Time Reversal

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time reversal

[′tīm ri‚vər·səl]
The replacement of the time coordinate t by its negative -t in the equations of motion of a dynamical system; the time reversal operator, a symmetry operator for a quantum-mechanical system, contains also the complex conjugation operator and a matrix operating on the spin coordinate.

Time Reversal


the mathematical operation of changing the sign of time in equations of motion that describe the development of a physical system in time. Such a change corresponds to a certain symmetry that exists in nature. Specifically, all fundamental interactions of elementary particles, with one exception (see below), have the property of T-invariance: time reversal, or the substitution t— t, does not change the form of the equations of motion. This means that for any possible motion of a system in nature there can occur time-reversed motion, wherein the system successively passes, in reverse order, through states symmetric to the states it traverses in “forward” motion. Such time-symmetric states have opposite directions for the velocities and projections of spins of all particles and the magnetic field. The consequences of T-invariance include certain relations between the probabilities of direct and inverse reactions, the prohibition of certain states of polarization of particles in reactions, and a zero electric dipole moment of elementary particles.

It follows from the general principles of modern quantum field theory that all processes in nature are symmetric with respect to the product of three operations: time reversal T, space inversion P, and charge conjugation C. The only experimentally observed processes in which violation of the combined inversion CP is observed are rare decays of the long-lived kL0-meson: the rare decay kL0 → 2 π and the lepton decays kL0 → π+ + e) + v̄e(v̄μ) and kL0 → π- + e++) + ve(vμ), in which weak (∼ 10-3) charge asymmetry has been detected. Theoretical analysis of experimental data on these decays leads to the conclusion that CPT-invariance is satisfied in the decays but T-invariance is violated. The nature of the forces that violate T-invariance has not been ascertained. These forces may be connected with the superweak interaction, which is a billion times weaker than the normal weak interaction.

Although elementary microprocesses, with the exception pointed out above, are reversible in time, the second law of thermodynamics dictates that macroscopic processes involving a very large number of particles proceed only in one direction— toward a state of thermodynamic equilibrium. Statistical physics explains this paradox by the set of microscopic states that corresponds to the state of macroscopic equilibrium being immeasurably greater than the set that corresponds to nonequilibrium states. Therefore, any disturbance, no matter how small, changes a system’s motion away from a state of equilibrium into motion toward equilibrium.


References in periodicals archive ?
Abdolali, 'Moving target tracking using time reversal method ',journal of Progress In Electromagnetic Research M, Vol.
Robustness of a time reversal system: From ultra-wide to narrow bandwidths", in Proc.
Leone, "Interferometric time reversal MUSIC for small scatterer localization," Progress In Electromagnetics Research, Vol.
Treeby, "Artifact trapping during time reversal photoacoustic imaging for acoustically heterogeneous media," IEEE Trans.
Time Reversal Mirror (TRM) technique is applied as an inversion method to reconstruct the image.
Bira van Kolck, of the University of Arizona, and his collaborators described that a property of the deuteron known as a magnetic quadrupole moment could reveal sources of a phenomenon known as time reversal violation.
Among the topics are the validity of random matrix theories for many-particle systems, the angular-momentum dependence of the density of states, group theory and the propagation of operator averages, electromagnetic sum rules by spectral distribution methods, compound-nuclear tests of time reversal invariance in the nucleon-nucleon interaction, strength functions and spreading widths of simple shell model configurations, and underlying symmetries of realistic interactions and the nuclear many-body problem.
During the late 19th century, scientists figured out that symmetries limit the number of ways atoms can be arranged into crystals, calculating 230 possible patterns* In the mid-20th century another kind of symmetry, time reversal, extended the number of possible arrangements in magnetic materials to 1,421.
Using a technique called Time Reversal Acoustics (TRA), the scientists hope to produce an amplified beam of sound loud enough to overwhelm an intruder but focused enough to spare the surrounding wildlife.
USE A WEEKLY MOISTURISING MASK For a weekly treat try The Sanctuary Time Reversal Mask, from pounds 1.
A white hole is connected to a black hole by a wormhole and hypothetically is the time reversal of a black hole.