Lifeboat Foundation

Ever have a day when everything went wrong? You predicted you would have a normal day, but your alarm clock didn’t ring. Already running late, you couldn’t find your briefcase or backpack. Staggering out the door, your car won’t start. Later, you find out you missed a surprise meeting or maybe a quiz. It’s not you, it’s the whole prediction game…

#1 – Observer effect; #2 – Heisenberg Uncertainty Principle; #3 – Quantum tunneling; #4 – Butterfly effect (last time); #5 – External perturbations (this time); #6 – Why care? Existentialism (next time); #7 -Why care? Time value of money

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Spoil Sport of Prediction #5: External perturbations

To figure out what happens next, you need to know where things are now. But you also need to know what outside influences will impinge on the system between “now,” and “next” whenever that is. Those influences can affect the evolution of the system – that’s why they’re called “influences.”

Imagine for example the Lorenz water wheel (or watch one: http://www.youtube.com/watch?v=zhOBibeW5J0). Given a predictable, steady stream of input water, it still spins forward and backward seemingly unpredictably due to the butterfly effect. But it gets worse if it is spinning in the rain. Now, the input is no longer steady and predictable. Every raindrop is another unpredictable butterfly whose tiny effects change the direction of the wheel at some future time. More generally, every external nudge to a system is like that butterfly.

Let’s identify some external influences likely to affect the future course of some systems of interest. If the system is a nuclear family whose dynamics we understand (let us assume), any attempt to simulate it will soon founder on the realities of day-to-day events that impinge on it. If a troublesome child commits nuisance vandalism, say, whether he is caught or not, and by whom (parents? victim? police?) will likely have great impact on the evolution of the family. Moving up in scale, if the system is a country, the results of the next election depend on much more than the political dynamics. They also hinge on unpredicted international events – even if the reaction of the country to each event could be predicted, whether and which events occur is also critical to know, yet unpredictable from the dynamics of the system itself. What about human affairs generally? There, unpredictability from external perturbations results from everything from butterflies flapping their wings, to volcanoes going off, to solar storms, and on and on.

Other systems have the same problem besides Lorenz water wheels and human dynamics, because every system – save one – is subject to external perturbations. The one exception is the entire universe in every detail, and you can’t simulate that. Even if you could, it generates its own perturbations that even in principle are unpredictable due to quantum tunneling, like when and in which direction a pencil perfectly balanced on its point will fall. Since those perturbations are impossibe to predict from within the system even in theory, they are effectively external to it.

But suppose you could control external perturbations, or at least control them enough? You’ve not won the prediction game yet…because until predicting is shown to be worth the effort, why do it? More on this next time…