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American physicist James Cronin won the 1980 Nobel Prize in physics for demonstrating the changes that occur when matter transforms into antimatter.
James W. Cronin, born in 1931, American physicist and Nobel laureate. Cronin shared the 1980 Nobel Prize for physics with American Val Logsdon Fitch for demonstrating that, unlike what was previously thought, symmetry is not always preserved when some elementary particles change in state from matter to antimatter.
Cronin was born in Chicago, Illinois, and received his bachelor's degree at Southern Methodist University in 1951. He earned both his master's and doctoral degrees in 1955 in physics from the University of Chicago. From 1955 to 1958 Cronin served as an assistant physicist at Brookhaven National Laboratory in Long Island, New York. He taught at Princeton University from 1958 to 1971 and in 1971, Cronin became professor of physics at the University of Chicago.
In 1964, Cronin, Fitch, and some of their colleagues were studying a subatomic particle called the K. meson, also known as a kaon. A meson is a particle that weighs approximately half as much as a proton. The kaon is an unstable, neutral (uncharged) meson that had been discovered in the interactions between particles from outer space and particles from the earth's atmosphere.
Physicists thought for some time that the universe followed three fundamental rules of symmetry. The first, the symmetry of charge conjugation (C), states that the result of an experiment should not change if all the particles in the experiment are switched from antimatter to matter or vice versa. The second rule of symmetry, parity (P), says that the result of an experiment should not change if the positions of all the particles in the experiment are completely reversed. The third rule, time-reversal (T), says that any reaction between elementary particles should occur equally well in either direction; that is, if two particles can come together to form one particle, the resulting particle should be able to decay to form the original two particles.
Before Cronin and Fitch started their experiment, the rules C and P had both been found to be untrue in some cases. Physicists tried to preserve the idea of symmetry by theorizing that any reaction had to be symmetric over a combination of CP; that is, if C symmetry was violated, P symmetry would have to be violated, too. Cronin and Fitch disproved this rule by showing that kaons do not preserve P, C, or CP symmetry some of the time. However, one rule of symmetry remains above suspicion: all reactions must be symmetric in the combination of charge conjugation, parity, and time (CPT).
Since every reaction has to be symmetric under CPT, Cronin and Fitch's result showed that T symmetry has to fail when CP symmetry fails. That means reactions that do not preserve CP symmetry cannot run backward. Some scientists think this explains how more matter than antimatter is created in the universe.