KentuckyFC writes: The idea that every physical event is a computation has spread like wildfire through science. That has triggered an unprecedented interest in unconventional computing such as quantum computing, DNA computing and even the ability of a single-celled organism, called slime mould, to solve mazes. However, that may need to change now that physicists have worked out a formal way of distinguishing between systems that compute and those that don’t. One key is the ability to encode and decode information. “Without the encode and decode steps, there is no computation; there is simply a physical system undergoing evolution,” they say. That means computers must be engineered systems based on well understood laws of physics that can be used to predict the outcome of an abstract evolution. So slime mould fails the test while most forms of quantum computation pass.