Which Electron Went Where?

Nick Brenton

Allan Franklin

Discussing the existence of electrons, philosopher of science Ian Hacking has written, “So far as I'm concerned, if you can spray them, then they are real.” He went on to elaborate this view: “We are completely convinced of the reality of electrons when we set out to build—and often enough succeed in building—new kinds of device that use various well-understood causal properties of electrons to interfere in other more hypothetical parts of nature.”

Sjaak Uitterdijk

Otto Stern and Walter Gerlach demonstrated in 1922 experimentally the “existence of space quantization in a magnetic field”, using their own words. The result of this experiment is later on used to introduce the so-called intrinsic spin angular momentum of elementary and other particles. This article describes what went wrong in the applied argumentation. In 1896 Zeeman and Lorentz showed experimentally and theoretically that atoms emit ‘shifted’ frequencies when exposed to an external magnetic field. This phenomenon has been used to demonstrate the existence of spinning electrons. However, it is shown that this demonstration is not convincing at all.

Jim Baggott

Despite the success of Schrödinger’s description of the H-atom, it became apparent that the spectrum of the simplest multi-electron atom—helium—could not be so readily explained. And the spectra of other atoms showed ‘anomalous’ splitting in a magnetic field. In 1920 Sommerfeld introduced a fourth quantum number. A few years later Pauli was led to the inspired conclusion that the electron must have a curious ‘two-valuedness’ characterized by a quantum number of ½, and went on to discover the exclusion principle. Perhaps this is because the electron possesses a self-rotation, leading to the notion of electron spin, potentially explaining why each orbital can accommodate only two electrons. Heisenberg traced this behaviour back to the symmetry properties of the wavefunctions. By observing which transitions in the spectrum of helium are allowed and which are forbidden, we can deduce the generalized Pauli principle, from which the exclusion principle follows.

Victor Francis Hess