calg November 2, 2018 When—and how—can a cellular automaton be rewritten as a lattice gas? aTommaso Toffoli, bSilvio Capobianco, and cPatrizia Mentrasti aElectrical and Computer Engineering, Boston University bSchool. Comp. Sci., Reykjavik University, Iceland cDip. di Matematica, Universit`adi Roma “La Sapienza” [email protected], [email protected], [email protected] Both cellular automata (CA) and lattice-gas automata (LG) provide finite algorithmic presentations for certain classes of infinite dynamical systems studied by symbolic dynamics; it is customary to use the term ‘cellular automaton’ or ‘lattice gas’ for the dynamic system itself as well as for its presentation. The two kinds of presentation share many traits but also display profound differences on issues ranging from decidability to modeling convenience and physical implementability. Following a conjecture by Toffoli and Margolus, it had been proved by Kari (and by Durand–Lose for more than two dimensions) that any invertible CA can be rewritten as an LG (with a possibly much more complex “unit cell”). But until now it was not known whether this is possible in general for noninvertible CA—which comprise “almost all” CA and represent the bulk of examples in theory and applications. Even circumstantial evidence—whether in favor or against—was lacking. Here, for noninvertible CA, (a) we prove that an LG presentation is out of the question for the vanishingly small class of surjective ones. We then turn our attention to all the rest—noninvertible and nonsurjective— which comprise all the typical ones, including Conway’s ‘Game of Life’. For these (b) we prove by explicit construction that all the one-dimensional ones are representable as LG, and (c) we present and motivate the conjecture that this result extends to any number of dimensions. The tradeoff between dissipation rate and structural complexity implied by the above results have compelling implications for the thermodynamics of computation at a microscopic scale. I do not know of any single instance where something useful argued conclusions. With as much irritation one could re- for the work on lattice gases has been borrowed from the tort, “How can lattice gases differ ‘in essence’ from cellular cellular automata field. Lattice gases differ in essence automata if they are merely a subset of them? What are from cellular automata. A confusion of the two fields dis- torts our thinking, hides the special properties of lattice these CA legacies that may ‘distort our thinking’ and ‘hide gases, and makes it harder to develop a good intuition. the special properties of lattice gases’? And aren’t there dy- (Michel H´enon[17], with specific reference to Wolfram[42]) namical systems that are much more naturally and easily modeled as cellular automata?” Today, with the benefit of twenty years’ hindsight—and especially after the results of the present paper—we are in a 1 Introduction position to defuse the argument. H´enon’s appeal could less belligerantly be reworded as follows: “Even though CA and Cellular automata (CA) provide a quick modeling route to LG describe essentially the same class of objects, for sound phenomenological aspects of nature—especially the emer- pedagogical reasons it may be expedient to deal with them arXiv:0709.1173v1 [nlin.CG] 7 Sep 2007 gence of complex behavior in dissipative systems. But in separate chapters—or even in separate books for different lattice-gas automata (LG) are unmatched as a source of audiences and applications. What is ox in the stable may fine-grained models of fundamental aspects of physics, es- well be beef on the table.” The bottom-line message is pecially for expressing the dynamics of conservative1 sys- that these two modeling approaches do not reflect mutually tems. exclusive strategies, but just opposite tradeoffs between the In the quote at the beginning of this paper, one may well structural complexity of a piece of computing machinery sympathize with H´enon’s annoyance: it turns out that dy- and its thermodynamic efficiency. namical behavior that is synthesized with the utmost nat- uralness when using lattice gases as a “programming lan- guage” become perversely hard to express in the cellular 2 Preview automata language. Yet, H´enon’s are visceral feelings, not Let C andLbe the sets of dynamical systems representable, 1A dynamics is called ‘conservative’ if it is the manifest expression of an invertible microscopic mechanism. It is called ‘dissipative’ if respectively, as cellular automata and lattice gases. Our the underlying mechanism is not invertible to begin with, or if its overall question is, How are these two sets related? On invertibility is de facto irrelevant because one is not capable or willing one hand, we shall see that any lattice gas can be trivially to maintain a strict accounting of microscopic states—perhaps owing rewritten as a cellular automaton—henceL ⊂C. As for the to lack of precise knowledge of the initial state and the evolution laws, impredictable influences on the part of the environment, or the sheer converse issue, i.e., how much of C is inL , let’s recall that size of the task. C is naturally partitioned into three classes which reflect 1 fundamental categorical properties, namely, (One must keep in mind that in a cellular automaton the t state set for qx is restricted to a finite alphabet A, while for descriptive name categorical property a differential equation it ranges over the whole continuum C+ invertible injective and surjective R. On the other hand, the local map f of a cellular automa- C◦ almost invertible noninjective but surjective ton is an unrestricted function of its arguments, while the C− locally lossy noninjective, nonsurjective corresponding map g of the finite-difference scheme can only see its arguments through “small differences”—qx+1 − qx, According to a conjecture made by one of us in 1990 and of the order of h, in space, and qt+1 − qt, of the order of k, proved by Kari in 1996 for the 1D and 2D cases and in time.) Durand–Lose in 2001 for the general case, the vanishingly + Though symbolic dynamics systems are generally infinite, small class C of invertible cellular automata is definitely both cellular automata and lattice-gas automata manage to “in.” For the other two classes, evidence has been lacking 2 provide finite algorithmic presentations for certain classes either way —which is particularly irritating for such decep- of them. These two kinds of presentation share a number tively simple CA as the one consisting merely of a row of of traits but also display profound differences—on issues and 2-input gates. ranging from decidability to physical implementability—to Here we first dismiss, as definitely “out,” the vanishingly the point that one might suspect that CA and LG specify ◦ small class C of almost invertible CA—where, while the quite different kinds of dynamical systems (see the H´enon system as a whole loses in one step a nonzero amount of quote at the beginning of this paper). information, the amount of loss per site is nonetheless zero. On the other hand, for a number of reasons, some empir- − What is left is the class C of locally lossy CA; these ical and some esthetical, we’ve long been entertaining the lose in one step a nonzero amount of information per site. notion—the hope, if you wish, since the evidence was ex- This class comprises almost all cellular automata. Here tremely scant either way—that in fact CA and LG provide we present a procedure for rewriting any one-dimensional just different presentations for the same kind of objects. In − C CA into an LG (which will have a more complex unit other words, letting C denote the set of dynamical systems cell, consisting of two layers and spanning more than one representable as cellular automata andL the set of those site of the original CA). We also show a procedure for a representable as lattice gases, one may ask, How are C and simple 2D example. Work is in progress on an analogous Lrelated? Are they disjoint, partially overlapping, or even construction (with a number of layers that increases with coincident? If the latter were true, how come that LG pre- the number of dimensions) hoped to be of general applica- sentations of even the very simplest CA are so hard to come bility. On the basis of these partial results, we propose the by? conjecture that—exception made for the paradoxical class ◦ C already dismissed above—the family of dynamical sys- t+1 tems presentable as cellular automata and that presentable as lattice gases coincide. t 3 Background Figure 1: A simple one-dimensional cellular automaton; this circuit can be thought of as a presentation of a dynamical system Symbolic dynamics studies a class of dynamical systems on the full shift[25]. For concreteness, assume that the site state which display topological continuity (reflecting locality of alphabet is binary and the local transition function at each site, interaction) and invariance under space and time transla- denoted by a shaded gate, is a logic and; the gates are fed by tion (the regularity of a “spacetime crystal”[39])—and can fanout (or “signal replicator”) nodes, denoted by dots. Is there thus be viewed as discrete, locally-finite versions of the par- an equivalent lattice gas? tial differential equations of field theories. For example, the local map of a simple one-dimensional cellular automaton, Take, for example, the simplest nontrivial CA, namely, which is a a recurrence relation of the form a string of 2-input and gates fed by fanout nodes (Fig. 1). Until now, no one had managed to exhibit a functionally t+1 t t qx = f(qx, qx+1) equivalent LG or, contrariwise, prove that such an LG can- not exist! What’s perhaps even more intriguing, this very is analogous to the well-known forward-time, forward-space question has never (as far as we know) been raised in the finite-difference scheme[32, p.