Trying to solve the 'ultraviolet catastrophe' in blackbody radiation, Max Planck proposed that energy is emitted and absorbed in discrete packets, or quanta, rather than as a continuous stream. He presented this idea in Berlin, deriving a formula that matched experimental data with startling accuracy. Planck himself doubted the physical reality of quanta at first, viewing them as a mathematical convenience, but the concept became the seed of quantum mechanics.
Physicists at the close of the 19th century faced an embarrassing failure: classical theory predicted that a heated black body should radiate infinite energy at short wavelengths, a nonsensical result dubbed the ultraviolet catastrophe. Max Planck, a conservative and methodical physicist in Berlin, had already found an empirical formula that fit the observed spectrum. The trouble was justifying it theoretically.
His solution, presented to the German Physical Society, was radical though he did not fully embrace its implications: he assumed that oscillators in the cavity walls could only exchange energy in discrete units proportional to frequency, related by what became known as Planck's constant, h. This was not, at the time, intended as a statement about the fundamental nature of light or matter — Planck called it 'an act of desperation,' a mathematical trick to make the numbers work.
It took Albert Einstein's 1905 paper on the photoelectric effect to argue that light itself travels in quantized packets, and further decades of work by Bohr, Heisenberg, and Schrödinger to build quantum mechanics into a full theory. But Planck's presentation is conventionally marked as quantum theory's birth date, a quiet technical talk that opened the door to the strangest and most successful physical theory ever developed.