Quantum mechanics introduces a bunch of new phenomena that seem strange-- wave nature for material particles, particle nature for light waves, entanglement correlations connecting particles over arbitrary distances-- but one of the most unusual has a rather innocuous name: "spin." This is an intrinsic angular momentum associated with fundamental particles, as if they were tiny spinning balls. While the name seems familiar, though, and some of the physics consequences follow naturally from the comforting analogy with tiny spinning balls, quantum-mechanical spin is sneakily one of the weirdest properties in quantum physics, and also the most important.

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