Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!sdd.hp.com!wuarchive!udel!haven!umd5!newton.cs.jhu.edu!callahan From: callahan@cs.jhu.edu (Paul Callahan) Newsgroups: comp.theory.cell-automata Subject: Re: Questions and comments about Conway's Life (long) Message-ID: Date: 17 Jan 91 17:59:24 GMT References: <133@gem.stack.urc.tue.nl> Reply-To: callahan@newton.cs.jhu.edu.UUCP (Paul Callahan) Organization: JHU Computer Science Deparment, Baltimore MD Lines: 64 In article <133@gem.stack.urc.tue.nl> angelo@gem.stack.urc.tue.nl (Angelo Wentzler) writes: >All very fine and very interesting, mr.Callahan, but could you please explain >to me what the following cell-groups are: >Spaceship,spark,glider,rake,block. I guess my first response has to be something like: if you don't know, then you probably won't be able to help me. I know that sounds horrible; it's just that I was writing a long article with limited time, and I didn't feel like spending too much timing giving my own definitions of "well known" patterns. All the patterns I mentioned are defined in either _Winning Ways for All Your Mathematical Plays_ by Berlekamp, Conway, and Guy or _The Recursive Universe_ by William Poundstone. I've seen some variations on names in the files provided with Xlife. To be brief: gliders and blocks are as you supposed (I don't know why you say that my notion of gliders is a larger pattern. Perhaps you could be more specific.) For the other constructions: Spaceships (called "fish" in Xlife files) come in three sizes: small, medium, and large. They are as follows ("sparks" shown capitalized): X..x. ....x (small) x...x .xxxx ..X... X...x. .....x (medium) x....x .xxxxx ..XX... X....x. ......x (large) x.....x .xxxxxx Spaceships move along x or y axes. Sparks have the property that they are produced by a spaceship, but can be removed without destroying it. They are useful for creating effects in which a spaceship passes by a pattern and affects it while escaping unharmed. A rake is an object that moves in the same way as a spaceship, but leaves a trail of gliders in its wake. These trails can be collided to produce stable objects, and other interesting things (such as a variety of "puffer train" effects). My goal was to determine if a set of rakes producing gliders at very wide intervals could be used to directly construct a Turing Machine within the Life universe. I know it _can_ be done, but I was interesting in knowing if an explicit construction had been given. My idea was to duplicate the finite head logic in every tape cell, though I now feel that that is ambitious even for the smallest known Universal Turing Machine (its state set and alphabet both number in the single digits--I don't have the exact numbers at my finger tips). I have another idea for an even simpler universal model of computation, which I will continue in the following posting. -- Paul Callahan callahan@cs.jhu.edu