PSPACE-Completeness of Reversible Deterministic Systems

TL;DR

This paper establishes the PSPACE-completeness of various reversible deterministic systems by developing a versatile proof framework and applying it to multiple models, including constraint logic, billiard balls, and motion planning gadgets.

Contribution

It introduces a general framework for proving PSPACE-completeness of reversible deterministic systems and applies it to four different models, fixing previous gaps and simplifying proofs.

Findings

Deterministic Constraint Logic is PSPACE-complete.

Reversible billiard ball model is PSPACE-hard with two balls.

Zero-player motion planning with certain gadgets is PSPACE-complete.

Abstract

We prove PSPACE-completeness of several reversible, fully deterministic systems. At the core, we develop a framework for such proofs (building on a result of Tsukiji and Hagiwara and a framework for motion planning through gadgets), showing that any system that can implement three basic gadgets is PSPACE-complete. We then apply this framework to four different systems, showing its versatility. First, we prove that Deterministic Constraint Logic is PSPACE-complete, fixing an error in a previous argument from 2008. Second, we give a new PSPACE-hardness proof for the reversible `billiard ball' model of Fredkin and Toffoli from 40 years ago, newly establishing hardness when only two balls move at once. Third, we prove PSPACE-completeness of zero-player motion planning with any reversible deterministic interacting $k$-tunnel gadget and a `rotate clockwise' gadget (a zero-player analog of branching hallways). Fourth, we give simpler proofs that zero-player motion planning is PSPACE-complete with just a single gadget, the 3-spinner. These results should in turn make it even easier to prove PSPACE-hardness of other reversible deterministic systems.