In 1922, the physicists Otto Stern and Walther Gerlach were able to prove experimentally that [[Electron|electrons]] can only be in exactly two [[spin]] states. This is extremely unusual compared to everyday measurements that we are familiar with: speed and temperature can take on a number of values. In classical physics, the concept of spin does not exist. Their experiment is based on an important finding: [[Spin|Spins]] react to [[Magnetic Field|magnetic fields]]. If the magnetic field is equally strong everywhere, then the spins align themselves like small compass needles only in the direction of the magnetic field. If the magnetic field becomes stronger or weaker depending on the location (in physics we speak of a gradient), then the spin is deflected. ![[stern-gerlach.excalidraw.light.svg]] For their experiment, Stern and Gerlach decided to work with silver atoms, an atomic species with a net spin. Silver had the practical advantage that its precipitation on a glass plate could be made clearly visible with the help of a sulfur-containing substance, as a black deposit was formed. As the silver atoms are exiting the oven, they are subjected to a magnetic field gradient: spins that point upwards are deflected upwards by the magnetic field in the Stern-Gerlach experiment, spins that pointing downwards are deflected downwards. Instead of a continuous distribution, only two points were detected on the screen. Spin is a [[Discreteness|discrete]] property: either the spin points in the direction of the magnetic field, or in the opposite direction. >[!read]- Further Reading >- [[Spin]] >- [[Magnetic Field]] >[!ref]- References >- W. Gerlach and O. Stern, Der experimentelle Nachweis der Richtungsquantelung im Magnetfeld, Z. Physik **9**, 349 (1922).