MILESTONES DOI: M i LESTo N E 2 10.1038/nphys857 Answers on a postcard 1922, Otto Stern and Walter Gerlach a postcard from Gerlach to Bohr demonstrated such a reality beyond congratulating him on the success of all reasonable doubt. his theory. Their experiment involved Although the Stern–Gerlach passing a collimated beam of silver experiment categorically disproved atoms through an inhomogeneous classical models of the atom, it magnetic field and onto a glass was also inconsistent with the slide where the deposits formed a Bohr–Sommerfeld model. In fact, the pattern. Classical models suggested observed splitting of the silver beam that the electron orbitals around the had nothing to do with the orbital nucleus of these atoms should be angular momentum, but was due randomly and continuously distrib- to the spin angular momentum of uted, and that a single, broad and the unpaired electron in the atomic continuous spot of silver should form structure of silver — something that in the centre of the slide. The Bohr– was not appreciated until years later, Postcard from Gerlach to Bohr. Image The fact that quantities such as Sommerfeld model, by contrast, following the introduction of the idea courtesy of Niels Bohr Archive, Copenhagen. energy, mass and charge come in predicted that space quantization of of electron spin by Wolfgang Pauli discrete and indivisible amounts, or these orbitals should cause the beam (Milestone 3). quanta, is now so fundamental to to be split into several discrete parts Not content with realizing what our understanding of the Universe in the inhomogeneous field, forming is perhaps the clearest and most that it is often taken for granted. It is discrete lines of silver deposits on the direct demonstration of the quantum difficult to appreciate fully just how slide. nature of atoms, Stern went on to counterintuitive an idea it was to Despite its elegant simplicity, the demonstrate and measure the quan- suggest that the angular momentum experiment almost never happened. tized spin of the proton — together of an object should take on similarly For one thing, for there to be any with the size of its magnetic moment quantized values. The roots of this observable splitting, the alignment of — for which he was awarded the idea lay in the 1913 description the beam and the centre of the mag- 1943 Nobel Prize in Physics. by Niels Bohr of the structure of netic field had to be just right. More Ed Gerstner, Senior Editor, the atom, later refined by Arnold prosaically, in the middle of a Nature Physics Sommerfeld in 1916, which sug- worsening economic depression, ORiGiNAL RESEARCH PAPERS gested that not only do electrons funding the construction of the Gerlach, W. & Stern, O. Der experimentelle exist in orbitals of well-defined size experiment proved to be almost as Nachweis der Richtungsquantelung im Magnetfeld. Z. Phys. 9, 349–352 (1922) | Frisch, R. and shape, but that the orientation of difficult. Thankfully, perseverance & Stern, O. Über die magnetische Ablenkung von these orbitals is strictly defined — a on the part of Stern and Gerlach, Wasserstoff-Molekülen und das magnetische characteristic known as space quan- and a cheque for several hundred Moment des Protons. I. Z. Phys. 85, 4–16 (1933) | Estermann, I. & Stern, O. Über die magnetische tization. In 1920, most physicists, dollars provided by Henry Goldman Ablenkung von Wasserstoff-Molekülen und das including Max Born, who was one of (co-founder of the investment firm magnetische Moment des Protons. II. Z. Phys. 85, 17–24 (1933) the architects of quantum mechan- Goldman Sachs), allowed them to FURTHER REAdiNG Friedrich, B. & Herschbach, ics, considered the idea to be more observe the splitting predicted by D. Stern and Gerlach: how a bad cigar helped a mathematical abstraction than quantum theory — a result that is reorient atomic physics. Phys. Today 56 (12), 53–59 (2003) a concrete physical reality. Yet, in now most famously recorded on natuRE MILESTONES | SPIN maRch 2008 | 1 © 2008 Nature Publishing Group .