parity

parity or space parity, in physics, quantity that refers to the relationship between an object or process and the image that it can produce in a mirror. For example, any right-handed object will produce a mirror-image counterpart that is identical to it in every way except that the mirror image is left-handed. A moving particle that spins in a clockwise manner, as would a right-handed screw advancing through space, will possess a mirror-image particle that is identical to it in every way except that it spins counterclockwise, as would a left-handed screw advancing through space. The law of conservation of parity implies that every real object or process has a mirror image that can also exist and that obeys the same physical laws. Although this concept has little significance in classical physics, it is of great importance in atomic and nuclear physics. From this law scientists inferred that all elementary particles and their interactions possessed mirror image counterparts that also exist. However, in 1956 T. D. Lee and C. N. Yang published a paper in which they argued that parity was not conserved in weak interactions. Their conjecture was verified the same year by C. S. Wu and coworkers at the U.S. National Bureau of Standards and other institutions in an experiment involving beta decay (see radioactivity). Parity is still conserved in the strong nuclear interactions and in the electromagnetic interactions. Formally, parity, P, is a quantity that expresses the behavior of the wave function of any system of particles when the spatial coordinates x, y, z, of the wave function are reflected through the origin to −x, −y, −z (see quantum theory). This mathematical operation is called the parity, or space-inversion, operation. See also symmetry.

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parity

In economics, equality in price, rate of exchange, purchasing power, or wages. In international exchange, parity exists when the exchange rate between two currencies makes the purchasing power of both currencies equal. Adjustments to maintain parity can occur in the marketplace as prices change in response to supply and demand, or through the intervention of national governments or international agencies such as the International Monetary Fund. In U.S. agricultural economics, the term parity is used for a system of regulating the prices of farm commodities, usually by government price supports and production quotas, to guarantee farmers the purchasing power they had in a past base period. Parity is also used in personnel administration to establish equitable wage rates for various classes of employees.

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parity

In physics, a property related to the symmetry of the wave function representing a system of fundamental particles. It plays an important role in quantum mechanics in the description of a physical system. Parity transformation replaces a system with a type of mirror image in which the spatial coordinates describing the system are inverted, so that the coordinates x, y, and z are replaced with −x, −y, and −z. If a system is identical to the original system after parity transformation, its parity is even. If the image is the negative of the original, its parity is odd. In either case, the physical observables of the system remain unchanged. In 1957 Chien-Shiung Wu (1912–1997) and coworkers made the surprising discovery that beta decay reactions do not conserve parity; in other words, the inverted image of the process does not exist in nature. This is a general property of the weak force.

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See parity checking and RAID.

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(storage, communications)

parity - An extra bit added to a byte or word to reveal errors in storage (in RAM or disk) or transmission. Even (odd) parity means that the parity bit is set so that there are an even (odd) number of one bits in the word, including the parity bit. A single parity bit can only reveal single bit errors since if an even number of bits are wrong then the parity bit will not change. Moreover, it is not possible to tell which bit is wrong, as it is with more sophisticated error detection and correction systems. See also longitudinal parity, checksum, cyclic redundancy check.