Billiard-Ball Computer: Reversible Logic via Elastic Collisions

Fredkin and Toffoli's 1982 "billiard-ball model of computation" showed that ordinary conservative mechanics — two hard spheres colliding elastically — can compute logic without ever erasing a bit, and therefore without paying the k_BT·ln 2 minimum energy that Landauer's principle demands of any irreversible operation. This simulator renders that primitive in real 3D: two equal-mass balls launch from opposite corners along crossing diagonal paths; whether they collide exactly at the crossing point is itself the logical AND of the two inputs, computed via the real equal-mass elastic-collision law rather than a lookup table. A live kinetic-energy readout stays flat through every collision, making the "no dissipation" claim something you can watch rather than take on faith — the same conservative-logic ideal that motivates ballistic and adiabatic nanocomputing device research today.