The Planck length, denoted P, is a unit of length, equal to 1.616229(38)×10−35 metres. It is a base unit in the system of Planck units, developed by physicist Max Planck. The Planck length can be defined from three fundamental physical constants: the speed of light in a vacuum, the Planck constant, and the gravitational constant. Planck time is the time required for light to travel in a vacuum a distance of 1 Planck length, which is approximately 5.39 × 10 −44 s.