Friedrich, Bretislav: How a Bad Cigar Assisted at the Birth of Quantum Mechanics

How a Bad Cigar Assisted at the Birth of Quantum Mechanics

In 1913, Otto Stern vowed to "quit physics if there was anything in this nonsense of Bohr's [model of the atom]. Seven years later, he proposed an experiment to put Bohr's theory to a crucial test. His aim was to decide "unequivocally between the quantum theoretical and classical views," by proving or disproving the existence of space quantization.
Space quantization emerged as a curious consequence of the Bohr model. Unlike the quantization of energy, which had, arguably, an observable counterpart in the atomic spectra, space quantization was considered a theoretical concept rather than an observable property. Nevertheless, Stern took the question of the existence of space quantization literally and, in his proposal, showed a way to challenge it by a direct experiment. The question arose from Stern's interest in magnetism and atomic theory, and put one to the service of the other. The experiment, accomplished by Stern and Walther Gerlach at Frankfurt after a two-year struggle, provided definitive evidence for the reality of space quantization.
The Stern-Gerlach experiment ranks among the dozen or so canonical experiments that ushered in the heroic age of quantum physics. Perhaps no other experiment is so often cited for elegant conceptual simplicity. From it emerged both new intellectual vistas and a host of useful applications of quantum science. Yet today, even among atomic physicists, we have found very few aware of historical particulars that enhance the drama of the story and the abiding lessons it offers. Among these particulars are a warm bed, a bad cigar, a timely postcard, a railroad strike, and an uncanny 'conspiracy of Nature' that rewarded Stern's and Gerlach's audacity. Their success in splitting a beam of silver atoms by means of a magnetic field startled, elated, and confounded pioneering quantum theorists, including several who beforehand regarded an attempt to observe space quantization to be naive and foolish.
Our narrative begins by briefly outlining the background of Stern and Gerlach and the historical context that brought them to collaborate at Frankfurt. We then describe the conception, vicissitudes, and reception of their iconic experiment and conclude with an epilogue tracing the divergent paths of the protagonists.