Reversing time with a jolt

Mirroring is not confined to spatial reflection. In the 1990s physicists began developing time-reversal mirrors, which send acoustic waves, water waves, or other waves back on the original trajectory from which they came (see the article by Mathias Fink, Physics Today, March 1997, page 34 ). The mirrors generally come in the form of antennas that essentially record the incoming waves and rebroadcast a time-reversed signal. Now Emmanuel Fort at ESPCI ParisTech and colleagues have created an antenna-free apparatus that generates time-reversed water waves with only a jolt.

In the group’s series of experiments, a plastic tip pierced the surface of water in a tank and produced an outward-moving circular wavepacket. After 60 ms, the tank was yanked downward by about 4 mm in 2 ms and with an acceleration about 20 times that of Earth’s gravity. Although the outgoing wave was unaffected, another wavepacket suddenly burst from the existing wave and collapsed inward, eventually reproducing the surface’s initial deformation from the plastic tip. The team observed the same time-reversal phenomenon in water disturbed by blowing air through holes in Plexiglas plates, which were shaped like a smiley face and the Eiffel Tower (see image above).

The researchers determined that wave propagation is so disrupted by the comparatively speedy jolt that the wave gets decoupled from its velocity vector. The jolt is analogous to hitting the pause button—and once the action resumes, wave propagation plays out both forward and backward in time. Because the wave equation is universal, a similar approach should achieve success with other kinds of waves, the researchers say. Next the team plans to use the technique to replicate the function of phase-conjugated mirrors, which flip the temporal switch on waves of a single frequency. Eventually, time-reversal schemes may be able to focus sound waves underwater or target cancerous tumors with light. (V. Bacot et al., Nat. Phys., 2016, doi: 10.1038/nphys3810 .)