Max Planck
Can energy come in countable pieces?
That time, that place
Heated iron glows red, then white as it grows hotter. How much light of each color comes out at each temperature had been measured precisely.
The trouble was that theory did not match. Calculated with the physics of the day, the energy ran to infinity at short wavelengths. Nothing of the sort happens in reality.
Why this question
Planck was a conservative man, and what he did did not begin from a bold idea.
He first found a formula that fit the measurements — almost by fitting it to them. Then he asked in reverse: what would have to be assumed for this formula to follow?
What was found
The assumption was this: energy is not exchanged in arbitrarily fine amounts but only in packets of a definite size.
Not poured like water but handed over in discrete coins. The size of one packet is proportional to the frequency.
With that, the calculation matched the measurements exactly.
And Planck did not believe it. He regarded the assumption not as a property of nature but as a device for getting the formula, and spent years afterward trying to find a way back to the older physics.
Energy could be divided as finely as you like
A calculation that matched the data only with the packet assumption in place
Five years later Einstein argued the packets were not a device but a property of light itself. Planck opposed him for a long time