Physica D 191 (2004) 178–192 Self-replicating loop with universal construction Daniel Mange, André Stauffer ∗ , Enrico Petraglio, Gianluca Tempesti Swiss Federal Institute of Technology, Logic Systems Laboratory, CH-1015 Lausanne, Switzerland Received 1 May 2003; received in revised form 18 November 2003; accepted 18 November 2003 Communicated by C.K.R.T. Jones Abstract After a survey of the theory and some realizations of self-replicating machines, this paper presents a novel self-replicating loop endowed with universal construction properties. Based on the hardware implementation of the so-called Tom Thumb algorithm, the design of this loop leads to a new kind of cellular automaton made of a processing and a control units. The self-replication of the “LSL” acronym serves as an artificial cell division example of the loop and results in a new and straightforward methodology for the self-replication of computing machines of any dimensions. © 2003 Elsevier B.V. All rights reserved. PACS: 84.30.Sk Keywords: Self-replication; Universal construction; Universal computation; Cellular automaton; Artificial cell division 1. Introduction and survey 1.1. Self-replicating loops The main goal of this paper is to present a new self-replicating loop endowed with universal construc- tion properties. While the early history of the theory of self-replicating machines is basically the history of von Neumann’s thinking on the matter [18], a practical implementation requires a sharply different approach. It was finally Langton, in 1984, who opened a second stage in this field of research. In order to construct a self-replicating automaton simpler than this of von Neumann, Langton [6] adopted more liberal criteria. He dropped the condition that the self-replicating unit ∗ Corresponding author. Tel.: +41-21-693-26-52; fax: +41-21-693-37-05. E-mail addresses: daniel.mange@epfl.ch (D. Mange), andre.stauffer@epfl.ch (A. Stauffer). must be capable of universal construction and compu- tation. Langton proposes a configuration in the form of a loop, endowed notably of a constructing arm and of a replication program or genome, which turns coun- terclockwise. After 151 time steps, the original loop (mother loop) produces a daughter loop, thus obtain- ing the self-replication of Langton’s loop. To avoid conflicts with biological definitions, we do not use the term “cell” to indicate the parts of a cellu- lar automaton, opting rather for the term “molecule”. In fact, in biological terms, a cell can be defined as the smallest part of a living being which carries the complete blueprint of the being, that is the being’s genome. According to the biological definitions of a cell, we end up with the following observations: • Langton’s self-replicating loop is a unicellular or- ganism: its genome requires 28 molecules and is 0167-2789/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physd.2003.11.003