The Undo
Button

Suppose I place a block at time T₀, regret it at T₁, and press Undo.

At the simplest level, the program only needs to remember the previous state of that coordinate. Air became stone; therefore replace stone with air.

But suppose the block interrupted flowing water.

Or another player saw me place it and consequently walked in another direction.

Or the block completed a redstone circuit, opening a door through which a creature passed.

What now constitutes the action that must be undone?

Restoring the block alone does not restore the previous world. The consequences of the block remain.

A perfect undo would therefore require increasingly large amounts of information about the world surrounding the original action. Every affected state would need its previous state preserved.

One might imagine enormous time cylinders attached to everything: histories containing enough information to restore each object, entity, interaction, and relationship to exactly what it had been.

The metaphor becomes absurd very quickly.

And that absurdity is useful.

In physical reality, an action is not an isolated event.

Move a cup and muscles contract. Forces act upon the cup. Air moves around it. Light reflects differently from its new position. Heat is exchanged. Sound waves propagate. Someone may see the movement. Their nervous system changes because they have seen it.

They might then say something.

I hear them.

I respond.

The original movement of the cup has become entangled with a continually expanding history of subsequent events.

To undo the movement perfectly, where should the reversal stop?

The problem is not simply that a perfect undo would require an extraordinary amount of information.

The deeper problem is that there is no obvious boundary around the event being undone.

The event has already become part of everything that followed from it.

This gives doing an interesting philosophical character.

An action may begin as possibility:

Before the action, alternatives remain conceivable.

Then comes Do.

At that point something becomes physically instantiated. Air has moved. A body has changed position. Words have been heard. The world from which the next moment emerges is no longer precisely the world that preceded the action.

In this sense, doing could be described as non-negotiated applied physics.

The interpretation can still change afterward. I can regret what I did, reinterpret it, apologize for it, compensate for it, celebrate it, forget it, or learn from it.

But none of those operations make the event unhappen.

They are new events.

The Archive was sent three photographs with the paper. No explanation accompanied them. They are reproduced here because each one records an action that has been taken, and none of the three can be withdrawn.

The Minecraft parable becomes stranger when we imagine that perfect reversal actually is possible.

Suppose the universe progresses:

and at T₂ someone presses the cosmic Undo button.

Everything is perfectly restored to T₀. Every particle returns to its former condition. Every interaction is reversed. Every body is restored. Every brain returns to precisely the state it occupied at T₀.

Then who remembers T₁ and T₂?

Nobody.

A memory of the reversal would itself be physical information produced during the interval that has supposedly been erased.

If I remember that the universe was reset, then something in me was not reset.

The reversal was therefore incomplete.

For a truly perfect reversal we seemingly require:

But from within the restored universe, the resulting experience would simply be:

No evidence of the discarded sequence remains.

This creates an extraordinary epistemological problem.

A perfectly erased event cannot leave evidence of having occurred, because the evidence would constitute something that had not been erased.

There appears to be one escape.

Something could exist outside the system being restored.

Imagine Minecraft again.

The game world can be returned to an earlier save while the player sitting at the computer remembers everything that happened after that save.

The player possesses information that the restored Minecraft world does not.

But this works precisely because the player is not contained within the rollback.

Applied to reality, this immediately becomes metaphysical.

For a witness to remember a complete restoration of the physical universe, that witness would somehow have to retain information independently of the physical states being restored.

Various philosophical and religious traditions have proposed things that resemble such a witness: consciousness, soul, God, an absolute observer, or realities beyond ordinary spacetime.

But the Undo problem does not prove any of them.

It merely exposes the requirement.

If information survives a complete physical reset, something carrying that information must not have been included in what we mean by "complete physical reset."

And then we must ask what that something is.

The question simply moves one level outward.

My friend's complaint about Minecraft therefore contains a surprisingly deep insight.

Perhaps the absence of Undo is not merely an inconvenience.

It may be part of what makes a world capable of becoming a history.

Actions matter because subsequent states inherit something from previous states. Choices acquire consequences. Events can become conditions for other events. The world accumulates structure.

A universe with a freely available perfect Undo would be profoundly different.

And yet even that statement contains a paradox.

If perfect Undo existed and included us, we could have used it innumerable times without ever knowing.

Every successful use would erase the observation that it had been used.

The absence of evidence could tell us nothing either way.

This leaves me with a peculiar form of agnosticism.

I cannot conclude that reality possesses an Undo function.

Nor can the thought experiment establish that time has secretly reversed, that consciousness exists outside physics, or that discarded timelines are real. Those would be additional metaphysical claims.

But something remarkable remains.

We can describe the conditions under which an event would necessarily remove its own evidence. We can recognize that a perfect reset, by definition, would reset the observer capable of detecting it.

Our ignorance therefore has structure.

And perhaps that is the more interesting discovery.

The fact that we can conceive of an event whose perfection would make its occurrence unknowable does not prove that the event occurs. It reveals instead that reality may contain questions for which observation from inside the system can never provide an answer.

There is something almost playful about that boundary.

A friend asks:

Years later the question has become:

Perhaps it wouldn't.

And if we were part of that universe—

neither would we.