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 required for light to travel, in a vacuum, a distance of 1 Planck length.[1] The unit is named after Max Planck, who was the first to propose it.

The Planck time is defined as:

t_P \equiv \sqrt{\frac{\hbar G}{c^5}} \approx 5.39124(27) \times 10^{-44} \mbox{ s}[2]

where:

\hbar = h / 2 \pi is the reduced Planck constant (sometimes h is used instead of \hbar in the definition[1])
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 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 relativity constant c, and the 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_P / t is large. Dimensional analysis suggests that the effects of both quantum mechanics and gravity will be important under these circumstances, requiring a theory of quantum gravity. Unfortunately, all of our scientific experiments and human experience happens over billions of billions of billions of Planck times, which makes it hard to directly probe the events happening at the Planck scale.

As of 2010, the smallest unit of time that was directly measured was on the order of 20 attoseconds (10−18 s), or about 3.7 × 1026 Planck times.[3]

Analysis of Hubble Space Telescope 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,[4] speculative theories of quantum gravity "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.[5] Other authors have disputed this, in particular Ng et al.[6], who stated that the blurring effect was overestimated by Lieu and Hillman by factors of between 1015 and 1030, 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

References

  1. ↑ 1.0 1.1 "Big Bang models back to Planck time". Georgia State University. 19 June 2005. http://hyperphysics.phy-astr.gsu.edu/hbase/astro/planck.html. 
  2. ↑ CODATA Value: Planck Time – The NIST Reference on Constants, Units, and Uncertainty.
  3. ↑ "Scientists Measure Shortest Interval of Time Ever". Neatorama. 29 July 2010. http://www.neatorama.com/2010/07/29/scientists-measure-shortest-interval-of-time-ever/. 
  4. ↑ Lieu, Richard; Hillman, Lloyd W. (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. http://www.journals.uchicago.edu/doi/pdf/10.1086/374350. Retrieved 2008-05-30. 
  5. ↑ "Hubble Pictures Too Crisp, Challenging Theories of Time and Space". Space.com. 2003-04-02. http://www.space.com/scienceastronomy/quantum_bits_030402.html. Retrieved 2008-05-30. 
  6. ↑ Ng, Y. Jack; Christiansen, W. A.; van Dam H. (2003-07-10). "Probing Planck-Scale Physics with Extragalactic Sources?". The Astrophysical Journal Letters (The American Astronomical Society) 591: L87–L89. doi:10.1086/377121. http://www.journals.uchicago.edu/doi/abs/10.1086/377121. Retrieved 2008-05-30. 
Planck's natural units
Base Planck units
Planck time · Planck length · Planck volume · Planck mass · Planck charge · Planck temperature
Derived Planck units
Planck energy · Planck force · Planck power · Planck density · Planck angular frequency · Planck pressure · Planck current · Planck voltage · Planck impedance · Planck momentum
Orders of magnitude (time), by powers of seconds
Negative powers

Planck time | <1 attosecond | Attosecond | Femtosecond | Picosecond | Nanosecond | Microsecond | Millisecond
Positive powers

Second | Kilosecond | Megasecond | Gigasecond | Terasecond | Petasecond | Exasecond and longer