Time Reversal Processes

Time Reversal phenomena can occur in different forms. Hereinafter, we show time reversal in biology, quantum physics and optics.

Reversing Cell Differentiation

Normally, the timeline of a cell goes from being undifferentiated and simple, to differentatiated and complex. Until fairly recently, cell differentiation was seen as final and irreversible. Once a cell became specialized, it was referred to as “terminally differentiated.” It was considered locked-in and unable to become any other cell type.

However, in 2006, scientists reported that they had turned a differentiated cell back into a stem cell with the potential to become any type of cell in the body. The difference between a stem cell and a differentiated cell is reflected in the cells’ DNA. In a stem cell, the DNA is arranged loosely, with its genes ready to spring into action. As signals enter the cell and differentiation begins, genes that will not be needed are shut down, and genes that will be required for a specialized function remain open and active.

Induced Pluripotent Stem Cells & DNA’s Bonding with Histones

Scientists noticed a small number of genes that were active only in stem cells and not in differentiated cells. Introducing just 4 of these genes back into differentiated cells made them behave like stem cells. The genes appear to be remodeling the cells’ DNA, unlocking the genes that were shut down during differentiation.  (1) The scientists named these cells “induced pluripotent stem cells,” or iPS cells. In 2012, they won the Nobel Prize in Physiology or Medicine for their work. (2)

In a stem cell, DNA is wrapped loosely around histone proteins . In a differentiated cell, segments of DNA that are not required for the cell’s specialized function are shut down and wrapped tightly around histone proteins. (*)

Reversing biological Time via Parabiosis

Connecting the circulatory system of old and young mice (parabiosis) has been documented to have rejuvenating effects on cells, tissues, organs, and functions.  (Source) A wide range of benefits are envisioned. Blood-based rejuvenation can come to totally change population health and aging. Survival may have to be balanced against reproduction, as reproductive age increases and biological aging slows down.

‘Arrow of time’ reversed in quantum experiment

In a quantum experiment, scientists reversed the arrow of time, the idea that natural processes run in one direction in time.  Like chilly air warming a mug, heat can spontaneously flow from a cold quantum particle to a hotter one under certain conditions. This phenomenon seems to reverse the “arrow of time,” the idea that natural processes run forward but not in reverse.

The existence of an arrow follows from the second law of thermodynamics. The law states that entropy, or disorder, tends to increase over time. That rule explains why it’s easy to shatter a glass but hard to put it back together, and why heat spontaneously flows from hot to cold but not the opposite direction.

The new result, however, “shows that the arrow of time is not an absolute concept, but a relative concept,” says study coauthor Eric Lutz, a theoretical physicist at the University of Erlangen-Nürnberg in Germany. Different systems can have arrows of time that point in different directions, as this quantum experiment showed.

Reversing the arrow of time was possible for the quantum particles because they were correlated, meaning their properties were linked in a way that isn’t possible for larger objects, a relationship akin to quantum entanglement.  This correlation means that the particles share some information.

In thermodynamics, just like in food, information has physical significance. “There’s order in the form of correlations,” says physicist David Jennings of the University of Oxford. This order is like fuel” that can be consumed to drive heat to flow in reverse.

Led by physicist Roberto Serra of the Federal University of ABC in Santo André, Brazil, the experimenters manipulated molecules of chloroform, which are made of carbon, hydrogen and chlorine atoms. The scientists prepared the molecules so that the temperature — judged by the probability of an atom’s nucleus being found in a higher energy state — was greater for the hydrogen nucleus than for the carbon. When the two nuclei’s energy states were uncorrelated, the heat flowed as normal, from hot hydrogen to cold carbon. But when the two nuclei had strong enough quantum correlations, heat flowed backward, making the hot nucleus hotter and the cold nucleus colder.

This phenomenon appears to be a process that compensates for the entropy decrease due to the reverse heat flow.  Scientists hope to use this  thermodynamics of quantum particles to create quantum engines that could perform tasks beyond the reach of typical machines, such as controlling the direction of heat flow on small scales. (3)

Time Reversal in Signaling Processing

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. TRM are known and used for decades in the optical domain, and are also used in the ultrasonic domain.

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 the target.

Another variation is to use a single transducer and an ergodic cavity. Intuitively, an ergodic cavity is one that will allow a wave originating at any point to reach any other point. An example of an ergodic cavity is an irregularly shaped swimming pool: if someone dives in, eventually the entire surface will be rippling with no clear pattern. If the propagation medium is lossless and the boundaries are perfect reflectors, a wave starting at any point will reach all other points an infinite number of times. This property can be exploited by using a single transducer and recording for a long time to get as many reflections as possible.


The time reversal technique is based upon a feature of the wave equation known as reciprocity: given a solution to the wave equation, then the time reversal (using a negative time) of that solution is also a solution. This occurs because the standard wave equation only contains even order derivatives. Some media are not reciprocal, but many  useful ones are approximately so, including sound waves in water or air, ultrasonic waves in human bodies, and electromagnetic waves in free space. The medium must also be approximately linear.

Time reversal techniques can be modeled as a matched filter. If a delta function is the original signal, then the received signal at the TRM is the impulse response of the channel. The TRM sends the reversed version of the impulse response back through the same channel, effectively autocorrelating it. This autocorrelation function has a peak at the origin, where the original source was. It is important to realize that the signal is concentrated in both space and time (in many applications, auto correlation functions are functions of time only).

Another way to think of a time reversal experiment is that the TRM is a “channel sampler”. The TRM measures the channel during the recording phase, and uses that information in the transmission phase to optimally focus the wave back to the source.


Mathias Fink of École Supérieure de Physique et de Chimie Industrielles de la Ville de Paris and his team have done numerous experiments with ultrasonic TRMs. An interesting experiment involved a single source transducer, a 96-element TRM, and 2000 thin steel rods located between the source and the array. (4) The source sent a 1 μs pulse both with and without the steel scatterers. The source point was measured for both time width and spatial width in the retransmission step. The spatial width was about 6 times narrower with the scatterers than without. Moreover, the spatial width was less than the diffraction limit as determined by the size of the TRM with the scatterers. This is possible because the scatterers increased the effective aperture of the array. Even when the scatterers were moved slightly (on the order of a wavelength) in between the receive and transmit steps, the focusing was still quite good, showing that time reversal techniques can be robust in the face of a changing medium.

In addition, José M. F. Moura of Carnegie Mellon University was leading a research team working to extend the principles of Time Reversal to electromagnetic waves, and they have achieved resolution in excess of the Rayleigh resolution limit, proving the efficacy of Time Reversal techniques. (5) Their efforts are focused on radar systems, and trying to improve detection and imaging schemes in highly cluttered environments, where Time Reversal techniques seem to provide the greatest benefit.


The strength of time reversal signal processing is that one need not know any details of the channel. The step of sending a wave through the channel effectively measures it, and the retransmission step uses this data to focus the wave. Thus one doesn’t have to solve the wave equation to optimize the system, one only needs to know that the medium is reciprocal. Time reversal is therefore suited to applications with inhomogeneous media. (6)

An attractive aspect of time reversal signal processing is the fact that it makes use of multipath propagation. Many wireless communication systems must compensate and correct for multipath effects. Time reversal techniques use multipath to their advantage by using the energy from all paths.


Just like the central dogma of molecular biology dealt with the detailed residue-by-residue transfer of sequential information (7), in the same way the central dogma that time is bent and relative (Einstein) or uni-directional and linear may need to eventually be fine-tuned.

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(1). Nature editorial staff (2014). Two retractions highlight long-standing issues of trust and sloppiness that must be addressed.Nature, 511, 5-6. doi: 10.1038/511005b Takahashi, K. & Yamanaka, S. (2006). Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors.Cell, 126(4), 663-676. doi: 10.1016/j.cell.2006.07.024
(2). In 2014, a group of scientists announced that they had turned differentiated cells back into stem cells simply by stressing them. This surprisingly simple process, they claimed, quickly and efficiently generates stem cells without using any kind of genetic manipulation. Unfortunately, the scientists’ claims about STAP cells didn’t hold up under scrutiny, and they ended up retracting their articles. iPS cells and other types of stem cells, however, are real. And researchers are working hard to find even more ways to coax differentiated cells back into a stem-like state.
(3).  K. Micadei et al. Reversing the thermodynamic arrow of time using quantum correlations. arXiv:1711.03323. Posted November 9, 2017.  E. Conover. Scientists peek inside the mind of Maxwell’s demon. Science News. Vol. 192, August 19, 2017, p. 14. E. Conover. Information is physical, even in quantum systems, study suggestsScience News. Vol. 189, May 28, 2016, p. 10. A. Grant. Ultrasmall engines bend second law of thermodynamicsScience News. Vol. 189, March 19, 2016, p. 18. A. Grant. The arrow of time. Science News. Vol. 188, July 25, 2015, p. 15.
(4).  Mathias Fink. Acoustic Time-Reversal Mirrors. Topics Appl. Phys. 84, 17-43. (2002) See also:  Mathias Fink. Time Reversal of Ultrasonic Fields–Part 1: Basic Principles. IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control. 39(5):pp 555–566. September 1992. Mathias Fink. Time-Reversed Acoustics. Scientific American November 1999. pp. 91-97.
(5). José M. F. Moura, Yuanwei Jin. “Detection by Time Reversal: Single Antenna”,IEEE Transactions on Signal Processing, 55:1, pp. 187-201, January 2007
(6).  Parvasi, Seyed Mohammad; Ho, Siu Chun Michael; Kong, Qingzhao; Mousavi, Reza; Song, Gangbing (1 January 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.
(7). This dogma was debunked in 1970 by Francis Criket who won a Nobel with his collogue Watson.  It states that such information cannot be transferred from protein to either protein or nucleic acid.
(*).  In this animation we’ll see the remarkable way our DNA is tightly packed up so that six feet of this long molecule fits into the microscopic nucleus of every cell. The process starts when DNA is wrapped around special protein molecules called histones. The combined loop of DNA and protein is called a nucleosome. Next the nucleosomes are packaged into a thread, which is sometimes described as “beads on a string”. The end result is a fiber known as chromatin. Now the chromatin fiber is coiled into a structure called a “solenoid”. This fiber is then looped and coiled yet again, leading finally to the familiar shapes known as chromosomes, which can be seen in the nucleus of dividing cells. Chromosomes are not always present. They form around the time cells divide when the two copies of the cell’s DNA need to be separated. At other times, as we can see now after the cell has divided, our DNA is less highly organized. It is still wrapped up around the histones, but not coiled into chromosomes.




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