# 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. The unit is named after Max Planck, who was the first to propose it.

The Planck time is defined as: $t_\mathrm{P} \equiv \sqrt{\frac{\hbar G}{c^5}}\approx 5.39106 (32) \times 10^{-44}\ \mathrm{s}$

where:

ħ = h2 π is the reduced Planck constant (sometimes h is used instead of ħ 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 unique combination of the gravitational constant G, the special-relativistic constant c, and the quantum constant ħ, to produce a constant with units of time. Because the Planck time comes from dimensional analysis, which ignores constant factors, there is no reason to believe that exactly one unit of Planck time has any special physical significance. Rather, the Planck time represents a rough time scale at which quantum gravitational effects are likely to become important.[clarification needed] The nature of those effects, and the exact time scale at which they would occur, would need to be derived from an actual theory of quantum gravity. All scientific experiments and human experiences occur over trillions upon trillions of Planck times, making any events happening at the Planck scale hard to detect. As of May 2010, the smallest time interval uncertainty in direct measurements is on the order of 12 attoseconds (1.2 × 10−17 seconds), about 2.2 × 1026 Planck times.