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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
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It is the reversed time version of the family considered in the previous example.
The reversed time version of the previous example, where F is formed by the Hurwitz matrices
If the head of the arrow of time can stand for either the past or the future, depending on the conceptual scheme, how can past and future be defined, and how can one distinguish normal time from reversed time? The answer that immediately comes to mind is chronology: the year 0 comes first, then the year 1000, then the year 2000.
If reversed time systematically undid the events that happened in the pre-Hobart phase, there would be no need for the Library to kill Peake in order to prevent his religious movement from splitting the United States into three nations.
For instance, in the real world, the behavior of the winners of World War I toward Germany has been seen as one of the multiple causes of World War II; but in reversed time, this explanation no longer holds, and the succession of the two wars becomes purely accidental, Once deprived of causality, history becomes like the sequence of events reported in the Annals of St Gall--a mere list that would make just as much sense if the events were dated in the reverse order:
While time travel narratives manipulate the fast forward or the fast backward button of the film of fictional history, they differ from reversed time narrative through the fact that they do not play the film backwards.
While reversed time stories and backward causality project the determinism of the past into the future, putting the world on a single predetermined track, La Moustache projects the indeterminism of the future into the past, placing the hero in a radically unstable world where knowledge becomes impossible, since knowledge is an awareness of facts that are either timeless, or forever written on the timeline of history.
The question of why authors resort to reversed time narration--only one of the paradoxes discussed in this article--has recently attracted much attention among narratologists (Richardson, Chatman 2009).