The hapless Humpty Dumpty often crops up as a metaphor for the second law of thermodynamics. Having fallen off his wall, Humpty really can’t be put back together again. Even if one could trace every broken bit of him back through time, it would be impossible to propel those fragments with precisely the right momentum to restore Humpty to his prelapsarian state.
But that’s not the case for whatever sound Humpty emits as he topples. To reverse sound, one doesn’t need to know the motion of every atom—just the phase, amplitude, and wavelength of the wave motion that entrains them. With the right equipment and a lossless medium, sound can be recorded, reversed, and rebroadcast to converge on the spot where it originated.
Called time-reversed acoustics, this remarkable technique has been around for about 10 years (see Mathias Fink’s article in Physics Today, Physics Today 0031-9228 50 3 1997...
