So I'm trying to ease back into the chaos theory posts. I thought that one good way of doing that was to take a look at one of the class chaos examples, which demonstrates just how simple a chaotic system can be. It really doesn't take much at all to push a system from being nice and smoothly predictable to being completely crazy.

This example comes from mathematical biology, and it generates a
graph commonly known as *the logistical map*. The
question behind the graph is, how can I predict what the stable
population of a particular species will be over time?

If there was an unlimited amount of food, and there were no predators, then it would be pretty easy. You'd have a pretty straightforward exponential growth curve. You'd have a constant, R, which is the growth rate. R would be determined by two factors: the rate of reproduction, and the rate of death from old age. With that number, you could put together a simple exponential curve - and presto, you'd have an accurate description of the population over time.

But reality isn't that simple. There's a finite amount of resources - that is, a finite amount of food for for your population to consume. So there's a maximum number of individuals that could possibly survive - if you get more than that, some will die until the population shrinks below that maximum threshold. Plus, there are factors like predators and disease, which reduce the available population of reproducing individuals. The growth rate only considers "How many children will be generated per member of the population?"; predators cull the population, which effectively reduces the growth rate. But it's not a straightforward relationship: the number of individuals that will be consumed by predators and disease is related to the size of the population!

Modeling this reasonably well turns out to be really simple. You
take the maximum population based on resources, P_{max}.
You then describe the population at any given point in time as
a *population ratio*: a *fraction* of
P_{max}. So if your environment could sustain one million
individuals, and the population is really 500,000, then you'd
describe the population ratio as 1/2.

Now, you can describe the population at time T with a recurrence relation:

P(t+1)= R × P(t) × (1-P(t))

That simple equation isn't perfect, but it's results are impressively close to accurate. It's good enough to be very useful for studying population growth.

So, what happens when you look at the behavior of that function
as you vary R? You find that below a certain threshold value, it
falls to zero. Cross that threshold, and you get a nice increasing
curve, which is roughly what you'd expect. Up until you hit R=3.
Then it splits, and you get an oscillation between two different
values. If you keep increasing R, it will split again - your
population will oscillate between 4 different values. A bit
farther, and it will split again, to eight values. And then things
start getting *really* wacky - because the curves
converge on one another, and even start to overlap: you've reached
chaos territory. On a graph of the function, at that point, the
graph becomes a black blur, and things become almost completely
unpredictable. It looks like the beautiful diagram at the top of
this post that I copied from
wikipedia (it's much more detailed then anything I could
create on my own).

But here's where it gets really amazing.

Take a look at that graph. You can see that it looks fractal.
With a graph like that, we can look for something called
a *self-similarity scaling factor*. The idea of a
SS-scaling factor is that we've got a system with strong
self-similarity. If we scale the graph up or down, what's the
scaling factor where a scaled version of the graph will exactly
overlap with the un-scaled graph/

For this population curve, the SSSF turns out to about 4.669.

What's the SSSF for the Mandelbrot set? 4.669.

In fact, the SSSF for nearly *all* bifurcating
systems that we see, and their related fractals, is virtually
always exactly 4.669. There's a basic structure which
underlies *all* systems of this sort.

What's *this sort*? Basically, it's a dynamical
system with a quadratic maximum. In other words, if you look at the
recurrence relation for the dynamical system, it's got a quadratic
factor, and it's got a maximum value. The equation for our
population system can be written: P(t+1) = R×P(t)-P(t)^{2},
which is obviously quadratic, and it will always produce a value
between zero and one, so it's got a fixed maximum. value, and Pick
any chaotic dynamical system with a quadratic maximum, and you'll
find this constant in it. Any dynamical system with those
properties will have a recurrence structure with a scaling factor
of 4.669.

That number, 4.669 is called the *Feigenbaum
constant*, after Mitchell Fiegenbaum, who first discovered it.
Most people *believe* that it's a transcendental
number, but no one is sure! We're not really sure of quite where
the number comes from, which makes it difficult to determine
whether or not it's really transcendental!

But it's damned useful. By knowing that a system is subject to recurrence at a rate determined by Feigenbaum's constant, we know exactly when that system will become chaotic. We don't need to continue to observe it as it scales up to see when the system will go chaotic - we can predict exactly when it will happen just by virtue of the structure of the system. Feigenbaum's constant predictably tell us when a system will become unpredictable.