Chaitin's Omega and an Algorithmic Phase Transition

TL;DR

This paper explores the connection between Chaitin's Omega, phase transitions in a statistical mechanical model of Turing machine computations, and their thermodynamic properties, revealing a critical point with unique convergence and divergence behaviors.

Contribution

It introduces a novel ensemble model of Turing machine computations exhibiting a phase transition at Omega, with detailed analysis of its thermodynamic and computational properties.

Findings

Identifies a first-order phase transition at the halting probability Omega.

Shows the free energy converges slowly to its critical value near the transition.

Proposes a computable approximation of the partition function with a super-logarithmic singularity.

Abstract

We consider the statistical mechanical ensemble of bit string histories that are computed by a universal Turing machine. The role of the energy is played by the program size. We show that this ensemble has a first-order phase transition at a critical temperature, at which the partition function equals Chaitin's halting probability $\Omega$. This phase transition has curious properties: the free energy is continuous near the critical temperature, but almost jumps: it converges more slowly to its finite critical value than any computable function. At the critical temperature, the average size of the bit strings diverges. We define a non-universal Turing machine that approximates this behavior of the partition function in a computable way by a super-logarithmic singularity, and discuss its thermodynamic properties. We also discuss analogies and differences between Chaitin's Omega and the partition functions of a quantum mechanical particle, a spin model, random surfaces, and quantum Turing machines. For universal Turing machines, we conjecture that the ensemble of bit string histories at the critical temperature has a continuum formulation in terms of a string theory.