Planck Constant
The Planck constant h connects the energy of a photon to its frequency (E = hν). It is the foundation of quantum mechanics and has defined the kilogram in the SI system since 2019.
physics.constants
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Exact Value (h)
Exactly defined since May 20, 2019 (SI redefinition). No measurement uncertainty.
Reduced Planck Constant (ℏ)
ℏ = h / (2π). Also called Dirac constant. Central to the Schrödinger equation.
Introduced
Max Planck derived the constant on December 14, 1900 from black-body radiation.
Defines SI Unit
Since 2019 the kilogram is defined via the fixed value of h (using a Kibble balance).
Relations
The Planck constant and the speed of light are both fundamental constants with exact values in the SI system.
Explanations
The Planck constant (h) is one of physics' most important numbers: it connects the energy of light particles directly to their frequency through the formula E = hν. With a value of 6.626 × 10⁻³⁴ J·s, it's incredibly tiny – but it's so important that since 2019 it has even defined the kilogram in the international measurement system!
Sources
CODATA-Referenzwert der Planck-Konstante.
Erklärung der Neudefinition des Kilogramms über die Planck-Konstante.
Related Cards
Provenance
Status
seed
Review
none
Last Updated
2026-02-17