.)

According to quantum theory, 1 Planck time should be the smallest unit of time physics can reason about in a meaningful way. But according to news reports, analyses of Hubble Space Telescope Deep Field images in 2003 raised a possible discrepancy. Very distant images that should have been blurry were not, contradicting the notion that Planck time is indeed the smallest measurable unit of time.

See also

• Orders of magnitude (time)

References

1. ^ "Big Bang models back to Planck time". Georgia State University. 19 June 2005. {{cite web}}: Check date values in: |date= (help)
2. CODATA Value: Planck Time – The NIST Reference on Constants, Units, and Uncertainty.
3. "Shortest time interval measured". BBC News. 25 February 2004. {{cite web}}: Check date values in: |date= (help)
4. "Fastest view of molecular motion". BBC News. 4 March 2006. {{cite web}}: Check date values in: |date= (help)
5. ^ "Hubble Pictures Too Crisp, Challenging Theories of Time and Space". Space.com. 2003-04-02. Retrieved 2008-05-30.
6. Lieu, Richard (2003-03-10). "The Phase Coherence of Light from Extragalactic Sources: Direct Evidence against First-Order Planck-Scale Fluctuations in Time and Space". The Astrophysical Journal. 585: L77–L80. doi:10.1086/374350. Retrieved 2008-05-30. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
7. Ng, Y. Jack (2003-07-10). "Probing Planck-Scale Physics with Extragalactic Sources?". The Astrophysical Journal Letters. 591. The American Astronomical Society: L87–L89. doi:10.1086/377121. Retrieved 2008-05-30. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

Template:Planckunits

v t e Time measurement and standards
Chronometry Orders of magnitude Metrology
International standards	Coordinated Universal Time offset UT ΔT DUT1 International Earth Rotation and Reference Systems Service ISO 31-1 ISO 8601 International Atomic Time 12-hour clock 24-hour clock Barycentric Coordinate Time Barycentric Dynamical Time Civil time Daylight saving time Geocentric Coordinate Time International Date Line IERS Reference Meridian Leap second Solar time Terrestrial Time Time zone 180th meridian
Obsolete standards	Ephemeris time Greenwich Mean Time Prime meridian
Time in physics	Absolute space and time Spacetime Chronon Continuous signal Coordinate time Cosmological decade Discrete time and continuous time Proper time Theory of relativity Time dilation Gravitational time dilation Time domain Time-translation symmetry T-symmetry
Horology	Clock Astrarium Atomic clock Complication History of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch stopwatch Water clock Sundial Dialing scales Equation of time History of sundials Sundial markup schema
Calendar	Gregorian Hebrew Hindu Holocene Islamic (lunar Hijri) Julian Solar Hijri Astronomical Dominical letter Epact Equinox Intercalation Julian day Leap year Lunar Lunisolar Solar Solstice Tropical year Weekday determination Weekday names
Archaeology and geology	Chronological dating Geologic time scale International Commission on Stratigraphy
Astronomical chronology	Galactic year Nuclear timescale Precession Sidereal time
Other units of time	Instant Flick Shake Jiffy Second Minute Moment Hour Day Week Fortnight Month Year Olympiad Lustrum Decade Century Saeculum Millennium
Related topics	Chronology Duration music Mental chronometry Decimal time Metric time System time Time metrology Time value of money Timekeeper

• Natural units
• Units of time
• Physical constants