Planck constant

Planck introduced h in December 1900 as an act of desperation: only by letting oscillators exchange energy in lumps of hf could he fit the blackbody spectrum. He expected the lumps to vanish from the finished theory. Instead they turned out to be real, and h became the scale that separates the quantum world from the classical one — every photon energy, every uncertainty relation, every energy level in every atom carries a factor of h.

On 20 May 2019 the SI turned Planck's fudge factor into the anchor of the mass scale. Fixing h at exactly 6.626 070 15 × 10⁻³⁴ J·s retired the platinum-iridium cylinder that had been the kilogram since 1889 and was quietly drifting by tens of micrograms; a Kibble balance now weighs mass against electrical power traceable to h. The unit J·s is the unit of action, identical to kg·m²/s — the same dimensions as angular momentum, which is why ħ measures spin.