It is speculated that the most natural units of mass, length, and time can be related to the fundamental physical constants associated with the theories of relativity, quantum mechanics, and gravitation. This originated from a proposal (ca. 1900) of Max Planck and is consequently designated as the Planck scale. The speed of light, is inextricably associated with electromagnetism and the special theory of relativity. Likewise, Planck's constant, most conveniently expressed as , is the essential parameter in quantum theory, while the gravitational constant is fundamental in both Newtonian gravity and the general theory of relativity.