Zack Savitsky in Quanta:
Before Erwin Schrödinger’s cat was simultaneously dead and alive, and before pointlike electrons washed like waves through thin slits, a somewhat lesser-known experiment lifted the veil on the bewildering beauty of the quantum world. In 1922, the German physicists Otto Stern and Walther Gerlach demonstrated that the behavior of atoms was governed by rules that defied expectations — an observation that cemented the still-budding theory of quantum mechanics.
“The Stern-Gerlach experiment is an icon — it’s an epochal experiment,” said Bretislav Friedrich, a physicist and historian at the Fritz Haber Institute in Germany who recently published a review and edited a book on the subject. “It was indeed one of the most important experiments in physics of all time.”
More here.