January 1925: Wolfgang Pauli announces the exclusion principle
The year 1925 was an important one for quantum physics, beginning with
Wolfgang Pauli’s January announcement of the exclusion principle. This
well-known principle, which states that no two identical fermion
particles can be in the same quantum state, provided for the first time a
theoretical basis for the structure of the periodic table of the
elements.
Wolfgang Pauli was born in Vienna in 1900, the same year that quantum
mechanics itself was born with Planck’s announcement of the idea of the
energy quanta. Pauli’s father was a physician and chemistry professor at
the University of Vienna, and his godfather was Ernest Mach. As a young
prodigy, when he found himself bored during class, Pauli would read
Einstein’s papers on relativity. By age 20 Pauli, then a student of
Arnold Sommerfeld at the University of Munich, had published papers on
relativity and written an encyclopedia article on relativity which
greatly impressed other physicists, including Albert Einstein himself.
Having learned classical mechanics and relativity, Pauli was
disconcerted by quantum mechanics upon being introduced to it by
Sommerfeld, and at first he found the subject rather confused.
Possibly because of his brilliance, Pauli’s professors and colleagues
tolerated some of his more annoying habits, such as his custom of
sleeping extremely late and rarely showing up for lectures before noon.
He was also extremely critical, and famous for deriding his colleagues’
less-than-coherent work as “not even wrong.” His tendency to criticize
often spurred others to clarify their ideas. Pauli also had such an
amazing propensity to cause accidents that scientists began to believe
that even to have him come close to one’s lab meant doom for the
experiment.
After receiving his doctorate in 1921 and spending
some time in Gottingen and then Copenhagen, Pauli took a position at the
University of Hamburg in 1923. He gave his first lecture there on the
periodic table of elements, which he found unsatisfactory because the
atomic shell structure was not understood. In 1913, Bohr had proposed
that electrons could occupy only certain quantized orbitals, but there
seemed to be no reason why all the electrons in an atom didn’t simply
crowd into the one lowest energy state. There was no convincing
explanation of the structure of the periodic table. Pauli had also
recently worked on trying to explain the anomalous Zeeman effect, (a
consequence of electron spin) and was convinced that the two problems
were somehow related.
In late 1924, Pauli made a big leap by
suggesting the idea of a adding a fourth quantum number to the three
that were then used to describe an electron’s quantum state. The first
three quantum numbers made sense physically, since they related to the
electron’s motion around the nucleus. Pauli called his new quantum
property of the electron a “two-valuedness not describable classically.”
Soon after making this proposal, Pauli realized that it could lead to
the solution of the problem of the closed orbitals.
Then in
January 1925, he announced the exclusion principle, stating that no two
electrons in an atom can occupy a state with the same values for the
four quantum numbers. Each electron had to be in its own unique state.
Other possibilities are excluded.
Pauli’s proposed fourth
quantum number puzzled physicists at the time, because no one could
explain its physical significance. Pauli himself was troubled by the
idea. Pauli was also bothered by the fact that he couldn’t give any
logical explanation for the exclusion principle or derive it from other
laws of quantum mechanics, and he remained unhappy about this problem.
Nonetheless, the principle worked–it explained the structure of the
periodic table and is essential for explaining other properties of
matter.
Later in 1925, Samuel Goudsmit and George Uhlenbeck,
inspired by Pauli’s work, interpreted the fourth quantum number as the
electron’s spin. Pauli originally applied the exclusion principle to
explain electrons in atoms, but later it was extended to any system of
fermions, which have half integer spin, but not to bosons, which have
integer spin.
In the two years after Pauli’s announcement of
his exclusion principle, the new quantum mechanics took off, with
Heisenberg’s formulation of matrix mechanics, and Schrödinger’s wave
mechanics, which was based on de Broglie’s idea that matter can have
wavelike properties.
In 1928 Pauli moved to Zurich. He spent
time during World War II in the United States, and returned to Zurich
after the war. In 1931, Pauli proposed the existence of a new particle,
the neutrino, as a solution to the apparent lack of energy conservation
in beta decay. After his many research accomplishments, he spent much of
his later years thinking about the history and philosophy of science.
Pauli always insisted on having a clear and coherent explanation of a
phenomenon, and always strove to find both an intuitive understanding of
an experiment and a rigorous mathematical scheme. Max Born once
commented that, “I knew he was a genius, comparable only to Einstein
himself. But he was a completely different type of man, who in my eyes,
did not attain Einstein’s greatness.” In 1945, Pauli was awarded the
Nobel Prize for the discovery of the exclusion principle. He died in
1958.
