What is spin in physics and why is it called angular momentum?
Spin is an intrinsic form of angular momentum carried by elementary particles, distinct from the orbital angular momentum that arises from a particle's motion around another object. It shares the same SI units as classical angular momentum but is quantized, meaning it can only take discrete values proportional to the reduced Planck constant. The name derives from the early idea that particles might be physically rotating, though that model was later shown to be inconsistent with known physics.
What did the Stern-Gerlach experiment reveal about electron spin?
The Stern-Gerlach experiment of 1922 sent silver atoms through an inhomogeneous magnetic field and produced exactly two discrete spots on the detector. Because silver atoms have no orbital angular momentum, the splitting could not be explained by classical physics and turned out to be the first direct experimental evidence of electron spin. The correct explanation was only given in 1927, when it was shown that the two spots correspond to the two possible spin states of the unmatched outer electron.
Who discovered electron spin and when was it published?
George Uhlenbeck and Samuel Goudsmit at Leiden University published the concept of electron spin in the autumn of 1925, under the advice of Paul Ehrenfest. Ralph Kronig had independently proposed the same idea in early 1925, but chose not to publish after Wolfgang Pauli criticized the model on the grounds that the electron's surface would need to exceed the speed of light.
What is the difference between fermions and bosons?
Fermions have half-integer spin values and obey the Pauli exclusion principle, meaning no two identical fermions can share the same quantum state simultaneously. Bosons have integer spin values and obey Bose-Einstein statistics, allowing them to occupy the same quantum state freely. Quarks, leptons, and electrons are fermions; photons, gluons, and the W and Z bosons are bosons.
When did Wolfgang Pauli prove the spin-statistics theorem?
Wolfgang Pauli proved the spin-statistics theorem in 1940. The theorem establishes that particles with half-integer spin must obey the Pauli exclusion principle while particles with integer spin do not. Pauli described the connection between spin and statistics as one of the most important applications of the special theory of relativity.
What are the practical applications of spin in physics and technology?
Spin underlies nuclear magnetic resonance spectroscopy used in chemistry, magnetic resonance imaging in medicine, electron spin resonance spectroscopy, and giant magnetoresistive drive-head technology in hard disks. Spin-orbit coupling produces the fine structure in atomic spectra that atomic clocks depend on. The Datta-Das spin transistor, proposed in 1990, launched the field of spintronics, which explores spin as a binary information carrier in electronics.