SFB/TRR 173 SPIN+X

What is Spin?

Spin is a given property of particles (electrons, protons, neutrons, etc.) such as mass or charge. Spin is significant for the structure of atoms and molecules and forms the basis of magnetism and of many chemical phenomena. There are two comparisons to classical physics that illustrate spin properties:

1. particles as tiny gyroscopes

 

In classical physics, any object that rotates, a so-called rotational momentum can be attributed. This depends on the mass and shape of the body and the speed with which it rotates. Spin also represents angular momentum. In a model conception one can describe small particles like electrons as spheres, which rotate around their own axis. their own axis. This notion is useful for explaining the behavior of the particles; however, it does not the measurable reality.

2. particles as tiny magnets

With the spin goes a magnetic field. One can in the model the particles like tiny bar magnets with a north and a south pole. Also the magnetic fields of bar magnets and particles with spin have the same shape, there is an important difference in their behavior: Bar magnets can rotate in arbitrary directions, but the orientation of quantum mechanical magnetic moments is "quantized": the spin of electrons for example can take only exactly two orientations in an external magnetic field, denoted "spin up" and "spin down" (for experimental evidence see „The Stern-Gerlach experiment").

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