Simply stated, the elementary particles that constitute matter are fermions and the elementary particles that constitute energy are bosons. Fermions have half-integer (1/2, 3/2, 5/2 …) spin while bosons have integer (0, 1, 2 . . .) spin.
Spin, or angular momentum, characterizes elementary particles, composite particles (known as hadrons) and atomic nuclei. Each type of particle has the same amount of spin, denoted by its quantum number.
Strangely, a particle’s direction of spin can reverse, but the amount of spin is immutable.
Fermions and bosons differ, as noted, in regard to whether the spin is whole or half integer, but we perceive them to differ markedly. The Pauli exclusion principle states that two otherwise identical fermions cannot simultaneously have the same quantum number, meaning that two electrons, for example, cannot occupy the same place at the same time. This prohibition does not, however, apply to bosons, the fundamental units of energy.
Fermions, with ½ integer spin, include protons, neutrons, electrons, neutrinos and quarks. They all conform to the Pauli exclusion principle. They have a continuous magnetic moment aligned with the direction of spin, which is the cause of accompanying electromagnetic interactions. Unlike fermions, any number of bosons can simultaneously exist in the same spatial location.
Wolfgang Pauli (1900-1958) was nominated by Albert Einstein for the Nobel Prize and received it in 1945 for his discovery of the exclusion principle and his explication of spin theory. In addition to being one of the key figures in the development of quantum mechanics, Pauli’s life was of great interest on many levels. Born into a prosperous Vienna family, he was well educated, completing at the age of 21 his doctoral dissertation on the quantum theory of ionized molecular hydrogen.
In 1930, he postulated the existence of the neutrino, then suffered a severe mental breakdown. He was treated by the renowned psychotherapist Carl Jung, whose methodology involved the analysis of dreams. Pauli experienced highly archetypal dreams, which he related to Jung, who had earlier broken with his colleague Sigmund Freud. Jung de-emphasized childhood trauma in favor of cultural archetypes and synchronicity, a concept which immediately got Pauli’s attention. Later, Jung analyzed more than 400 of Pauli’s dreams in his book, Psychology and Alchemy.
Pauli made a complete recovery and was later noted for his genial personality and good humor. The relationship between Werner Heisenberg and Pauli was strained, however, despite the compatibility in their theoretical conclusions. Another great physicist, Paul Dirac, as we shall see in another article, figured prominently in the ongoing dialog that was twentieth-century quantum theory.
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