Max Planck launched quantum theory on December 14, 1900

Max Planck introduced the quantum on 14 December 1900, when he told the German Physical Society in Berlin that hot objects give off radiation in discrete packets of energy, each a multiple of a tiny unit now written h. Historians widely treat that lecture as the birth of quantum theory.

The problem he solved was blackbody radiation: how much light a hot object emits at each frequency. Wilhelm Wien’s formula worked at high frequencies, but by October 1900 new measurements showed it failing completely at low ones.

Three dates, one discovery

• 7 October 1900: after a Sunday visit from his Berlin colleague, the experimentalist Rubens, who brought the new low-frequency data, Planck works out the right curve that same evening. JQI’s Luis Orozco argues this, not December, is quantum mechanics’ true birthday.
• 19 October: Planck presents the formula to the German Physical Society. It fits the data and is hailed as correct, but to him it is a “lucky guess”.
• 14 December: he derives it from first principles, at a price: the oscillators in a hot body must absorb and emit energy only in discrete quanta.

Planck later described that step as “an act of desperation”, and he was a reluctant revolutionary. Physicists adopted his formula at once but took years to notice the radical idea underneath; it was Einstein’s explanation of the photoelectric effect, among other results, that made quanta impossible to ignore. Planck received the 1918 Nobel Prize in Physics “for the discovery of energy quanta”.

His constant has since become one of the foundations of measurement itself. Since 2019, h is no longer measured but fixed by definition at exactly 6.626 070 15 × 10⁻³⁴ joule-seconds, and the kilogram is defined from it, replacing Le Grand K, the platinum-iridium cylinder in a vault near Paris that had defined it for more than a century.