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**Time reversal** may refer to:

- In physics, T-symmetry - the study of thermodynamics and the symmetry of certain physical laws where the concept of time is reversed — i.e. where a mirror (reverse) transformation is applied to a forward direction time state.
- Time reversal signal processing, a signal processing technique popular in acoustics, and currently being applied to electromagnetics, that results from the reversal in time of a system's response signal.
- Time travel, theorised and speculative concepts about travelling physically through time.

**T-symmetry** or **time reversal symmetry** is the theoretical symmetry of physical laws under the transformation of time reversal:

**Time Reversal Signal Processing** is a technique for focusing waves. A Time Reversal Mirror (TRM) is a device that can focus waves using the time reversal method. TRMs are also known as time reversal mirror arrays, as they are usually arrays of transducers, but they do not have to be arrays. TRM are known and used for decades in the optical domain, and are also used in the ultrasonic domain.

**Time travel** is the concept of movement between certain points in time, analogous to movement between different points in space by an object or a person, typically using a hypothetical device known as a **time machine**. Time travel is a widely-recognized concept in philosophy and fiction. The idea of a time machine was popularized by H. G. Wells' 1895 novel *The Time Machine*.

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A **mirror image** is a reflected duplication of an object that appears almost identical, but is reversed in the direction perpendicular to the mirror surface. As an optical effect it results from reflection off of substances such as a mirror or water. It is also a concept in geometry and can be used as a conceptualization process for 3-D structures.

**Charge, parity, and time reversal symmetry** is a fundamental symmetry of physical laws under the simultaneous transformations of charge conjugation (C), parity transformation (P), and time reversal (T). CPT is the only combination of C, P, and T that is observed to be an exact symmetry of nature at the fundamental level. The **CPT theorem** says that CPT symmetry holds for all physical phenomena, or more precisely, that any Lorentz invariant local quantum field theory with a Hermitian Hamiltonian must have CPT symmetry.

In general relativity, a **white hole** is a hypothetical region of spacetime which cannot be entered from the outside, although matter and light can escape from it. In this sense, it is the reverse of a black hole, which can only be entered from the outside and from which matter and light cannot escape. White holes appear in the theory of eternal black holes. In addition to a black hole region in the future, such a solution of the Einstein field equations has a white hole region in its past. However, this region does not exist for black holes that have formed through gravitational collapse, nor are there any known physical processes through which a white hole could be formed. Although information and evidence regarding white holes remains inconclusive, the 2006 GRB 060614 has been proposed as the first documented occurrence of a white hole.

In mathematics, **deconvolution** is an algorithm-based process used to reverse the effects of convolution on recorded data. The concept of deconvolution is widely used in the techniques of signal processing and image processing. Because these techniques are in turn widely used in many scientific and engineering disciplines, deconvolution finds many applications.

**Thrust reversal**, also called **reverse thrust**, is the temporary diversion of an aircraft engine's thrust so that it is directed forward, rather than backward. Reverse thrust acts against the forward travel of the aircraft, providing deceleration. Thrust reverser systems are featured on many jet aircraft to help slow down just after touch-down, reducing wear on the brakes and enabling shorter landing distances. Such devices affect the aircraft significantly and are considered important for safe operations by airlines. There have been accidents involving thrust reversal systems, including fatal ones.

**Loschmidt's paradox**, also known as the **reversibility paradox**, **irreversibility paradox** or *Umkehreinwand*, is the objection that it should not be possible to deduce an irreversible process from time-symmetric dynamics. This puts the time reversal symmetry of (almost) all known low-level fundamental physical processes at odds with any attempt to infer from them the second law of thermodynamics which describes the behaviour of macroscopic systems. Both of these are well-accepted principles in physics, with sound observational and theoretical support, yet they seem to be in conflict; hence the paradox.

**Incremental dating** techniques allow the construction of year-by-year annual chronologies, which can be temporally fixed or floating.

A mathematical or physical process is **time-reversible** if the dynamics of the process remain well-defined when the sequence of time-states is reversed.

**Electrodialysis reversal** (**EDR**) is an electrodialysis reversal water desalination membrane process that has been commercially used since the early 1960s. An electric current migrates dissolved salt ions, including fluorides, nitrates and sulfates, through an electrodialysis stack consisting of alternating layers of cationic and anionic ion exchange membranes. Periodically, the direction of ion flow is reversed by reversing the polarity of the applied electric current.

**Reversal symmetry** is a voting system criterion which requires that if candidate A is the unique winner, and each voter's individual preferences are inverted, then A must not be elected. Methods that satisfy reversal symmetry include Borda count, the Kemeny-Young method, and the Schulze method. Methods that fail include Bucklin voting, instant-runoff voting and Condorcet methods that fail the Condorcet loser criterion such as Minimax.

In physics, a **symmetry** of a physical system is a physical or mathematical feature of the system that is preserved or remains unchanged under some transformation.

In organic chemistry, a **ring flip** is the interconversion of cyclic conformers that have equivalent ring shapes that results in the exchange of nonequivalent substituent positions. The overall process generally takes place over several steps, involving coupled rotations about several of the molecule's single bonds, in conjunction with minor deformations of bond angles. Most commonly, the term is used to refer to the interconversion of the two chair conformers of cyclohexane derivatives, which is specifically referred to as a **chair flip**, although other cycloalkanes and inorganic rings undergo similar processes.

In quantum mechanics, the **Kramers degeneracy theorem** states that for every energy eigenstate of a time-reversal symmetric system with half-integer total spin, there is at least one more eigenstate with the same energy. In other words, every energy level is at least doubly degenerate if it has half-integer spin. The law is named for the Dutch physicist H. A. Kramers.

**MUSIC** is an algorithm used for frequency estimation and radio direction finding.

**Periodic current reversal** (PCR) is a current modulation technique that can be used in various electrochemical processes.
In the most basic sense PCR simply means that the electric current applied to the cell is reversed periodically - but unlike pure AC the average (nett) current is still biased in one direction.

In particle physics, **CP violation** is a violation of **CP-symmetry** : the combination of C-symmetry and P-symmetry. CP-symmetry states that the laws of physics should be the same if a particle is interchanged with its antiparticle while its spatial coordinates are inverted. The discovery of CP violation in 1964 in the decays of neutral kaons resulted in the Nobel Prize in Physics in 1980 for its discoverers James Cronin and Val Fitch.

In applied mathematics, a **bit-reversal permutation** is a permutation of a sequence of *n* items, where *n* = 2^{k} is a power of two. It is defined by indexing the elements of the sequence by the numbers from 0 to *n* − 1 and then reversing the binary representations of each of these numbers. Each item is then mapped to the new position given by this reversed value. The bit reversal permutation is an involution, so repeating the same permutation twice returns to the original ordering on the items.

**Phase conjugation** is a physical transformation of a wave field where the resulting field has a reversed propagation direction but keeps its amplitudes and phases.