Lankide:Saralaka/Proba orria

Spin

Spina (ingelesetik: spin, "bira") partikula subatomikoen propietate intrintseko bat da, horren ondorioz oinarrizko partikula bakoitzak balio jakin bateko berezko momentu angeluarra duena. 1925an Ralph Kronig, George Uhlenbeck eta Samuel Goudsmitek sartu zuten mekanika kuantikoan. 1920an kimikari analitikoek atomoen elektroiak deskribatzeko, zenbaki kuantikoez gain, laugarren kontzeptu bat beharrezkoa zuten, elektroiaren spina. Honek, bere ardatzarekiko biratzean eremu magnetiko bat sortzen du, spin izenekoa.

Gerora, spinaren kontzeptua protoi, neutroi eta antipartikulentzako zabaldu zen.

Historia

1922 SternGerlach: Elektroiaren spinaren lehen froga esperimental zuzena 1922ko Stern-Gerlach esperimentua izan zen.

CASTELLANO

Si bien la teoría cuántica de la época no podía explicar algunas propiedades de los espectros atómicos, los físicos Goudsmit y Uhlenbeck descubrieron que, añadiendo un número cuántico adicional —el «número cuántico de espín»— se lograba dar una explicación más completa de los espectros atómicos. La primera evidencia experimental de la existencia del espín se produjo con el experimento realizado en 1922 por Otto Stern y Walther Gerlach, aunque su interpretación no llegara sino hasta 1927.​ Pronto, el concepto de espín se amplió a todas las partículas subatómicas, incluidos los protones, los neutrones y las antipartículas.

INGLÉS

(1) The existence of electron spin angular momentum is inferred from experiments, such as the Stern–Gerlach experiment, in which silver atoms were observed to possess two possible discrete angular momenta despite having no orbital angular momentum. The existence of the electron spin can also be inferred theoretically from the spin–statistics theorem and from the Pauli exclusion principle—and vice versa, given the particular spin of the electron, one may derive the Pauli exclusion principle.

(2) Spin was first discovered in the context of the emission spectrum of alkali metals. In 1924, Wolfgang Pauli introduced what he called a "two-valuedness not describable classically" associated with the electron in the outermost shell. This allowed him to formulate the Pauli exclusion principle, stating that no two electrons can have the same quantum state in the same quantum system.

The physical interpretation of Pauli's "degree of freedom" was initially unknown. Ralph Kronig, one of Landé's assistants, suggested in early 1925 that it was produced by the self-rotation of the electron. When Pauli heard about the idea, he criticized it severely, noting that the electron's hypothetical surface would have to be moving faster than the speed of light in order for it to rotate quickly enough to produce the necessary angular momentum. This would violate the theory of relativity. Largely due to Pauli's criticism, Kronig decided not to publish his idea.

In the autumn of 1925, the same thought came to Dutch physicists George Uhlenbeck and Samuel Goudsmit at Leiden University. Under the advice of Paul Ehrenfest, they published their results. It met a favorable response, especially after Llewellyn Thomas managed to resolve a factor-of-two discrepancy between experimental results and Uhlenbeck and Goudsmit's calculations (and Kronig's unpublished results). This discrepancy was due to the orientation of the electron's tangent frame, in addition to its position.

Despite his initial objections, Pauli formalized the theory of spin in 1927, using the modern theory of quantum mechanics invented by Schrödinger and Heisenberg. He pioneered the use of Pauli matrices as a representation of the spin operators and introduced a two-component spinor wave-function. Uhlenbeck and Goudsmit treated spin as arising from classical rotation, while Pauli emphasized, that spin is a non-classical and intrinsic property.

Pauli's theory of spin was non-relativistic. However, in 1928, Paul Dirac published the Dirac equation, which described the relativistic electron. In the Dirac equation, a four-component spinor (known as a "Dirac spinor") was used for the electron wave-function. Relativistic spin explained gyromagnetic anomaly, which was (in retrospect) first observed by Samuel Jackson Barnett in 1914 (see Einstein–de Haas effect). In 1940, Pauli proved the spin–statistics theorem, which states that fermions have half-integer spin, and bosons have integer spin.

In retrospect, the first direct experimental evidence of the electron spin was the Stern–Gerlach experiment of 1922. However, the correct explanation of this experiment was only given in 1927.

STERNGERLACH

At the time of the experiment, the most prevalent model for describing the atom was the Bohr-Sommerfeld model, which described electrons as going around the positively charged nucleus only in certain discrete atomic orbitals or energy levels. Since the electron was quantized to be only in certain positions in space, the separation into distinct orbits was referred to as space quantization. The Stern–Gerlach experiment was meant to test the Bohr–Sommerfeld hypothesis that the direction of the angular momentum of a silver atom is quantized.

Note that the experiment was performed several years before George Uhlenbeck and Samuel Goudsmit formulated their hypothesis about the existence of electron spin in 1925. Even though the result of the Stern−Gerlach experiment has later turned out to be in agreement with the predictions of quantum mechanics for a spin-¹⁄₂ particle, the experimental result was also consistent with the Bohr–Sommerfeld theory.

https://www.lorentz.leidenuniv.nl/history/spin/goudsmit.html