The CNRS2 and Le Nouvel Economiste have awarded the prize for the best joint work between a laboratory and a company for the second year in a row. The Grand Prix went to the "Waves and Acoustics" Laboratory -- a CNRS, ESPCI3 and Paris VII University research unit -- headed by Mathias Fink, and Snecma. The time-reversing mirrors engineered by this team sample and record an incidence acoustic field and then emit it backwards. The potential applications of the technology are countless. A highly efficient device that does not destroy the titanium components of plane engines has already been tested as part of a major joint effort with Snecma. A lithotriptor that can track and destroy a moving kidney stone has also been successfully engineered. Other applications are now being developed, especially in acoustics and outside-body surgery.
"No one has ever seen a block of matter explode and the fragments spontaneously gather to form the original block, or see a drop of ink re-form after dilution in a glass of water," says Mathias Fink. Many physicists have been obsessed with reversing time, i.e., challenging the irreversibility of macroscopic physical phenomena. Although the equations of standard mechanics and quantum mechanics can be reversed on a microscopic scale, macroscopic phenomena have always proved to be irreversible because of the huge amount of particles involved.
However, since relatively little data is required for the full description of a wave field in wave physics, it becomes feasible to conduct time-reversing experiments with sound waves. The equations describing wave propagation are "invariant by reversing the direction of time." In other words this means that the answers to the equations are always the same whether time is moving from the past to the future or vice versa. The only prerequisite is to disregard the dissipation of energy into heat; this is usually the case with sonic or ultrasonic frequencies, i.e., ranging from a few hertz to some dozen megahertz.
Mathias Fink, who began to address the subject in 1987, came up with the idea of "reversing in time" the ultrasounds emitted by an object. He recorded the emitted wave and analyzed it to produce a reversed wave which in turn propagated backwards and re-focused on the initial emitter. He used reversed "acoustic retinas", so to speak, to do this. The "retinas" are in fact a network of piezoelectric transducers that can act either as a microphone or as a loud-speaker. In other words, the transducers can act as sensors by generating an electric current when they pick up a sound wave but they can also act as emitters by generating a sound wave when they are charged with electric current.
That is how the first time-reversing mirror was engineered. It is a device consisting of a system of piezoelectric transducers. Each transducer is connected to an electronic chain including an amplifier and an analog-to-digital converter. The sound signals picked up by the transducer travel through the chain and are stored in the memories. The signals are then reversed by reading the memories backwards and a "time-reversed" wave is reconstituted.
Unlike optical mirrors where imaginary projections of reflected light rays converge on the vanishing point, the beams do converge on a point in time-reversing mirrors.
The time-reversing mirror is an amazing device that has opened the door to vast prospects, including applications already in the works. "The most advanced application to date is the non-destructive testing of materials," adds Mathias Fink, and indeed, a time-reversing mirror has been tested as part of a major joint effort with Snecma. The goal of the testing was to detect defects in titanium alloys. The alloys have an extremely heterogeneous microstructure that generates a loud background echo making it impossible to see defects with a weak contrast. However, these tiny defects can cause jet engines to explode, explaining why the French company was so keen on assessing the detection capabilities of a time-reversing mirror that could pinpoint defects half the size of those detected by a General Electric engineered technology. Indeed the American company's technique was considered as the most efficient until now.
The most promising applications are undoubtedly in the medical field. Actually, the first time-reversing mirrors designed by Mathias Fink's team were for a new type of lithotriptor, a device using ultra-sound to destroy kidney stones. Ultrasounds or X-rays are commonly used to pinpoint kidney stones. 70% of the several thousand ulstrasounds emitted to destroy a kidney stone miss the target because when the body moves, so does the stone. However the time-reversing mirror has solved this problem by using a group of the mirror's components to light the designated area. The kidney stone then reflects the signals and the mirror picks up, automatically amplifies and sends the signals back to the source, i.e., the stone.
A 64 track time-reversing mirror has already been engineered with support from ANVAR. The mirror works in real-time, uses a network of transducers with a 20 cm. diameter, and sends back 1,000 signals per second, resulting in promising clinical results. "The machine we have designed has worked remarkably well in hospital and showed that the ultrasonic beam keeps tracking the moving stone," explains Mathias Fink.
Other applications, now being researched at the "Wave and Acoustics" Laboratory, will eventually be engineered. For instance, focusing ultrasound waves through the skull for surgical purposes is being researched with the neurosurgery ward at the Val de Grâce Hospital in Paris. Project goal is to use ultrasound hyperthermia to home in on and destroy a small tumor pinpointed by MRI (Magnetic Resonance Imaging).
The technique is being considered for mine-detection and more generally for underwater acoustics. Other fields for exploration are underwater telecommunications where the time-reversing technique that can compensate for any defects in the propagation channel might play a major role in the future. In the long run, time-reversal could be used to project stereophonic sound or create a sound traveling to just one person. Last, the technique is used in hydrodynamics for the accurate measuring of turbulent flows, and is expected to help us to acquire a better understanding of how turbulence forms.
Indeed, as Mathias Fink says, "Time-reversal has a wealth of applications." A modest man, he quickly adds, "It's the simplest idea on earth: reverse time and send it back."
ESPCI - Laboratoire Ondes et Acoustique
Mathias Fink
Phone: 40.79.44.71., Fax: 40.79.44.25.
Email: mathias.fink@espci.fr
Web: http://www.loa.espci.fr/loa/english/loa.htm
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