The Stern-Gerlach Experiment, 1921–1940: From the Old Quantum Theory to Spin

Beginning in 1921, Otto Stern and Walther Gerlach collimated a beam of silver atoms (magnetic moments), and sent them through an inhomogeneous magnetic field. Their experiment, which showed a splitting of their atomic beam into two components and permitted a calculation of the Bohr magneton, was initially interpreted in the context of the “old quantum theory” — essentially, the Bohr-Sommerfeld picture of the atom in which electrons were in circular or elliptical orbits around the nucleus. In 1916, Arnold Sommerfeld augmented this picture when he developed a theory of space quantization, in which magnetic moments took discreet, quantized orientations with respect to an arbitrary direction in space—here, an external magnetic field. Stern and Gerlach adopted Sommerfeld’s theory, and saw their beam splitting as an example of space quantization. The magnetic moment was thought to be due to the orbital angular momentum of the silver atoms in the beam.



After the introduction of spin and modern quantum mechanics in the mid-1920s, the Stern-Gerlach experiment and the closely related Zeeman effect were reinterpreted in this new theoretical context. Now, the orbital angular momentum of the silver atom became zero, and the magnetic moment was due entirely to the intrinsic angular momentum (spin) of the electron.



I will address the following questions: When and how did this reinterpretation take place?

And how did physics textbooks, both introductory and advanced, talk about these topics in the late 1920s and 1930s? The answers are surprising.