1957 Nobel Prize in Physics
Reason for Award
for their penetrating investigation of the so-called parity laws, which led to important discoveries regarding elementary particles (Phys. Rev. 104 (1956) 254-258; Phys. Rev. 106 (1957) 340-345; Phys. Rev. 105 (1957) 1413-1417)
Laureates
United States of America
United States of America
Explanation
Imagine looking at our world in a mirror; left and right swap places. Scientists once believed the laws of nature would look the same in that mirror. Dr. Yang and Dr. Lee suggested that a special force, called the weak force, might break this rule. Later experiments showed that nature really can tell right from left in weak interactions. This surprising result forced scientists to rewrite their textbooks.
Related Keywords
parity
Parity is the symmetry that asks whether physical laws stay the same under spatial inversion (x→−x, y→−y, z→−z). It is conserved in classical mechanics and electromagnetism, so it was once assumed universal. If no experiment can distinguish left from right, parity holds. Experiments in 1957 proved that the weak interaction violates this symmetry, shaking fundamental assumptions. The concept now extends to studies of CP symmetry, baryon number and other invariances.
weak interaction
The weak interaction is the fundamental force responsible for β-decay and neutrino scattering. It is far weaker than electromagnetism and acts over extremely short ranges. It provided the first evidence of parity violation and is described theoretically by the V–A current structure. Its mediator particles are the W and Z bosons, discovered experimentally in the 1980s. Weak processes influence Big-Bang nucleosynthesis and solar energy production.
beta decay
Beta decay is a process in which a nucleus emits an electron (or positron) and an antineutrino (or neutrino) and transmutes into another element. It proceeds via the weak interaction; a typical case is a neutron converting into a proton inside the nucleus. In 1957 the cobalt-60 experiment measured an asymmetry in electron emission, giving decisive evidence for parity violation. The correlation between electron momentum and nuclear spin was strongly left-handed. Beta decay also finds applications in radiation therapy and radiometric dating.
cobalt-60
Cobalt-60 is a radioactive isotope with a half-life of about 5.27 years, undergoing β-decay. Its nuclear spin of 5/2 allows significant alignment in low-temperature, high-field environments, making it ideal for parity tests. In Wu’s experiment, Co-60 atoms embedded in a cobalt chloride crystal were oriented with a magnetic field at millikelvin temperatures. Statistical analysis of emitted electrons directly revealed parity violation. Cobalt-60 is still widely used as a γ-ray source in industry and medicine.
pion decay
The pion (π) is the lightest meson and decays mainly into a muon and a neutrino. This decay proceeds via the weak interaction, and parity violation manifests in the spin-momentum correlations of the decay products. The Garwin-Lederman sequential π→μ→e experiment measured electron angular distributions and supported the V–A structure. Pions also mediate the residual nuclear force between protons and neutrons, providing tests of quantum chromodynamics. Modern neutrino beam facilities rely on pion decay as a primary neutrino source.
symmetry violation
When physical laws are not invariant under a certain transformation, we speak of symmetry violation. Parity violation is a classic example, showing that nature distinguishes left from right. Later discoveries of CP and T violation provide keys to explaining why matter dominates over antimatter in the universe. Symmetry breaking is crucial not only in particle physics but also in condensed-matter systems like superconductors and liquid crystals. As a principle linking group theory to field theory, it profoundly influences all areas of physics.
CP symmetry
CP symmetry combines charge conjugation (C) and parity (P) transformations. While P is violated in weak interactions, CP was once thought to remain intact. In 1964 a slight CP violation was discovered in neutral kaon decays, prompting the inclusion of a complex phase in the Standard Model. CP violation is one of the necessary conditions for the cosmological baryon asymmetry, linking particle physics with cosmology. Ongoing experiments measure CP violation in B mesons and neutrinos with increasing precision.
Standard Model
The Standard Model is a quantum field theory that unifies electromagnetism, the weak force and the strong force. Yang and Lee’s work supplied the crucial V–A structure for weak interactions, an essential ingredient in model building. The SU(2)×U(1) gauge theory achieves electroweak unification, while the Higgs mechanism endows W and Z bosons with mass. Numerous experimental tests confirm its predictions, making it the foundation of high-energy physics. Yet incorporation of gravity and explanation of dark matter remain open, stimulating Beyond-Standard-Model research.