“Natural units” of measurement (i.e. designed so that certain fundamental physical constants are normalized to 1), named after the German physicist Max Planck who first proposed them in 1899. They were an attempt to eliminate all arbitrariness from the system of units, and to help simplify many complex equations in modern physics. Among the most important are the Planck Energy, the Planck Length, the Planck Time and the Planck Temperature.