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Definition
Related concepts: Duality, Daosim, Complementarity, Symmetry, Triality, Ying Yang Theory,
Parity
in physics, property important in the quantum-mechanical description of a physical system. In most cases, it relates to the symmetry of the wave function representing a system of fundamental particles. A parity transformation replaces such a system with a type of mirror image. Stated mathematically, the spatial coordinates describing the system are inverted through the point at the origin; that is, the coordinates x, y, and z are replaced with -x, -y, and -z. In general, if a system is identical to the original system after a parity transformation, the system is said to have even parity. If the final formulation is the negative of the original, its parity is odd. For either parity the physical observables, which depend on the square of the wave function, are unchanged. A complex system has an overall parity that is the product of the parities of its components.Until 1956 it was assumed that when an isolated system of fundamental particles interacts the overall parity remains the same, or is conserved. This conservation of parity implied that, for fundamental physical interactions, it is impossible to distinguish right from left and clockwise from counterclockwise. The laws of physics, it was thought, are indifferent to mirror reflection and could never predict a change in parity of a system. This law of the conservation of parity was explicitly formulated in the early 1930s by the Hungarian-born physicist Eugene P. Wigner and became an intrinsic part of quantum mechanics.
In attempting to understand some puzzles in the decay of particles
called K mesons, the Chinese-born physicists Tsung-Dao
Lee and Chen
Ning Yang proposed in 1956 that parity is not always conserved. For
subatomic particles, three basic interactions are important: electromagnetic,
strong, and weak. Lee and Yang showed that there was no evidence that parity
conservation applies to the weak
interaction. The fundamental laws governing weak interactions should
not be indifferent to mirror reflection, and, therefore, weak interactions
should show some measure of built-in right- or left-handedness that might
be experimentally detectable. In experiments conducted by the Chinese-born
physicist Chien-Shiung
Wu in 1956 and announced by her in 1957, it was conclusively proved
that the electrons
ejected along with antineutrinos from certain unstable cobalt nuclei in
the process of beta decay, a weak interaction, are predominantly left-handed—that
is to say, the spin rotation of the electrons is that of a left-handed
screw. Nevertheless, it is believed on strong theoretical grounds (i.e.,
the CPT theorem) that when the operation of parity reversal P is joined
with two others, called charge conjugation C and time reversal T, the combined
operation does leave the fundamental laws unchanged.
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by David Y. Gao