Photograph of George Uhlenbeck (left) and Samuel Goudsmit, Dutch-American physicists, taken in 1926. The academic lives of Uhlenbeck (b.1900) and Goudsmit (1902-1978) were closely intertwined, both having gained their PhD at Leiden in 1927. This resulted from their work in particle physics. In 1925 they discovered electron spin, the fourth quantum dimension used to describe the electron. Spin is the intrinsic angular momentum of a particle, other quantum dimensions are mass, charge and orbital angular momentum. All elementary particles exhibit spin, described as fractions of a quantum amount based on Planck's constant.