One of the key concepts in engineering theory is metastable equilibrium. Systems are designed to resist forces, but a large shock can cause catastrophe.
The classic example of this is a marble resting on the dish. The marble can move in any direction but will come back to rest in the middle of the dish – unless it is pushed hard. Then it is given enough energy to seek a new equilibrium position. Maybe the new equilibrium position is inside a larger dish. Maybe it’s on the floor, rolling straight towards a heating vent.
The principle at work here is minimization of potential energy. Every object at every scale seeks to minimize its energy level. It explains the throwing off of photons from excited electrons in a neon light, it explains the shape of water condensate, it governs the flow of hot gas up a chimney, and, unfortunately, it means that our buildings fall down in high winds.
You can never prevent minimization of potential energy because you can’t stop entropy. However, you can slow it down. You can trick systems into finding a local minima, just like the marble was tricked into the middle of the saucer. This is called metastability. The system is not at its preferred state, but a further investment of energy is needed to push it over the edge. Until that energy is provided the system will remain in its metastable state.
This concept is not only useful in structural engineering, it is broadly applicable. For instance, we can use the principles to discuss why sustainability is important. If we look at the ecological system here in the Midwest, we see that everywhere people are constantly altering small aspects of our environment. None of these actions by itself cause much damage. But if we consider the sum total of all of the actions, we realize that a destabilizing force is being applied.
An ecological system is merely metastable. Most people believe that humans can act as responsible stewards of the environment (e.g. recent tuna conservation debate). The current theories of resource management assume that we can study natural systems and determine where the tipping points are. As long as we don’t push nature over the edge then we can optimize our utility of it.
The problem is that balancing nature on the edge means only a small shock will lead to disaster. History is full of civilizations who have learned too late that nature should not be pushed too far. A recent study pointed out that the Nazca civilization may have been decimated by a combination of over-harvesting Huarango trees before a severe El-Nino event. The old forests are now deserts, having suffered a complete ecological collapse in CE500. The people kept pushing that marble towards the edge, never expecting the strong shock that forced it over.
We are now playing the same game on a global scale. We don’t have to think too hard to find the next shock to the system. Climate change is expected to be capped at a 2degC change, but could go higher if politicians don’t find a way forward in Copenhagen (current rate is 6degC – BBC). This rapid climate change could force our ecological systems over the edge and hurtling out of control.
Not only will these changes devastate our natural resources, especially for those areas fenced in by human development, it will cause our carefully cultivated croplands major problems. Imagine trying to curb world hunger and disease when global crop capacity decreases by 30%.
As an engineer, I am familiar with the effects of upsetting metastability. Our industry is always studying disasters and trying to learn from them. Of course, the disasters leave human tragedy in their wake. Society buries its dead. Survivors return to the scene of the tragedy and face a pile of debris that was once the source of their community. Amid all the calls to rebuild, everyone begins to doubt if what was lost could ever be replaced. We must remember that certain things can never be replaced.