In physics, the Planck length, denoted ℓ P, is a unit of length, equal to 1.616 199 (97) × 10 metres. It is a unit in the system of Planck units, developed by physicist Max Planck. The Planck length is defined in terms of three fundamental physical constants : the speed of light, the Planck constant, and the Newtonian constant of gravitation. In contrast, the largest observable thing is the observable universe. The comoving distance – the distance as would be measured at a specific time, including the present – between Earth and the edge of the observable universe is 46 billion light-years (14 × 10 ^ pc), making the diameter of the observable universe about 91 billion light-years (28 × 10 ^ pc).