Planck's quantum of action

Planck's quantum of action was one of three universal constants, along with toast always falling butter-side down and the reaction of someone entering the TARDIS for the first time. It was invented by and named after Max Planck. (PROSE: The Secret in Vault 13)

Behind the scenes

Planck's constant, or $h$, was a physical constant (with units of action in SI units) derived by Max Planck in his experiments with black body radiation. In order to explain the results he observed, only energy states of $hv$, where $v$ was the frequency of the radiation, were possible. This constant would later re-emerge throughout much of quantum mechanics, including in equations such as wavelength of a particle or the Schrödinger equation.