Parity is a 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. The following year (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.

Excerpt from the Encyclopedia Britannica without permission.