Spin

Spin is intrinsic angular momentum carried by quantum particles, distinct from motion through space. It is quantized and gives particles characteristic magnetic and statistical properties.

Spin quantum number: A quantum number specifying a particle’s intrinsic angular momentum in units of the reduced Planck constant. It labels the magnitude of spin and distinguishes particle types such as electrons and photons.
Angular momentum: A conserved physical quantity associated with rotational motion or symmetry. Spin is the intrinsic contribution to a system’s total angular momentum.
Electron spin resonance: A spectroscopy technique that detects transitions between electron spin states in a magnetic field. Resonant spin transitions reveal unpaired electrons in materials and molecules.
Orbital angular momentum: Angular momentum associated with a particle’s spatial wavefunction or motion around a center. Unlike spin, it arises from spatial degrees of freedom rather than an intrinsic property.
Spin–statistics theorem: A relativistic quantum-field-theory theorem linking integer spin to bosonic statistics and half-integer spin to fermionic statistics. It explains why spin determines how identical particles behave under exchange.
Spin projection: The component of a particle’s spin along a chosen measurement axis. Measurements reveal a quantized component, not a classical spinning motion.
Quantum state: A mathematical description containing the probabilities for outcomes of quantum measurements. Spin is represented by quantum states whose components predict measurement results.
Nuclear magnetic resonance: A technique that probes nuclei by driving transitions between their spin states with radio-frequency fields. Nuclear spin responses provide the signals used in spectroscopy and imaging.
Classical spinning top: A macroscopic rigid body rotating about an axis and carrying mechanical angular momentum. Spin shares angular-momentum mathematics but is not literal rotation of a tiny object.
Pauli exclusion principle: The rule that identical fermions cannot occupy the same quantum state. Half-integer spin makes the exclusion principle apply to electrons and other fermions.

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