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.

@mcnees The sad thing is, this sequence of events has been virtually absent from physics for the last 50+ years.

(Feel free to share exceptions!)