Back in the late 19th century a lot of scientists thought physics was almost over. They thought most of the big stuff had been figured out and only a few details remained.
But then Max Planck was commissioned to try and make light bulbs more efficient and this led him to look at electromagnetic radiation. Specifically, he wanted to know how the intensity of radiation depended on its frequency.
What he discovered was that radiation was not emitted smoothly, like a ramp perhaps, but that it came in discrete chunks, more like a staircase. He found that the energy of radiation was its frequency multiplied by a specific number that seemed to define a minimum chunk size.
The specific number in question is now called Planck’s Constant, which is $6.62607015×10^-34$. That’s a ludicrously tiny number but it’s a chunk nevertheless. ‘Chunks’ would never do as a name for something in science so they were instead called quanta and these were the early roots of quantum mechanics.
Planck was quite a conservative chap and really didn’t fancy all this quantum nonsense and said:
My unavailing attempts to somehow reintegrate the action quantum into classical theory extended over several years and caused me much trouble.
It’s probably fair to say he didn’t really grasp the whole import of what he discovered.
Then in 1905, along came Albert Einstein who took the world by storm with four ground-breaking papers that year: he explained Brownian motion as evidence of atoms, he changed our entire concept of time, light and motion with the Special Theory of Relativity, he related mass and energy through his famous $E = mc^2$ and, pertinent to Planck, he theorised that all energy came in discrete chunks via his explanation of something called the photoelectric effect. Talk about having a good year.
At first a lot of people thought Einstein was merely a rebellious upstart and they didn’t pay too much attention to his papers, but eventually they realised that a quantum nature explained a great many things and in the 1920s quantum mechanics got firmly underway.
Neils Bohr, a Dane, placed himself at the centre of the quantum mechanics revolution and his exchanges with Einstein became the stuff of legend. Einstein was always reluctant to accept quantum mechanics as the final say on elementary particle physics. He didn’t like the uncertain, statistical, probabilistic nature of quantum theory and thought it was just a stepping stone to a more precise, deterministic theory that lay below. Whilst Einstein’s challenges often troubled the Dane, Bohr managed to successfully counter most of them in time. The so-called Copenhagen interpretation of quantum mechanics developed by Bohr and his cohorts is still used today.
Many great scientists were involved in the development of quantum mechanics: Werner Heisenberg, Erwin Schrodinger, Wolfgang Pauli, Paul Dirac, Max Born and Eugene Wigner to name but a few.
The thing is, so strange was this new science compared to what they were used to, they had to make up a lot of it as they went along. They often worked on instinct as much as anything, at least initially, but they also had the intellectual nous to back up those instincts with testable theories.
A Belgian industrialist called Ernest Solvay started organising a series of conferences for scientists in 1911 and they took place every few years or so. In 1927 they had such a conference entitled Electrons and Photons and the prominent scientists of the day were invited. The discussions were mainly about the then new theory of quantum mechanics.
I’m of the opinion that this particular 1927 conference was possibly the greatest meeting of minds the world has ever seen. There were 29 attendees and 17 of those either had or would go on to get Nobel Prizes, mostly for their contributions to quantum theory.
There were many big men of science long before 1927 of course: Galileo, Copernicus, Newton, Maxwell and the like, and there have been some truly great scientists since then: Feynman, Bell, Wheeler and Gell-Man to name but a few. But I think 1927 was the epoch.
The Rapports et Discussions are still available for that conference, as long as you read French.