Planck constant

Syllable breakdown

Dictionary meaning

a fundamental physical constant, denoted h, that relates the energy of a photon to its frequency; its value is approximately 6.626 × 10⁻³⁴ joule-seconds, and it sets the scale at which quantum effects become significant; introduced by Max Planck in 1900 to resolve the ultraviolet catastrophe in blackbody radiation, the Planck constant underpins all of quantum mechanics; the reduced Planck constant ℏ (h-bar), equal to h divided by 2π, appears throughout quantum mechanics in expressions for angular momentum quantisation, the Heisenberg uncertainty principle, and Schrödinger's equation; in 2019, the Planck constant was used to redefine the kilogram in the International System of Units