>"Months later, Schrödinger — inspired by Louis de Broglie’s idea that matter behaves like a wave — proposed an entirely different, but mathematically equivalent, formulation of particle behavior based on the better-known mathematics of waves."
[...]
More surprising results unfurled when Dirac extended his equation to describe an electron interacting with an electromagnetic field. Experimentalists had confirmed that the electron’s intrinsic angular momentum, or spin, was equal to 1/2, but theoreticians couldn’t figure out how to properly incorporate it into their theories. With his new equation, Dirac had found, almost as an afterthought, that the spin emerged naturally.
[...]
The Dirac equation was simple and elegant, yet dense with implications. Perhaps its most profound feature was that,
instead of producing two components for negative and positive spin states, it produced four: a negative and positive spin state for each of two particles with positive and negative energy states."
Related:
https://en.wikipedia.org/wiki/Chirality
https://en.wikipedia.org/wiki/Angular_momentum
https://en.wikipedia.org/wiki/Noether%27s_theorem