Planck units may sometimes be semi-humorously referred to by physicists as ""God's units"". They eliminate human centred arbitrariness from the system of units: some physicists argue that communication with extraterrestrial intelligence would have to use such a system of units to make common reference to scale. Unlike the meter and second, which exist as fundamental units in the SI system for historical reasons, the Planck length and Planck time are linked at a fundamental physical level.