Planck's constant (h)

Planck's constant (symbol h) is the fundamental physical constant of quantum theory, fixing the size of the quantum of action. It relates the energy E of a quantum of electromagnetic radiation to its frequency by E = h nu, the relation Max Planck introduced in 1900 to explain black-body radiation. Its reduced form, written h-bar, equals h divided by 2 pi and appears throughout quantum mechanics, including the uncertainty principle.

#Value and the SI

Since the 2019 revision of the International System of Units, Planck's constant has an exact defined value of 6.62607015 times 10 to the minus 34 joule seconds, with no measurement uncertainty. Fixing this number is what now defines the kilogram: the unit of mass is derived from h together with the definitions of the second and the metre. Because energy and frequency are proportional through h, the constant sets the scale at which quantized, non-classical behavior becomes significant.