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Much of my work has had its origin in the notion that science should treasure its own history, that historical scholarship should treasure science, and that the full understanding of each is deficient without the other.
-Gerald Holton
The Advancement of Science, and Its Burdens]
IN THIS ESSAY WE REVISIT A TREASURED EPISODE from the heroic age of atomic physics. The story centers on an experiment, elegantly simple in its conception, extraordinarily startling in its outcome, and extremely fruitful in its legacy. From it emerged both new intellectual vistas and a host of useful applications of quantum science. Yet this germinal experiment, carried out at Frankfurt in 1921-22 by Otto Stern and Walther Gerlach, is not at all familiar except to physical scientists.2 Even among them we have found very few aware of historical particulars that enhance the drama of the story and the abiding lessons it offers about how science works. These particulars include a bad cigar that amplified a puny signal, a postcard from New York that offset the huge inflation then rising in Germany, and an uncanny "conspiracy of Nature" that rewarded the audacity of the experimenters, despite the inadequacies of a fledgling theory that had led a skeptical Otto Stern to devise his crucial test.
We begin by describing the historical context of the experiment, chiefly stemming from the atomic model proposed by Niels Bohr in 1913, nowadays referred to as the "old quantum theory." Our intent is to provide an account accessible to anyone with only vague memories of high-school physics or chemistry. But as background we need to discuss a few concepts, to show how Stern's interest was whetted by the tantalizing, partial successes and patent failures of Bohr's model when confronted with atomic spectra and magnetism. Stern came to focus on the idea of space quantization. This was one of the most peculiar inferences from the old quantum theory, the notion that the quasiplanetary electron orbits postulated by Bohr could have only certain discrete orientations in space. Even Stern's theoretical colleagues who had invoked this idea regarded it as merely a mathematical construct, devoid of physical reality.
We next trace the conception, preparation, vicissitudes, and realization of the Stern-Gerlach experiment. It showed unequivocally that space quantization was...