Spin
A property that electrons and other particles are born with, making them behave like tiny magnets. For an electron it takes only two values: "up" or "down".
In 1925 two young physicists at Leiden, George Uhlenbeck and Samuel Goudsmit, proposed that the "fourth number" Pauli had said each electron in an atom needs was a sign that the electron was spinning[1]. A spinning charge is a tiny magnet, and that brought order to the anomalous Zeeman effect, the puzzling way spectral lines split in a magnetic field[2].
Taken literally, though, a spinning ball would need its surface to move faster than light. Today spin is understood not as anything actually rotating but as a property the electron is simply born with. In 1928 Dirac's equation, which combines relativity with quantum mechanics, was shown to produce spin naturally.
Because spin has only two values, it works together with the Pauli exclusion principle to decide how electrons fill up an atom, and how two electrons pair up to hold atoms together in a chemical bond. The split of a beam of silver atoms into two in the 1922 Stern–Gerlach experiment was, it later turned out, caused by electron spin as well.
Related terms
- Pauli exclusion principleThe rule that no two electrons in the same atom can be in exactly the same state. Set out by Wolfgang Pauli in 1925.
- Stern–Gerlach experimentThe 1922 experiment showing that a beam of silver atoms passed through a magnetic field splits cleanly in two instead of smearing out: direct evidence that atoms can point only in certain directions.
- Zeeman effectThe splitting of spectral lines into several when atoms are placed in a magnetic field.
Sources
- G. E. Uhlenbeck, S. Goudsmit, “Ersetzung der Hypothese vom unmechanischen Zwang durch eine Forderung bezüglich des inneren Verhaltens jedes einzelnen Elektrons,” Naturwissenschaften 13, 953–954 (1925).
- G. E. Uhlenbeck, S. Goudsmit, “Spinning Electrons and the Structure of Spectra,” Nature 117, 264–265 (1926).