Spin One-Half: Feynman's Physics Explained
Particles carry an intrinsic angular momentum called spin. The electron is a spin-one-half particle, meaning its spin along any axis can only be +half or -half a unit — the simplest non-trivial quantum system. It has a bizarre property: you must rotate it a full 720 degrees, not 360, to bring its state back to the start. The mathematics of spin-one-half underlies the Pauli principle, magnetism, and the structure of matter.
The big idea
An electron must turn twice around to come back to where it started.
Think about it
What does it tell you about quantum objects that turning one all the way around (360 degrees) does not return it to its original state?
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