A Smiley Became Static in 500 Steps. I Ran Time Backwards.

About this piece

A tiny rule on 2 by 2 squares turns the smiley into static in 500 steps. Run the rule backwards and every one of 14,400 cells comes home.

The rule: in every 2 by 2 square, 2 lit cells flip all four, and 3 lit cells (1, on every other step) spin it half a turn. The squares shift by one cell every step. In 500 steps the smiley is static, yet 2,838 cells stay lit at every step.

Run the rule backwards and the face returns, cell for cell. Change one cell in the static first, and you get a different past with no face. Undo that cell and the smiley is back. 5,000 out and 5,000 back works too.