ℏ

Planck's eponymous constant ℏ (pronounced h-bar) was introduced by Max Planck to explain experimental observations of the radiation spectra of black bodies. Planck's constant was broadened in meaning by Albert Einstein, who introduced the quantum of electromagnetic energy, now called a photon. Planck's constant ℏ relates a photon's energy to its radial frequency:

E = ℏω

The currently accepted value for ℏ is 1.054571817×10^-34 J-s (according to the 2019 International Bureau of Weights and Measures).

Planck's constant is ubiquitous in quantum mechanics, where it relates energy to frequency and momentum to wavevector, in other words relating kinematic properties to wave properties. Planck's constant most famously appears in Heisenberg's uncertainty relations, placing limits on the precision of simultaneous measurements of position and momentum, or of energy and time. More fundamentally, it appears in Schrödinger's wave formulation of quantum mechanics and Dirac's relativistic wave equation for the electron. Planck's constant also appears in many fundamental natural constants such as the fine-structure constant, the quantum of conductance and the quantum of magnetic flux.