Molecular beams entwined with quantum theory: A bouquet for Max Planck

In an era when the fledgling quantum theory was “uncertain and even gave contradictory answers,” Otto Stern undertook to employ molecular beams to test directly fundamental aspects of the theory. During 1921-1935, this led to five decisive experiments reviewed here, resulting in the discovery or demonstration of space quantization, de Broglie matter waves, anomalous magnetic moments of the proton and neutron, recoil of an atom on emission of a photon, and the limitation of scattering cross-sections for molecular collisions imposed by the uncertainty principle.