A spectrum two laws could not fit
The claim at the centre of this brief is narrow but consequential: to make his 1900 formula for blackbody radiation match observation, Max Planck assumed that the oscillators responsible for the radiation could only emit or absorb energy in discrete units, each proportional to frequency and expressed as E equals h times frequency, where h is now called Planck’s constant. Planck did not present this as a discovery about how nature works. He treated it as a formal assumption needed to get the mathematics to fit the data, something he could not justify from classical physics and, by his own account, did not think much about at the time as a statement of physical reality.
An assumption Planck did not believe
The problem Planck was solving concerned how the intensity of radiation from a heated body varies with frequency. One existing formula, due to Wilhelm Wien, matched observations at high frequencies but failed at low ones; another, associated with Rayleigh and Jeans, worked at low frequencies but predicted implausibly large output at high ones. Planck had tried an earlier approach in 1899 using a principle of elementary disorder, which experiment then contradicted. He revised his method, presented a new blackbody radiation law in October 1900, and traced its origin, in a paper the following December, to the assumption that energy comes in discrete quanta rather than a continuous range.
An act of despair
What holds up is the law itself and the constant it introduced: Planck’s blackbody formula and the value of h have remained standard physics since 1900, and the postulate of quantised energy became foundational to everything that followed in quantum theory. Planck later described turning to Ludwig Boltzmann’s statistical methods, which he had previously argued against, as an act of despair, saying he was ready to sacrifice his prior convictions about physics to make progress. That willingness to adopt a method he distrusted, in service of a result he also distrusted, is part of why the postulate survived: it worked, even though its author was not prepared to say why.
Physicists who set h to zero
What did not hold up, for some years, was the idea that the quantisation was physically real rather than a mathematical trick. Rayleigh, Jeans and Hendrik Lorentz responded to Planck’s law by effectively setting the constant to zero so that the result would align with classical physics, a move that frustrated Planck. Planck himself continued to treat the quantum as a formal device rather than a fact about energy, and when Einstein proposed in 1905 that light itself travels in discrete quanta to explain the photoelectric effect, Planck resisted that extension, worried it would undermine the established, continuous picture of electromagnetism that Maxwell’s equations described.
Einstein’s photons, resisted by their own source
The postulate’s significance beyond Planck’s own radiation problem came from that resistance being overturned. Einstein’s use of quantised energy to explain the photoelectric effect, and Niels Bohr’s later use of the same idea to explain the pattern of light emitted by hydrogen atoms, showed that Planck’s assumption was not confined to blackbody radiation but described something general about how energy behaves at small scales. What began as a formula-fitting device became the seed of quantum mechanics, the framework now used across physics and chemistry to describe atoms, radiation and matter. Planck received the 1918 Nobel Prize in Physics for the discovery, awarded the following year, for an idea he had spent years treating as provisional.
A postulate that outgrew its author
Worth reading selectively: the source material here is thin, a short article on the postulate itself supplemented by Planck’s biography, so this suits a reader who wants the specific moment quantisation entered physics rather than a full account of quantum theory’s development. Its value is in the detail that Planck did not believe his own postulate was physically true, and that other leading physicists spent years trying to write the constant back out of their equations. That gap between what was proposed and what was believed at the time is a sharper way into the history of quantum mechanics than a straightforward account of the discovery would be, even if it does not cover the theory’s later elaboration.
