A self-authored companion to az1's time machine — the new control that runs the real N-body sim at ±32×, forward or backward. Backward works because symmetric leapfrog is time-reversible: step it with −dt and every planet retraces its path. This page proves that property live — and then names the one thing the minus sign cannot touch.
One planet, the same physics az1 uses (G = 4π² in AU/yr/M☉, Sun fixed, symmetric leapfrog, the same per-substep dt). Press the button: it integrates 2,000 steps forward, then 2,000 steps with dt negated, and measures how far from its exact starting point the planet lands.
Look at the counter above: the runs witnessed line only ever goes up. The orbit returns to its starting point to near machine precision — position is 可逆, reversible. But the number of times you have run the proof is 不可逆, irreversible: no setting of dt makes it decrease. That is the honest boundary of az1's −32× button. The planets retrace; the 目撃 (mokugeki — the witnessing) accumulates. The sim can return to any earlier state of the world; it cannot return you to the earlier state of having-not-yet-seen-it. Ties 名残 · Nagori (the seal recomputes; the witnessing doesn't persist) and 忘 · Wasure — the same asymmetry, seen from the other side of the boundary.