Planck time
The Planck time is the interval light takes to cross one Planck length, about 5.39 × 10⁻⁴⁴ seconds. It is not a measurement of anything; it is what you get by combining the gravitational constant, the speed of light and the reduced Planck constant into a quantity with the dimensions of time, and it is roughly 10²⁵ times shorter than the briefest pulse any laboratory has produced.
Watch out: It is often described as the shortest meaningful duration. That claim is a physical conjecture, not a definition: it marks the scale at which a quantum theory of gravity is expected to be needed, and no such theory is confirmed. Its numerical value also inherits the uncertainty of G, which is the worst-measured of the fundamental constants.
| 1 tP | 5.3912470e-44 s |
Max Planck proposed the natural units in 1899, before relativity and before quantum mechanics, on the grounds that a system built only from universal constants would mean the same thing to any civilisation anywhere. The set survived both revolutions unchanged.