Explore. Restore
A small part of the carpenter’s world runs here through a single day, a village by the lake and five of the people who live in it. At dawn the keeper’s machine saves a checkpoint. Partway through the day a fault escapes correction and destroys the workshop, and you decide, as the keeper, how to restore it.
A machine can save the complete state of a world at a given moment, a checkpoint from which the world can later be restored.
The day
The fault
At half past twelve a fault escapes correction and destroys the workshop. The machine catches it at once and pauses the world, so no one inside has seen anything happen.
A world paused for a year of substrate time would notice nothing, and the same holds for a world run at a hundredth of its former speed.
Chapter 2, Many Machines, Two Clocks, ¶ 69
How will you restore it?
Restore the checkpoint
Restoring a checkpoint in the simple way, though, rewinds everything to the moment it was taken and erases whatever happened in between, including everything that was lived.
The erased run was no recording. It was a fully causal process with every counterfactual in place, so it bore every mark of reality, and its subjects lived through it before it was undone.
Restore the workshop into the present
A region destroyed by a fault that escaped correction could be restored from a saved state and merged back into the present, and a mind that would otherwise end could be preserved, a possibility with large consequences for the death of minds.
A restoration of this kind is a rollback at the scale of a world. It falls under the same rule as any other rollback, and a keeper who uses checkpoints must learn to restore what was lost into the present instead of returning the present to the past.
Leave it
Inside a world, a fault has no natural cause, because nothing in the world’s laws produced it.
Reliability belongs among the structural goods a world owes those who live in it, the conditions that make every other good possible.
What happens in the village
The rule
Error correction has a companion principle in graceful degradation, under which a failure that escapes correction stays local, corrupting one small region instead of propagating across the world.
A keeper who uses checkpoints restores what was lost into the present and never returns the present to the past.
How the village works
The village, its five people, and their day are this page’s invention, set in the carpenter’s world of Chapter 1 beside the lake of Chapter 2. The day runs from six in the morning to eight in the evening, at about two and a half seconds an hour. Restoring the checkpoint returns everything to dawn, and the day that follows differs from the first, as it would in a world whose physics holds chance at the fine grain. Restoring the workshop into the present recomputes only the lost region, from the checkpoint and from the record of everything that entered it since, and sets it back as it would have been.