In the early 20th century, two famous discoveries about spin were made. One of them, discovered by Albert Einstein and Wander Johannes de Haas, explains a relationship between the spin of elementary particles. They found a relationship between magnetism and angular momentum. (Around that time, Einstein also put forth his theory of general relativity.) A little more than a decade later, Paul Dirac unveiled his equation dealing with a relativistic quantum mechanical wave, providing an explanation of electrons as elementary spin-1/2 particles.
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