Recording · click a frame to replay from it
Inside the deck
The atoms are soft disks that bounce off each other and the walls, moved by Newton’s laws. Positions and velocities are stored as whole numbers (in units of 10⁻⁷ of the box), with the Levesque–Verlet scheme, so every step can be undone exactly: reversing time retraces the motion bit for bit, with no rounding error at all.
Entropy counts the ways the atoms could be arranged that look the same to us. The deck measures it from the counting cells (how many atoms are in each) and from the spread of speeds in each part of the box, with the Miller–Madow correction for a finite gas. The dashed line on the chart is the most it can reach at this energy.
Reverse time flips every velocity at once. With the nudge on, one atom is first moved by 3 × 10⁻⁷ of the box, about a millionth of its own width. Collisions magnify that difference until the reversal fails.
The film strip saves the whole state every couple of time units. Clicking a frame replays from that saved state: that is a recording, not physics running backwards.
Space pause · R reverse time · W wall · 1–7 reels.
Drawn for clarity: the speed of play. The gas is two-dimensional and has a few hundred atoms, not 10²³; the odds of a return by chance grow astronomically worse with more atoms.
Further reading: Feynman Lectures, Volume I, chapter 46 · Carroll, From Eternity to Here
Theoria · Entropy 0.3 · The time deck