Spin (physics)

Spin is an <a href="/facts/Intrinsic_and_extrinsic_properties/5WJ1zN7q">intrinsic</a> form of <a href="/facts/Angular_momentum/7TuSVFxq">angular momentum</a> carried by <a href="/facts/Elementary_particle/CTFDM97U">elementary particles</a>, and thus by <a href="/facts/List_of_particles/FwR0Ab6u">composite particles</a> such as <a href="/facts/Hadron/C0sd1zty">hadrons</a>, <a href="/facts/Atomic_nucleus/orpIm5Xl">atomic nuclei</a>, and atoms.: <a href="https://archive.org/details/introductiontoqu00grif_190/page/n194">183</a>–184  Spin is quantized, and accurate models for the interaction with spin require <a href="/facts/Relativistic_quantum_mechanics/DhkDNnHf">relativistic quantum mechanics</a> or <a href="/facts/Quantum_field_theory/VoewsNJV">quantum field theory</a>.
The existence of <a href="/facts/Electron/L0KBo5cW">electron</a> <a href="/facts/Spin_angular_momentum/VW94scuE">spin angular momentum</a> is <a href="/facts/Inferred/RBbe95bP">inferred</a> from experiments, such as the <a href="/facts/Stern%25E2%2580%2593Gerlach_experiment/PCWPE56v">Stern–Gerlach experiment</a>, in which silver atoms were observed to possess two possible discrete angular momenta despite having no orbital angular momentum. The relativistic <a href="/facts/Spin%25E2%2580%2593statistics_theorem/j2qqVpez">spin–statistics theorem</a> connects electron spin quantization to the <a href="/facts/Pauli_exclusion_principle/lsBKGeHD">Pauli exclusion principle</a>: observations of exclusion imply half-integer spin, and observations of half-integer spin imply exclusion.
Spin is described mathematically as a vector for some particles such as photons, and as a <a href="/facts/Spinor/sKFNnEwP">spinor</a> or <a href="/facts/Bispinor/wm8UNWxJ">bispinor</a> for other particles such as electrons. Spinors and bispinors behave similarly to <a href="/facts/Euclidean_vector/bCLrdksy">vectors</a>: they have definite magnitudes and change under rotations; however, they use an unconventional "direction". All elementary particles of a given kind have the same magnitude of spin angular momentum, though its direction may change. These are indicated by assigning the particle a <a href="/facts/Spin_quantum_number/5Nh2H8gr">spin quantum number</a>.: <a href="https://archive.org/details/introductiontoqu00grif_190/page/n194">183–184</a> 
The <a href="/facts/International_System_of_Units/S7YLglNz">SI units</a> of spin are the same as classical angular momentum (i.e., <a href="/facts/Newton_(unit)/Ral9XgNl">N</a>·<a href="/facts/Metre/hPhzc1CV">m</a>·<a href="/facts/Second/jtFxnJXx">s</a>, <a href="/facts/Joule/9ndq9jD2">J</a>·s, or <a href="/facts/Kilogram/lU2zbPk8">kg</a>·m2·s−1). In quantum mechanics, angular momentum and spin angular momentum take discrete values proportional to the <a href="/facts/Planck_constant/qGHAe3qq">Planck constant</a>. In practice, spin is usually given as a <a href="/facts/Dimensionless/TINrpQIq">dimensionless</a> spin quantum number by dividing the spin angular momentum by the <a href="/facts/Reduced_Planck_constant/qGHAe3qq">reduced Planck constant</a> ħ. Often, the "spin quantum number" is simply called "spin".

Spin (physics) open-in-new

Spin (physics)