Planck’s quantum hypothesis would revolutionize physics, but he initially thought it wasn’t real.

He suspected that the interaction of matter and radiation was tremendously complicated but still governed by the physics known at the time — what we now call “classical physics.”

Invoking quanta of radiation to derive the blackbody emission spectrum was, it seemed to Planck, just a mathematical trick that somehow encapsulated all that complication.

If so, it was a trick that *worked*.

Previous efforts to derive the blackbody emission spectrum using classical physics gave nonsensical results at high frequencies. Replace continuous emission with discrete quanta and things work out perfectly.

Evidence for the reality of quanta began with Einstein’s explanation of the photoelectric effect in 1905. The proposal that radiation of frequency f exists only in discrete chunks of energy E=hf was the first result of Einstein’s “Miracle Year.”

Here's a great APS Landmarks entry where you can read more about Compton Scattering and the reality of photons.

https://physics.aps.org/story/v13/st8

This sequence of events was a not uncommon narrative in the transformative years of late 19th and early 20th century physics. It would go something like this:

1. An experimental result doesn’t make sense

2. A physicist proposes a radical solution.

3. The solution seems too weird to be real. Everyone assumes it must be a convenient "trick."

4. Eventually, everyone realizes it wasn't a trick, and reality is in fact weirder than they expected.

Planck’s hypothesis was in fact a deep statement about the nature of Nature. Electromagnetic adiation of frequency f comes in discrete chunks of energy E=hf. It's not an artifact of a complicated interaction with matter.

Nowadays we refer to the proportionality constant h as "Planck's constant." In the SI system it takes the value h=6.626 x 10⁻³⁴ kg*m²/s, or 4.136 x 10⁻¹⁵ eV*s.

As far as we can tell, h is a fundamental constant that is hardwired into our Universe.

The results of Planck’s December 14th lecture were submitted to Annalen der Physik a few weeks later, on January 7th, and published not long after that.

You can see the original (in German) here:
https://onlinelibrary.wiley.com/doi/epdf/10.1002/andp.19013090310

An English translation of Planck’s paper, taken from Dirk ter Haar’s “The Old Quantum Theory,” is available here:
https://web.archive.org/web/20230529024105/http://hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Planck%20(1900),%20Distribution%20Law.pdf

Something that isn’t as widely known is that “h” is not the only fundamental constant that was introduced during Planck’s presentation to the German Physical Society #OTD in 1900!

Planck was investigating emission and absorption by blackbodies, so he needed a way to relate energies and temperatures. Thus, he had to introduce what we now refer to as “Boltzmann’s constant.”

That’s right, the factor of “k” that appears in the formula “S = k log W” — the formula engraved on Boltzmann’s memorial! — was actually introduced by Max Planck. Boltzmann expressed his results in terms of the ideal gas constant R.

Image: user Daderot on Wikipedia