Planck time

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:

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

where:

$\hbar =h/2\pi$ is the reduced Planck constant (sometimes $h$ is used instead of $\hbar$ 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

According to quantum theory, 1 Planck time should be the smallest unit of time physics can reason about in a meaningful way. As of 2006, the smallest unit of time that was directly measured was on the order of 1 attosecond (10⁻¹⁸ s), or about 10²⁶ Planck times.

Analyses 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, 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. Other authors have disputed this, in particular Ng et al., who state that the blurring effect was overestimated by Lieu and Hillman by factors of between $10^{15}$ and $10^{30}$, 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

• Orders of magnitude (time)