James Watson Cronin
Born: 29 September 1931, Chicago, IL, USA
Affiliation at the time of the award: University of Chicago, Chicago, IL, USA
Prize motivation: "for the discovery of violations of fundamental symmetry principles in the decay of neutral K-mesons"
Field: particle physics
Prize share: 1/2
For a long time, physicists assumed that different types of symmetries characterise nature. In a kind of mirror-world, the physical laws should be the same if right and left are exchanged and if matter is replaced by antimatter. The left-right symmetry had already been shown to be violated when James Cronin and Val Fitch in 1964 discovered that in the decay of the neutral K-meson the matter-antimatter symmetry is also violated. They could also show that symmetry under time-reversal is not valid: reactions going backward in time are not identical to those going forward.