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...
