Engineering relies increasingly on computer modelling of systems and structures. Because of the expense of prototypes for testing, modelling offers clear economic advantages. However, this may not be as reliable as one would wish, because of uncertainty in the physics being modelled. This may be due to the inherent randomness or unpredictability of nature or may be a result of ignorance. Even if physics is deterministic, predictability can be lost – as in Chaos Theory, where the fluttering of a butterfly’s wings can appear to have catastrophic consequences. Uncertainty also commonly arises in living organisms, e.g. the behaviour of human tissue depends on highly uncertain material properties; this will be illustrated by reference to models of heart valves and ‘shaken baby’ syndrome.

Simplified stress analysis of infant brain showing effect of fontanelle.

Computational model of human heart valve system showing internal stresses.