A beautiful model of reversible computing is the billiard ball model, summarized here: http://en.wikipedia.org/wiki/Billiard-ball_computer http://en.wikipedia.

A beautiful model of reversible computing is the billiard ball model, summarized here: http://en.wikipedia.org/wiki/Billiard-ball_computer http://en.wikipedia.org/wiki/Billiard-ball_computer The punchline is that (idealized, friction free) billiard balls + billiard ball "mirrors" (for the balls to bounce off) are enough to do universal computation! To reverse the computation, just reverse all the billiard balls.