Eugene Wigner introduced the concept of parity

Person: Eugene Wigner
Lived: 1902–1995
Field: Theoretical physics
From: Hungarian-American

The story

In 1927, at the request of Arnold Sommerfeld, the young Eugene Wigner went to the University of Göttingen to assist the great mathematician David Hilbert. Their collaboration was disappointing. The aged Hilbert's interests had shifted to logic, and MacTutor records that the two met only five times during the year. But Wigner, working largely on his own, used the time to produce papers of great depth and significance.

In one of them, "On the Conservation Laws of Quantum Mechanics", he introduced the new concept of parity. Parity describes how a quantum system behaves under mirror reflection, when left and right are exchanged. If the laws governing a system make no distinction between left and right, its states can be classified by their parity, and that parity is conserved. Wigner applied this and related symmetries, such as the equivalence of running time forwards and backwards, to the behaviour of electrons in atoms, and the Nobel Committee later credited this work with important discoveries.

At the end of the 1930s he extended his symmetry methods to atomic nuclei, and the conservation of parity became one of the principles physicists relied on in nuclear and particle physics. For three decades it was generally assumed to hold in every kind of process.

That assumption was eventually tested. In 1956 the theorists Tsung-Dao Lee and Chen-Ning Yang pointed out that while parity conservation had been verified for the strong and electromagnetic forces, it had never been tested for the weak interaction. In 1957 Chien-Shiung Wu and her colleagues found a clear violation of parity in the beta decay of cobalt-60, quickly confirmed by a second team at Columbia University. When Wigner received the Nobel Prize in 1963, the presentation speech noted that his work on left-right symmetry had been an important preliminary to this deeper understanding.

Parity is now a basic concept in atomic, nuclear and particle physics. Wigner's precise formulation gave physicists a way to ask whether nature treats left and right alike, and the answer that came three decades later, that the weak force does not, became a landmark of twentieth-century physics.

Why it matters

Parity is now a basic concept in atomic, nuclear and particle physics, used to predict and explain how particles and atoms behave.

About Eugene Wigner

Budapest-born Nobel physicist who brought symmetry and group theory into quantum mechanics, helped launch the Manhattan Project and designed the first plutonium production reactors.

Read the full profile

Eugene Wigner

1902–1995 · Theoretical physics

View profile →