Max Planck was the first quantum mechanic. He began as an "old school" mechanic, thoroughly steeped in Newtonian mechanics and Maxwell's electrodynamics; those laws conveniently sorted physics into the "corpuscular" and the "ethereal" domains, a dichotomy that corresponded to things having mass and things lacking mass, i.e., radiant light. Higgs had something to say about this later.
Isaac Newton had solved the age-old riddle of why apples fall, allowing astronomers to predict the motions of the heavens. Miracle, mystery and authority. Newton also dabbled in light, describing reflection and interference--along with his famous prism experiments--but the mathematical laws governing light propagating "through the ether" were first described by a Scotsman, James Clerk Maxwell. Newton's Laws of Gravitation governed masses while Maxwell's electrodynamics ruled the waves. Planck came along in 1900 and sort of melded the two theories at their interface.
Physics then had a big unsolved problem called "black body radiation." Heating kilns and ovens made the inside walls glow--first red, then yellow, and finally white hot. Glass makers and potters could even gauge an oven's temperature based on its color inside. Adding more heat to an oven made the walls give off progressively higher energy light but there was a limit: ovens would not begin to emit UV light. Light bulbs were another 19th century invention that used heat (electrical resistance) to produce light and Planck was motivated in part by practical concerns.
What Planck did can be summarized visually with a plot of light intensity versus wavelength:
Planck's theoretical curves (berechnet) agreed beautifully with experiment (beobachtet). Note that there are seven different non-overlapping curves corresponding to progressively higher temperatures. Prior attempts to predict the same phenomenon, based on classical electrodynamics, had failed. These attempts are neatly summarized in this graphic:
The green line corresponds to Planck's law and to reality; the red line, Rayleigh-Jeans Law, only worked at low frequencies (long wavelengths), while the blue line, Wien's Law, worked only at the high frequencies (short wavelengths). As an aside, the red line's straight up ascent was later referred to as the "Ultraviolet Catastrophe" as a sort of metaphor for the failure of classical theory to account for the reality of Planck's Law. But Planck did more than meld two theories--he invented anew.
The newer science of thermodynamics and Maxwell Boltzmann in particular had shown that tiny invisible yet indivisible atoms could statistically sum to bulk properties. The details are grounded in probabilities rather than certainties, much like my Parable Of The Gas. What Planck did was to apply Boltzmann-like statistical mechanics to the problem of black body radiation.
Planck viewed a red-hot oven (black body radiator) as material in equilibrium with light--sort of a transubstantiation of the ethereal and corpuscular. He named the nexus--the unseen--"resonators" and counted them in a statistical way, describing their behavior mathematically. This was all well before anyone knew or even thought that atoms were held together by electrons--atoms were still thought to be amorphous blobs. That something as seemingly seamless as light should be treated like discrete masses when it interacted with matter was an assumption but it proved key to deriving the solution to the black body problem. There was no other way to explain the behavior. What Planck did was revolutionary, but he did not do it because he understood why--he did it because his theory fit experiment. Werner Heisenberg later stated Planck's insight most succinctly and in most certain terms:
Radiant heat is not a continuous flow and indefinitely divisible. It must be defined as a discontinuous mass made up of units all of which are similar to one another.Around the time of Planck's insight, another, younger German physicist appeared on stage. He was then a Swiss patent examiner and barely known, preoccupied with developing his own theories of relativity, but his elastic mind intuitively wrapped around what even Planck had trouble fully accepting and generalizing. Einstein took Planck's teachings and explained the photoelectric effect--why blue light but not red light could make certain metals conduct electricity. It seemed counter intuitive that even the most intense red light could not do what the faintest of blue light could do. Einstein explained that only blue light was energetic enough to knock electrons free. There were energy thresholds and band gaps at the atomic level. Discontinuities and E=hv.
According to Thomas Kuhn, Planck needed the goading of Einstein and Paul Ehrenfast afterwards to fully realize what he had done. Certainly Planck's older contemporaries were doubters too. In Planck's words:
A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.Planck and Einstein remained close friends throughout the 1930's. Planck, conservative Christian, and Einstein, agnostic Jew, enjoyed making music together when not discussing physics in Wilhelmine and Weimar Berlin--while it lasted (maybe they did discuss the physics of music--frequencies, harmonics, metered beats). Planck tried in vain to intervene on his friend's behalf during the rise of the Nazi regime but he ultimately failed.
There was even greater sadness for Planck besides the exile of his great friend Einstein; there was the trial of his eldest son Erwin at the hands of Roland Freisler, whom I described back here. The Nazis executed Erwin Planck just a few heartbreaking months before the whole regime finally collapsed. The older Planck never recovered from the loss of Erwin and died just two years later in 1947. Here they are during happier times:
|Max and son Erwin Planck|
|Erwin Planck vor dem Gesezt in 1945|
The elder Planck's life sort of tracked a shape: A half-century of slow triumph peaked in the 1930's and then precipitously declined, much like the shape of one of his triumphant black body radiation curves which conquered physics. Post-war Germany honored Planck by renaming the Kaiser-Wilhelm-Gesellschaft (its premier scientific society) the Max-Planck-Gesellschaft.