parity

parity¹

/par"i tee/, n.

1. equality, as in amount, status, or character.

2. equivalence; correspondence; similarity; analogy.

3. Finance.

a. equivalence in value in the currency of another country.

b. equivalence in value at a fixed ratio between moneys of different metals.

4. Physics.

a. a property of a wave function, expressed as +1 or -1 and noting the relation of the given function to the function formed when each variable is replaced by its negative, +1 indicating that the functions are identical and -1 that the second function is the negative of the first.

b. Also called intrinsic parity. a number +1 or -1 assigned to each kind of elementary particle in such a way that the product of the parities of the particles in a system of particles multiplied by the parity of the wave function describing the system is unchanged when particles are created or annihilated.

5. a system of regulating prices of farm commodities, usually by government price supports, to provide farmers with the same purchasing power they had in a selected base period.

6. Computers. the condition of the number of items in a set, particularly the number of bits per byte or word, being either even or odd: used as a means for detecting certain errors.

[1565-75; < LL paritas. See PAR¹, -ITY]

parity²

/par"i tee/, n. Obstet.

1. the condition or fact of having borne offspring.

2. para⁵ (def. 1).

[1875-80; < L par(ere) to bring forth (cf. PARENT) + -ITY]

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I

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.

II

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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▪ economics

      in economics, equality in price, rate of exchange, purchasing power, or wages.

      In international exchange, parity refers to the exchange rate between the currencies of two countries making the purchasing power of both currencies substantially equal. Theoretically, exchange rates of currencies can be set at a parity or par level and adjusted to maintain parity as economic conditions change. The adjustments can be made in the marketplace, by price changes, as conditions of supply and demand change. These kinds of adjustment occur naturally if the exchange rates are allowed to fluctuate freely or within wide ranges. If, however, the exchange rates are stabilized or set arbitrarily (as by the Bretton Woods Conference of 1944) or are set within a narrow range, the par rates can be maintained by intervention of national governments or international agencies (e.g., the International Monetary Fund).

      In U.S. agricultural economics, the term parity was applied to a system of regulating the prices of farm commodities, usually by government price supports and production quotas, in order to provide farmers with the same purchasing power that they had in a selected base period. For example, if the average price received per bushel of wheat during the base period was 98 cents, and, if the prices paid by farmers for other goods quadrupled, then the parity price for wheat would be $3.92 per bushel.

▪ particle physics

      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 (conservation law) 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 (Wigner, Eugene Paul) and became an intrinsic part of quantum mechanics.

      In attempting to understand some puzzles in the decay of subatomic particles (subatomic particle) called K-mesons (meson), the Chinese-born physicists Tsung-Dao Lee (Lee, Tsung-Dao) and Chen Ning Yang (Yang, Chen Ning) proposed in 1956 that parity is not always conserved. For subatomic particles three fundamental interactions (fundamental interaction) are important: the electromagnetic (electromagnetism), strong (strong force), and weak (weak force) forces. Lee and Yang showed that there was no evidence that parity conservation applies to the weak force. The fundamental laws governing the weak force should not be indifferent to mirror reflection, and, therefore, particle interactions that occur by means of the weak force should show some measure of built-in right- or left-handedness that might be experimentally detectable. In 1957 a team led by the Chinese-born physicist Chien-Shiung Wu (Wu, Chien-Shiung) announced conclusive experimental proof that the electrons (electron) ejected along with antineutrinos (neutrino) 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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