# 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:

Any time-independent structures (e.g. critical points or attractors) which the dynamics give rise to must therefore either be self-symmetrical or have symmetrical images under the involution π.

## Physics

In physics, the laws of motion of classical mechanics exhibit time reversibility, as long as the operator π reverses the conjugate momenta of all the particles of the system, i.e. (T-symmetry).

In quantum mechanical systems, however, the weak nuclear force is not invariant under T-symmetry alone; if weak interactions are present reversible dynamics are still possible, but only if the operator π also reverses the signs of all the charges and the parity of the spatial co-ordinates (C-symmetry and P-symmetry). This reversibility of several linked properties is known as CPT symmetry.

Thermodynamic processes can be reversible or irreversible, depending on the change in entropy during the process.

## Stochastic processes

A stochastic process is time-reversible if the joint probabilities of the forward and reverse state sequences are the same for all sets of time increments { *τ*_{s} }, for *s* = 1, ..., *k* for any *k*:^{[1]}

A univariate stationary Gaussian process is time-reversible. Markov processes can only be reversible if their stationary distributions have the property of detailed balance:

Kolmogorov's criterion defines the condition for a Markov chain or continuous-time Markov chain to be time-reversible.

Time reversal of numerous classes of stochastic processes has been studied, including Lévy processes,^{[2]} stochastic networks (Kelly's lemma),^{[3]} birth and death processes,^{[4]} Markov chains,^{[5]} and piecewise deterministic Markov processes.^{[6]}

## Waves and optics

Time reversal method works based on the linear reciprocity of the wave equation, which states that the time reversed solution of a wave equation is also a solution to the wave equation since standard wave equations only contain even derivatives of the unknown variables.^{[7]} Thus, the wave equation is symmetrical under time reversal, so the time reversal of any valid solution is also a solution. This means that a wave's path through space is valid when travelled in either direction.

Time reversal signal processing is a process in which this property is used to reverse a received signal; this signal is then re-emitted and a temporal compression occurs, resulting in a reverse of the initial excitation waveform being played at the initial source.

## See also

## Notes

- ↑ Tong (1990), Section 4.4
- ↑ Jacod, J.; Protter, P. (1988). "Time Reversal on Levy Processes".
*The Annals of Probability*.**16**(2): 620. doi:10.1214/aop/1176991776. JSTOR 2243828. - ↑ Kelly, F. P. (1976). "Networks of Queues".
*Advances in Applied Probability*.**8**(2): 416–432. doi:10.2307/1425912. JSTOR 1425912. - ↑ Tanaka, H. (1989). "Time Reversal of Random Walks in One-Dimension".
*Tokyo Journal of Mathematics*.**12**: 159. doi:10.3836/tjm/1270133555. - ↑ Norris, J. R. (1998).
*Markov Chains*. Cambridge University Press. ISBN 0521633966. - ↑ Löpker, A.; Palmowski, Z. (2013). "On time reversal of piecewise deterministic Markov processes".
*Electronic Journal of Probability*.**18**. arXiv:1110.3813. doi:10.1214/EJP.v18-1958. - ↑ Parvasi, Seyed Mohammad; Ho, Siu Chun Michael; Kong, Qingzhao; Mousavi, Reza; Song, Gangbing (19 July 2016). "Real time bolt preload monitoring using piezoceramic transducers and time reversal technique—a numerical study with experimental verification".
*Smart Materials and Structures*.**25**(8): 085015. doi:10.1088/0964-1726/25/8/085015. ISSN 0964-1726.

## References

- Isham, V. (1991) "Modelling stochastic phenomena". In:
*Stochastic Theory and Modelling*, Hinkley, DV., Reid, N., Snell, E.J. (Eds). Chapman and Hall. ISBN 978-0-412-30590-0. - Tong, H. (1990)
*Non-linear Time Series: A Dynamical System Approach*. Oxford UP. ISBN 0-19-852300-9

- Isham, V. (1991) "Modelling stochastic phenomena". In: