The Lorenz Attractor in 3D Images by Paul Bourke April 1997 PovRay code by Marcus Fritzsch May 2002 This image appeared in the Nature journal 31 August 2000, pp 949 as part of an article titled The Lorenz Attractor Exists, written by Ian Stewart. It was created as part of an OpenGL interactive viewer and rendered on a farm of Dec Alphas using PovRay. Appeared in the book "What Shape is a Snowflake" by Ian Stewart, page 177. Appeared in Wiedzaizycie #7, July 2003, page 49.

The so called "lorenz attractor" was first studied by Ed N. Lorenz, a meterologist, around 1963. It was derived from a simplified model of convention in the earths atmosphere. It also arises naturally in models of lasers and dynamos. The system is most commonly expressed as 3 coupled non-linear differential equations.

One commonly used set of constants is a = 10, b = 28, c = 8 / 3. Another is a = 28, b = 46.92, c = 4. "a" is sometimes known as the Prandtl number and "b" the Rayleigh number.

The series does not form limit cyles nor does it ever reach a steady state. Instead it is an example of deterministic chaos. As with other chaotic systems the Lorenz system is sensitive to the initial conditions, two initial states no matter how close will diverge, usually sooner rather than later.

While the equations look simple enough they lead to wonderous trajectories, some examples of which are illustrated below.

PovRay 3.5 macro by Marcus Fritzsch

// N = number iterations
// h, a, b, c: initial parameters
// x0, y0, z0: start-location
// rad = radius of the spheres that trace the attractor

#macro lorenz(h, a, b, c, x0, y0, z0, N, rad)
// use it like: 
// lorenz(0.001099, 10, 28, 8/3, 0.0001, 0.0001, 0.0001, 350000, 0.04)
   #local i = 0;
   union {
   #while (i < N)
       #local x0 = x0 + h * a * (y0 - x0);
       #local y0 = y0 + h * (x0 * (b - z0) - y0);
       #local z0 = z0 + h * (x0 * y0 - c * z0);
       #if (i > 100)
           sphere { 
              <x0,y0,z0>, rad 
              pigment { color rgb <i/N,i/N,1> } 
           }
       #end
       #local i = i + 1;
   #end
   }
#end

A physical model simulating the Lorenz equations has been attributed to Willem Malkus and Lou Howard around 1970. It consists of leaking cups on the rim of a larger wheel as shown in the diagram on the right. Liquid flows from the pipe at the top, each cup leaks from the bottom. Under different input flow rates you should be able to convince yourself that under just the right flow rate the wheel will spin one way and then the other chaotically. Of course you can always build it.....

C source

#include "stdio.h"
#include "stdlib.h"
#include "math.h"

#define N 10000

int main(int argc,char **argv)
{
   int i=0;
   double x0,y0,z0,x1,y1,z1;
   double h = 0.01;
   double a = 10.0;
   double b = 28.0;
   double c = 8.0 / 3.0;

   x0 = 0.1;
   y0 = 0;
   z0 = 0;
   for (i=0;i<N;i++) {
      x1 = x0 + h * a * (y0 - x0);
      y1 = y0 + h * (x0 * (b - z0) - y0);
      z1 = z0 + h * (x0 * y0 - c * z0);
      x0 = x1;
      y0 = y1;
      z0 = z1;
      if (i > 100)
         printf("%d %g %g %g\n",i,x0,y0,z0);
   }
}

PovRay rendering by Marcus Fritzsch