Basic Constituents of Matter
Molecules are built up from the atom , which is the basic unit of any chemical element .
..... Click the link for more information. 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 radioactivity, spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation . The energy produced by radioactivity has important military and industrial applications.
..... Click the link for more information. ). 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 quantum theory, modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
..... Click the link for more information. ). This mathematical operation is called the parity, or space-inversion, operation. See also symmetry symmetry, generally speaking, a balance or correspondence between various parts of an object; the term symmetry is used both in the arts and in the sciences.
..... Click the link for more information. .
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.
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.
parity
See parity checking and RAID.
parity
parity [′par·əd·ē]
Parity (quantum mechanics)
A physical property of a wave function which specifies its behavior under simultaneous reflection of all spatial coordinates through the origin, that is, when x is replaced by -x, y by -y, and z by -z. If the single-particle wave function &psgr; satisfies Eq. (1), it is said to have even parity. If, on the other hand, Eq. (2) holds, the wave function is said to have odd parity. These two expressions can be combined in Eq. (3),
The conservation of parity follows from the inversion symmetry of space, that is, the invariance of the Schrödinger equation H&psgr; = E&psgr; (the wave equation satisfied by the wave function &psgr;) to the inversion of space coordinates, r → - r . The parity (or inversion) operator, which changes r to - r , has the alternative interpretation that the coordinate values remain unchanged but the coordinate axes are inverted; that is, the positive x axis of the new frame points along the old negative x axis, and similarly for y and z. If the original frame was right-handed, then the new frame is left-handed. [A cartesian coordinate system (frame, for short) is called right-handed if it is possible to place the right hand at the origin and point the thumb and first and second fingers along the positive x, y, and z axes, respectively.] Thus, parity would be conserved if the statement of physical laws were independent of the handedness of the coordinate system that was being used. Of course, the fact that most people are right-handed is not a physical law but an accident of evolution; there is nothing in the relevant laws of physics which favors a right-handed over a left-handed human. The same holds for optically active organic compounds, such as the amino acids. However, the statement that the neutrino is left-handed is a physical law. See Neutrino
All the strong interactions between hadrons (for example, nuclear forces) and the electromagnetic interactions are symmetrical to inversion, so that parity is conserved by these interactions. As far as is known, only the weak interactions fail to conserve parity. Thus parity is not conserved in the weak decays of elementary particles (including beta decay of nuclei); in all other processes the weak interactions play a small role, and parity is very nearly conserved. Likewise, in energy eigenstates, weak interactions can be neglected to a very good approximation, and parity is very nearly a good quantum number, so that each atomic, nuclear, or hadronic state is characterized by a definite value of parity, and its conservation in reactions is an important principle. See Fundamental interactions, Weak nuclear interactions
One of the selection rules which follows from parity conservation is the following: A spin zero boson cannot decay sometimes into two &pgr; mesons and sometimes into three &pgr; mesons, because these final states have different parities, even and odd respectively. But the positive K meson is observed to have both these decay modes, originally called the Θ and the &tgr; mesons, respectively, but later shown by the identity of masses and lifetimes to be decay modes of the same particle. This &tgr;-Θ puzzle was the first observation of parity nonconservation. In 1956, T. D. Lee and C. N. Yang made the bold hypothesis that parity also is not conserved in beta decay. They reasoned that the magnitude of the beta-decay coupling is about the same as the coupling which leads to decay of the K meson, and so these decay processes may be manifestations of a single kind of coupling. Also, there is a very natural way to introduce parity nonconservation in beta decay, namely, by assuming a restriction on the possible states of the neutrino (two-component theory). They pointed out that no beta-decay experiment had ever looked for the spin-momentum correlations that would indicate parity nonconservation; they urged that these correlations be sought.
In the first experiment to show parity nonconservation in beta decay, the spins of the beta-active nuclei cobalt-60 were polarized with a magnetic field at low temperature; the decay electrons were observed to be emitted preferentially in directions opposite to the direction of the 60Co spin. The magnitude of this correlation shows that the parity-nonconserving and parity-conserving parts of the beta interaction are of equal size, substantiating the two-component neutrino theory.
It was at first somewhat disconcerting to find parity not conserved, for that seemed to imply a handedness of space. But this is not really the situation; the saving thing is that anti-60Co decays in the opposite direction. Thus, after all, there is nothing intrinsically left-handed about the world, just as there is nothing intrinsically positively charged about nuclei. What really exists here is a correlation between handedness and sign of charge.
| (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. |
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