# Parity

# Parity

Parity is both an operation and an intrinsic property used to describe particles and their wavefunctions (mathematical representations of one or more particles) in quantum mechanics (a branch of physics focusing on particles smaller than an atomic nucleus).

The parity operation is a combination of a left-right trade (mirror reflection) with a top-bottom switch. This combination is also called a spatial inversion. How objects behave under a parity operation defines their intrinsic parity. All microscopic particles have an intrinsic parity that helps us tell them apart. An object or group of objects that is the same before and after a parity operation is called parity invariant. A parity invariant object has “even” or “+1” intrinsic parity. If the parity of an object changes due to a parity operation, it has “odd” or “-1” intrinsic parity.

Even though people do not obey reflection symmetry (their right and left sides are different), scientists believed that the laws of physics were parity invariant. In 1956 a Chinese-American scientist named Tsung Dao Lee figured out that the idea of parity invariance had not been tested in relation to one of the fundamental forces of physics, the weak force (responsible for spontaneous decays of some microscopic particles). This prompted Lee and a colleague, Chen Ning Yang, to think of a clever experiment to test the parity invariance of the weak force. Later in 1956 Dr. Chien-Shiung Wu carried out this difficult experiment using a radioactive (spontaneously decaying) element called Cobalt. Wu observed the direction of electrons (smallest naturally-occurring charged particles) coming out of the Cobalt due to its radioactive decay. She found that the electrons did not come out the way she expected. Thus this experiment was not parity invariant. Since it tested the weak force, this meant the weak force was not parity invariant either. This result was so important that Yang and Lee won the 1957 Nobel Prize in physics for it. Now we know when we see parity invariance the weak force is the culprit. All other fundamental forces are parity invariant.

Parity is often studied along with charge conjugation. Charge conjugation changes a particle into its opposite, or antiparticle, by changing the sign of its electric charge. Even though parity is not conserved by itself, it was thought that the combination of parity (P) and charge conjugation was conserved. In 1964, however, physicists J. H. Christenson, J. W. Cronin, V. L. Fitch, and R. Turlay discovered that CP conservation is not obeyed by studying the decays of particles called Kaons. Scientists know that the laws of nature must obey conservation of the combination of parity, charge conjugation, and time symmetry (T), because of the way they are formulated. This is called CPT symmetry. CP symmetry is not conserved, it follows that time symmetry must not hold, so that total combination, CPT, can be conserved.

Lesley Smith

# Parity

# Parity

Parity is both an operation and an intrinsic property used to describe particles and their wavefunctions (mathematical representations of one or more particles) in **quantum mechanics** (a branch of **physics** focusing on particles smaller than an atomic nucleus).

The parity operation is a combination of a left-right trade (mirror reflection) with a top-bottom switch. This combination is also called a spatial inversion. How objects behave under a parity operation defines their intrinsic parity. All microscopic particles have an intrinsic parity that helps us tell them apart. An object or group of objects that is the same before and after a parity operation is called parity **invariant** . A parity invariant object has "even" or "+1" intrinsic parity. If the parity of an object changes due to a parity operation, it has "odd" or "-1" intrinsic parity.

Even though people do not obey reflection **symmetry** (their right and left sides are different), scientists believed that the laws of physics were parity invariant. In 1956 a Chinese-American scientist named Tsung Dao Lee figured out that the idea of parity invariance had not been tested in **relation** to one of the fundamental forces of physics, the weak **force** (responsible for spontaneous decays of some microscopic particles). This prompted Lee and a colleague, Chen Ning Yang, to think of a clever experiment to test the parity invariance of the weak force. Later in 1956 Dr. Chien-Shiung Wu carried out this difficult experiment using a radioactive (spontaneously decaying) element called Cobalt. Wu observed the direction of electrons (smallest naturally-occurring charged particles) coming out of the Cobalt due to its **radioactive decay** . She found that the electrons did not come out the way she expected. Thus this experiment was not parity invariant. Since it tested the weak force, this meant the weak force was not parity invariant either. This result was so important that Yang and Lee won the 1957 Nobel Prize in physics for it. Now we know when we see parity invariance the weak force is the culprit. All other fundamental forces are parity invariant.

Parity is often studied along with charge conjugation. Charge conjugation changes a particle into its opposite, or **antiparticle** , by changing the sign of its **electric charge** . Even though parity is not conserved by itself, it was thought that the combination of parity (P) and charge conjugation was conserved. In 1964, however, physicists J. H. Christenson, J. W. Cronin, V. L. Fitch, and R. Turlay discovered that CP **conservation** is not obeyed by studying the decays of particles called Kaons. Scientists know that the laws of nature must obey conservation of the combination of parity, charge conjugation, and **time** symmetry (T), because of the way they are formulated. This is called CPT symmetry. Because CP symmetry is not conserved, it follows that time symmetry must not hold, so that total combination, CPT, can be conserved.

Lesley Smith

# parity

par·i·ty^{1 } / ˈparitē/ •
n. 1. the state or condition of being equal, esp. regarding status or pay: *parity of incomes between rural workers and those in industrial occupations.* ∎ the value of one currency in terms of another at an established exchange rate. ∎ a system of providing farmers with consistent purchasing power by regulating prices of farm products, usually with government price supports.2. Math. (of a number) the fact of being even or odd. ∎ Physics the property of a spatial wave equation that either remains the same (even parity) or changes sign (odd parity) under a given transformation. ∎ Physics the value of a quantum number corresponding to this property. ∎ Comput. a function whose being even (or odd) provides a check on a set of binary values.par·i·ty^{2 } •
n. Med. the fact or condition of having borne children. ∎ the number of children previously borne: *very high parity (six children or more).*

# parity

**parity** (symbol P) In physics, term used to denote space-reflection symmetry. The principle of conservation of parity states that physical laws are the same in a left- and right-handed co-ordinate system. This was regarded as inviolable until 1956, when US physicists Chen Ning Yang and Tsung-Dao Lee showed that it was transgressed by certain interactions between elementary particles. Parity is also used in information theory to denote a coding method employed in message transmission to detect errors.

# parity

**parity** A function that is computed to provide a check on a group of binary values (e.g. a word, byte, or character) by forming the modulo-2 sum of the bits in the group. The generated sum, a redundant value, is called the *parity bit*. The parity bit is 0 if the number of 1s in the original group was even. The parity bit is 1 if the number of 1s in the original group was odd.

The parity computation just defined will cause the augmented group of binary values (the original group plus the parity bit) to have an even number of 1s; this is called *even parity*. In some cases, hardware considerations make it desirable to have an odd number of 1s in the augmented group, and the parity bit is selected to cause the total number of 1s to be odd; this is called *odd parity*. See also parity check.

# Parity

# PARITY

*Equality in amount or value. Equivalence of prices of farm products to the prices existing at some former date (the base period) or to the general cost of living; equivalence of prices of goods or services in two different markets. The relationship between two currencies such that they are exchangeable for each other at the par or official rate of exchange.*

# parity

**parity ( pa-riti) n.** a term used to indicate the number of pregnancies a woman has had that have each resulted in the birth of an infant capable of survival. See also grand multiparity.

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