= Why is the spin of the electron half?
https://physics.stackexchange.com/questions/266359/when-we-say-electron-spin-is-1-2-what-exactly-does-it-mean-1-2-of-what
More interestingly, how is that implied by the <Stern-Gerlach experiment>?
https://physics.stackexchange.com/questions/266359/when-we-say-electron-spin-is-1-2-what-exactly-does-it-mean-1-2-of-what/266371\#266371 suggests that half could either mean:
* at limit of large `l` for the <Schrödinger equation solution for the hydrogen atom> the difference between each angular momentum is twice that of the eletron's spin. Not very satisfactory.
* it comes directly out of the <Dirac equation>. This is satisfactory. :-)