Reading Einstein’s 1905 paper on the quantization of light for the first time, I was floored by the following passage in section 1:
Eine Anzahl Elektronen sei ferner an voneinander weit entfernte Punkte des Raumes gekettet durch nach diesen Punkten gerichtete, den Elongationen proportionale Kräfte. Auch diese Elektronen sollen mit den freien Molecülen und Elektronen in konservative Wechselwirkung treten, wenn ihnen letztere sehr nahe kommen. Wir nennen die an Raumpunkte geketteten Elektronen “Resonatoren”; sie senden elektromagnetische Wellen bestimmter Periode aus und absorbieren solche.
which is translated by Arons and Peppard as:
Furthermore, let there be a number of electrons which are bound to widely separated points by forces proportional to their distances from these points. The bound electrons are also to participate in conservative interactions with the free molecules and electrons when the latter come very close. We call the bound electrons “oscillators”; they emit and absorb electromagnetic waves of definite periods.
A linear force with distance (similar to a spring) gives a parabolic potential; Einstein is (crudely) describing the Quantum Harmonic Oscillator a couple of decades before anybody solved it or realized how important it was! In some ways, this even anticipates the Quantum Field Theory of the 1940s-1960s with its emphasis on modeling the electromagnetic field as a set of non-interacting (“widely separated”) QHOs!
Energy and Time 