Time Reversal
Time reversal may refer to: *Reverse motion – a visual effect in which reversing the order of the frames of a film or video makes time appear to run backward *Reverse tape effects – an audio effect in which reversing the direction of an audio recording renders sounds backward *T-symmetry (or time reversal symmetry) – the expected symmetry of physical laws independent of whether time runs forward or backward *Time reversibility – the ability of some processes to operate in either direction of time *Time reversal signal processing – a technique for focusing acoustic and electromagnetic waves by reversing in time a system's response signals *Time travel 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 with the use of a hypothetical device known as a time machine. Time travel is a ...

Reverse Motion
Reverse motion (also known as reverse motion photography or reverse action) is a special effect in cinematography whereby the action that is filmed is shown backwards (i.e. time-reversed) on screen. It can either be an in-camera effect or an effect produced with the use of an optical printer. There are various reasons why this technique may be adopted, such as for comedic effect (destruction reversal) or for safety reasons (a car stopping just in time may be filmed starting at the stop point). Uses Artistic There are several uses for reverse action. Some are artistic in nature. For example, reverse action can be used for comedic effect. Or it can be used to bring things "back to life" on screen, by filming a process of destruction or decay in reverse. Sometimes it is necessary, for the intended effect to be achieved, for all actions other than the one that it is intended to reverse to be performed, during shooting, backwards—actors have to perform their actions and dialogue backw ...
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Reverse Tape Effects
Reverse tape effects are special effects created by recording sound onto magnetic tape and then physically reversing the tape so that when the tape is played back, the sounds recorded on it are heard in reverse. Backmasking is a type of reverse tape effect. History In 1877, Thomas Edison invented the phonograph, a device allowing sound to be recorded and reproduced on a rotating cylinder with a stylus (or "needle") attached to a diaphragm mounted at the narrow end of a horn. Emile Berliner invented the familiar lateral-cut disc phonograph record in 1888. In addition to recreating recorded sounds by placing the stylus on the cylinder or disc and rotating it in the same direction as during the recording, one could hear different sounds by rotating the cylinder or disc backwards. In 1878, Edison noted that, when played backwards, "the song is still melodious in many cases, and some of the strains are sweet and novel, but altogether different from the song reproduced in the right ...
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T-symmetry
T-symmetry or time reversal symmetry is the theoretical symmetry of physical laws under the transformation of time reversal, : T: t \mapsto -t. Since the second law of thermodynamics states that entropy increases as time flows toward the future, in general, the macroscopic universe does not show symmetry under time reversal. In other words, time is said to be non-symmetric, or asymmetric, except for special equilibrium states when the second law of thermodynamics predicts the time symmetry to hold. However, quantum noninvasive measurements are predicted to violate time symmetry even in equilibrium, contrary to their classical counterparts, although this has not yet been experimentally confirmed. Time ''asymmetries'' generally are caused by one of three categories: # intrinsic to the dynamic physical law (e.g., for the weak force) # due to the initial conditions of the universe (e.g., for the second law of thermodynamics) # due to measurements (e.g., for the noninvasive measu ...
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Time Reversibility
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. A deterministic process is time-reversible if the time-reversed process satisfies the same dynamic equations as the original process; in other words, the equations are invariant or symmetrical under a change in the sign of time. A stochastic process is reversible if the statistical properties of the process are the same as the statistical properties for time-reversed data from the same process. Mathematics In mathematics, a dynamical system is time-reversible if the forward evolution is one-to-one, so that for every state there exists a transformation (an involution) π which gives a one-to-one mapping between the time-reversed evolution of any one state and the forward-time evolution of another corresponding state, given by the operator equation: :U_ = \pi \, U_\, \pi Any time-independent structures (e.g. critical points ...
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Time Reversal Signal Processing
Time Reversal Signal Processing has three main uses: creating an optimal carrier signal for communication, reconstructing a source event, and focusing high-energy waves to a point in space. 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 since they are usually arrays of transducers. TRM are well-known and have been used for decades in the optical domain. They are also used in the ultrasonic domain. Overview If the source is passive, i.e. some type of isolated reflector, an iterative technique can be used to focus energy on it. The TRM transmits a plane wave which travels toward the target and is reflected off it. The reflected wave returns to the TRM, where it looks as if the target has emitted a (weak) signal. The TRM reverses and retransmits the signal as usual, and a more focused wave travels toward the target. As the process is repeated, the waves become more and more focused on ...
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