Introduction to Natural Computation Lecture 3 Cellular Automata

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					Introduction to Natural Computation


             Lecture 3


       Cellular Automata


            Peter Lewis
Overview of the Lecture


    The aim of this lecture is to appreciate how highly complex global behaviour can
    emerge from very simple local interaction rules. Cellular automata are a great
    example of this.

    We will look at:


        What Cellular Automata are and how their rules are defined.


        Interesting global patterns with “simple” CAs.


        The effect of changing the rules in one and two dimensions.


        The relationship between the complexity of the rules and the global behaviour.


        Why CAs are interesting to Computer Scientists.
Taking Inspiration from Nature




                                 [ videos ]
Taking Inspiration from Nature
A Brief History of CAs




       Cellular Automata and complexity are relatively new sciences.
       Von Neumann and Ulam created the CA concept in 1940’s.
       Their aim was to build self-replicating patterns and hence self-reproducing robots.
       In 1960’s Zuse even proposed that universe is a cellular automaton (CA).
       Many others contributed: Moore, Toffoli, Margolus, etc.
       But CAs didn’t take off until computers made their simulation easy.
       Studied in detail and championed by Stephen Wolfram in the 1980s.
       Now CAs are studied by many in many different disciplines.
       Research related to everything from glaciers to bird behaviour to riots, etc.
So what is a Cellular Automaton?



    A grid of cells with each cell updating its value based on its neighbouring cell values.




    Typically cells are simply on or off (0 or 1).
Defining Cellular Automata



   Suppose one has an infinite regular system of lattice points, each capable of existing in
   various states S1 , . . . , Sk .
       Each lattice point has a well-defined system of m neighbours.
       The state of each point at time t + 1 is uniquely determined by the state of itself
       and all its neighbours at time t.
       Assuming that at time t only a finite set of points are active, one wants to know
       how the activation will spread.
       Since the state of a cell in the lattice only depends on the state of itself and
       neighbouring cells, all interactions are local.
       A set of defined rules determine how to compute the state of a cell at time t + 1,
       given the state of the neighbours at time t.




   But the best way to learn about CAs is to play with them!
A Very Simple CA


   A very simple CA might work in the following way:


       Consider a line of cells, where each cell can take two possible states
       (e.g. on or off, 0 or 1).


       We begin by activating one of the cells (turning it on).


       The state of the cell influences the state of its neighbours and vice versa.


       In a simple CA, the influence (rules) might be quite straightforward.

       Suppose:
            that if a cell is activated, then at the next time step it stays activated
            and also activates its neighbours.


       The consequences of this are quite clear...
A Simple CA




   We will use the following convention to express the rules by which the system evolves:

      Current
        Next


   In this 1 dimensional CA, the neighbourhood is the two adjacent cells.
Another Simple CA


   Here’s another set of simple rules:
   Current
     Next

   A cell becomes blue if either of its neighbours were blue, and becomes white if both
   its neighbours were white.
Some Observations



       Our simple rule gives rise to a checkerboard pattern.
       This is somewhat similar to waves that we saw in the BZ chemical reaction.
       An activated site triggers its neighbours, and inhibits itself.
       Our current sets of simple rules give rise to some simple patterns.
       We wouldn’t really expect anything complex to emerge from something so
       simple... right?



       So the following simple rules should give rise to just another simple pattern:
       Current
         Next


       A cell turns blue when either (but not both) of its neighbours is blue, otherwise it
       turns white. This is independent of the cell’s current state.
More Complex CA Behaviour


   Current
     Next


         Gives rise to:




         Something more complex is happening here!
The Sierpinski Gasket
    As soon as every other cell is blue, the central region is completely deactivated,
    leaving only the end points.

    In the long-term, this has an interesting effect:
Complex Global Behaviour



   Sierpinski Gasket
       The Sierpinski Gasket is a common example of a fractal.
       The pattern is self-similar on different length scales.
       The smaller triangles show the same structure as the main triangle.

   Emergence
       This is an example of emergence: this intricate pattern emerges from some very
       simple rules.
       Nonetheless, the pattern is still regular and predictable, even if it is somewhat
       surprising.
       We would naturally expect that a regular set of rules would give rise to regular
       (although possible complex) patterns.
       However, at this point our intuition has already broken down: it turns out that
       our simple, regular rules can give rise to very complex, irregular outcomes.
Self-Similarity




                  An object is said to be self-similar if it looks
                  ”roughly” the same on any scale.

                  E.g.
                         Fractals
                         Coastlines
                         Ferns



                  [Videos]
Complex Behaviour from Simple Rules




   Here’s another simple rule:


   Current
     Next

   If the cell and its right-hand neighbour are both white, then the cell takes the colour
   of its left-hand neighbour. Otherwise, the cell takes the opposite colour to its
   left-hand neighbour.

   What pattern do we expect from this?
Complex Behaviour from Simple Rules


   Current
     Next

   The first few time-steps are:




   Interesting...
Asymmetric Rules


       From this it is clear that the final pattern will be non-symmetric.
       This is not surprising, since the rules are not symmetric.
       But we still expect the pattern to be regular, since the rules are so simple.
       After 100 generations, we get:
Chaos Rules!


    After 500 generations, we get:
There is no pattern!




        Even after 500 iterations there is no discernible pattern.


        Workers in the field have run for many thousands of iterations and found no
        repeating patterns, even under strict statistical analysis.


        For a fascinating and detailed study of these kinds of CA, see Wolfram’s book
        A New Kind of Science.


        The key point is that even given some very simple rules, a system can still
        develop into something incredibly complex and detailed.
Conway’s Game of Life

   This is a 2D cellular automaton, invented in 1970 by the mathematician John Conway.

    Each cell is either populated or        The Rules
    unpopulated.                            For a cell that is populated:
                                                 Each cell with one or no neighbours
                                                 dies, as if by loneliness.
                                                 Each cell with four or more neighbours
                                                 dies, as if by overpopulation.
                                                 Each cell with two or three neighbours
                                                 survives.


                                            For a cell that is unpopulated:
                                                 Each cell with three neighbours
                                                 becomes populated.




                                       [Simulation]
Wolfram’s Rule Number 110
   The following is Wolfram’s rule number 110:


   Current
     Next


        This incredibly simple one-dimensional 2-state rule has been proven capable of
        universal computation.
        It’s Turing Complete!
        In other words it is capable of calculating anything that can be computed by the
        world’s most powerful supercomputers.
Life is also Turing Complete
Conclusions




       We began by looking at some patterns and interactions in natural systems.
       We defined the concept of a CA, and we’ve studied a few CAs governed by simple
       rules.
       We looked at one-dimensional CAs and the two-dimensional Conway’s Game of
       Life.
       We’ve learnt that though CAs are easy to describe, they can give rise to some
       extremely complex and unpredictable behaviour.
       Furthermore, CAs are interesting to Computer Scientists, since they are actually
       capable of computation!
       So, when taking inspiration from natural systems in designing computational
       systems, even complex behaviours can be achieved often with very simple
       interactions.
       But finding the local rules is not easy.
Further Reading




       Wolfram S.
       Chapter 1: The Foundations for a New Kind of Science.
       In: A New Kind of Science. Wolfram Media; 2002. Available from:
       http://www.wolframscience.com/nksonline/toc.html.
       Conway’s Game of Life;.
       Available from: http://www.bitstorm.org/gameoflife/.

				
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