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Conservation of parity.

Physicists had worked out conservation laws that dictated conservation of energy, momentum, angular momentum, and electric charge, among others. In every case, this meant that the total quantity of that property in a closed system (one that did not interact with objects outside the system) could not change, no matter what happened within the system. The assumption was that such conservation laws were universal.

The study of subatomic particles showed that these conservation laws held in the subatomic realm as well. In addition, new conservation laws were discovered, such as the conservation of parity. Parity was the quality of being either odd or even. Just as in numbers, odd parity plus odd parity equaled even parity; even parity plus even parity equaled even parity; but odd parity plus even parity equaled odd parity. Each particle was assigned a particular parity, either odd or even, so that the total of all the particles in a closed system was either odd or even. No matter what happened to the particles within the system, if it began even, it ended even, and if it began odd, it ended odd. At least, so it was assumed.

Then trouble arose with kaons. Sometimes kaons broke down to two pions, which together had even parity; and sometimes to three pions, which together had odd parity. It was concluded that there were two kinds of kaons, one with even parity and one with odd. However, no one could detect any difference between the two kinds of kaons or predict which one a particular kaon would be.

In 1956 two Chinese physicists, Yang Chen Ning (b. 1922) and Lee Tsungdao (b.1926), suggested that there was only one kind of kaon, but that since kaons broke down through the weak interaction, and since in the weak interaction parity was not necessarily conserved, then a kaon could break down into either two or three pions indiscriminately. They pointed out that if parity was conserved in the weak interaction, then in certain particle changes, electrons would come out in equal amounts, left and right. If parity was not conserved, then electrons would come out predominantly in one direction. The experiment was performed, and the electrons came out predominantly in one direction.

This meant that although parity seemed to be conserved in the strong interaction and the electro-magnetic interaction, it was not conserved in the weak interaction. As a result, Yang and Lee received the Nobel Prize for physics in 1957.

This did not mean, by the way, that conservation of parity really broke down altogether. It might merely mean that parity had to be combined with another property for both to be conserved. for instance, if particles gave off electrons predominantly in one direction, antiparticles gave them off predominantly in the other direction. The combination was called C-P (charge conjugation and parity), so scientists decided there was a law of C-P conservation.

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Author:Asimov, Isaac
Publication:Asimov's Chronology of Science & Discovery, Updated ed.
Article Type:Reference Source
Date:Jan 1, 1994
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