Exhibition · Series XI · Computation Itself
Reversal Needs the Missing Bit
What information must a process retain in order to be exactly reversible?
What to notice
The forward rule alone loses information. Retaining one additional layer of prior state changes the update law: the process can now be stepped backward exactly.
Everyday analogy. Overwrite a document without keeping its previous version and exact undo may be impossible. Keep the missing prior state, and reversal becomes possible because the information was never discarded.
Pre-rendered from an explicit second-order reversible cellular-automaton construction. The additional retained state is part of the rule, not hidden metadata.