Mathias Fink has invented a time machine of sorts. Although Fink’s device--he calls it a time-reversal mirror--won’t hurl you into the past or future, it does play tricks with time, or more precisely, with sound. If you were to stand in front of Fink’s mirror at the Waves and Acoustics Laboratory at the University of Paris and speak into it, you would hear a peculiar echo: anything you said would come back to you exactly reversed. Your hello would echo--almost instantaneously--as olleh. Fink is quick to point out that his invention, quirky as it might seem, is no mere toy. It may in the near future lead to a vastly improved ultrasound imaging device that could diagnose problems as diverse as kidney stones and faulty aircraft parts.
Ultrasound frequencies lie above the audible range, and like X- rays they penetrate solids. Unlike X-rays, though, ultrasound waves reflect off soft-tissue boundaries, ...
