The Stern-Gerlach experiment, named after Otto Stern and Walther Gerlach, is a celebrated experiment in 1920 on deflection of particles, often used to illustrate basic principles of quantum mechanics. It can be used to demonstrate that electrons and atoms have intrinsically quantum properties, that measurement in quantum mechanics affects the particles measured, and that quantum states are necessarily described by complex numbers.
Description
The Stern-Gerlach experiment involves sending a beam of spin 1/2 particles through an inhomogeneous magnetic field. The magnetic field is strongly increasing in one direction, which is usually called the "z" direction. Each spin-1/2 particle has a spin magnetic moment . The inhomogeneous magnetic field causes the particle to be deflected either up or down, and thus measures the magnetic moment of the particle.
For a spin-1/2 particle, the spin can take only two possible values. The spin is said to be quantized, and the phenomenon is known as the spin quantization .
In the classical (nonquantum) case, a "spinning" particle will enter with a spin in some unknown direction. If the direction is randomly chosen, then a random amount of the spin will be aligned in the +/-Z direction. Thus, some particles will be deflected upwards or downwards greatly, some will be deflected upwards or downwards moderately, and some may hardly be deflected at all.
However, in the Stern-Gerlach experiment, we witness that all particles are deflected either up or down by the same amount. The particles either have spin 
or 
, with no intermediate values.
Details
History
The Stern-Gerlach experiment was performed in Frankfurt, Germany in 1922 by Otto Stern and Walther Gerlach. At the time, Stern was an assistant to Max Born at the University of Frankfurt's Institute for Theoretical Physics , and Gerlach was an assistant at the same university's Institute for Experimental Physics .
At the time of the experiment, the most prevalent model for describing the atom was the Bohr model, which described electrons as going around the positively-charged nucleus only in certain discrete orbitals or energy levels. Since the electron was quantized to be only in certain positions in space, the separation into distinct orbits was referred to as space quantization .
External link
Stern-Gerlach Experiment Java Applet Animation
References
- Friedrich, Bretislav and Dudley Herschbach, "Stern and Gerlach: How a Bad Cigar Helped Reorient Atomic Physics". Physics Today, December 2003. Available online at [1].