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.

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 math.x, math.y, and math.z are replaced with −math.x, −math.y, and −math.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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Parity is a concept of equality of status or functional equivalence. It has several different specific definitions.

  • Parity (physics), the name of the symmetry of interactions under spatial inversion
  • Parity (mathematics) indicates whether a number is even or odd
  • Parity (medicine), the number of times a woman or female animal has given birth
  • Parity Pricing (marketing)
  • Parity bit, in computing, a very simple example of an error detecting code
  • Purchasing power parity, in economics, an estimate of the exchange rate required to equalise the purchasing power of different currencies, given the prices of goods and services in the countries concerned
  • Interest rate parity, in finance, refers to the notion that the differential in interest rates between two countries is equal to the differential between the forward exchange rate and the spot exchange rate
  • Base parity, in human genetics, the G/C ratio in regions of DNA
  • Put–call parity, in financial mathematics, defines a relationship between the price of a European call option and a European put option—both with the identical strike price and expiry
  • Parity (sports) refers to engineering an equal playing field in which all teams can compete, regardless of their economic circumstances
  • In demography, the number of reproductive events (births)
  • Potty parity attempts to equalize the waiting times of males and females in restroom queues by designating or building more women's restrooms, giving them more facilities to use
  • A tactic in othello
  • Militarily, it refers to having a comparable force as one's enemy, such as geopolitically as in mutual assured destruction
  • Parity is a case on a Rubik's Revenge cube that may not occur on 3x3 Rubik's Cube
  • Grid parity of renewable energy.

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