Waves That Return to the Past.

It's no use crying over spilt milk, as the saying goes. To err may be human; but in the world of wave mechanics, the clock can be turned back; spilt milk can be returned to the saucer. This phenomenon is called a time-reversal wave, and it can be applied to light waves, ultrasound waves, or any other kind of wave.

Consider a uniform set of plane waves. As the waves travel along, they meet various boundaries, which reflect and refract the waves. If the surfaces of these boundaries are perfectly flat, the waves retain their original shape and maintain their original course, but ...

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