# Planck time

The Planck time is the natural unit of time, denoted by tP. It is considered the smallest possible measurement of time.

[itex]t_P = \sqrt{\frac{\hbar G}{c^5}} \approx[itex] 5.391 × 10-44 seconds

where:

[itex]\hbar[itex] is Dirac's constant
G is the gravitational constant
c is the speed of light in a vacuum

The Planck time is the time it would take a photon travelling at the speed of light to cross a distance equal to the Planck length. This is the "quantum of time", the smallest measurement of time that has any meaning, (roughly 10-43 seconds). Within the framework of the laws of physics as we understand them today, we can neither measure nor discern any difference between the universe at the time it first came into existence and the universe anything less than 1 Planck time later.

The estimated age of the Universe (4.3 × 1017 s) is 8 × 1060 Planck times.

## A simple definition

Planck's time is how long it takes for light to travel Planck's length.

The Planck length is the smallest amount of length with significance, as below this length quantum laws reign, meaning all measurements are nonsense. The speed of any object is limited by Einstein's laws of relativity, which state that nothing can travel faster than the speed of light in a vacuum (denoted [itex]c[itex]).

The distance [itex]d[itex] traveled by an object traveling at constant speed [itex]s[itex] in time [itex]t[itex] is given by the equation [itex]d = st[itex]. Since we know the Planck length and the speed of light, we can insert these values into the equation to get the Planck time. Thus the Planck time is equal to 1.616 × 10-35 m / (299,792,458 m/s) [itex]\approx[itex] 5.39 × 10-44 seconds.

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