Unconventional computing is computing by any of a wide range of new or unusual methods. It is also known as alternative computing. The term unconventional computation was coined by Cristian S. Calude and John Casti and used at the First International Conference on Unconventional Models of Computation in 1998. Background The general theory of computation allows for a variety of models. Computing technology first developed using mechanical systems and then evolved into the use of electronic devices. Other fields of modern physics provide additional avenues for development. Computational model Computational models use computer programs to simulate and study complex systems using an algorithmic or mechanistic approach. They are commonly used to study complex nonlinear systems for which simple analytical solutions are not readily available. Experimentation with the model is done by adjusting parameters in the computer and studying the differences in the outcome. Operation theories of the model can be derived/deduced from these computational experiments. Examples of computational models include weather forecasting models, earth simulator models, flight simulator models, molecular protein folding models, and neural network models. Mechanical computing Historically, mechanical computers were used in industry before the advent of the transistor. Mechanical computers retain some interest today both in research and as analogue computers. Some mechanical computers have a theoretical or didactic relevance, such as billiard-ball computers, while hydraulic ones like the MONIAC or the Water integrator were used effectively. While some are actually simulated, others are not. No attempt is made to build a functioning computer through the mechanical collisions of billiard balls. The domino computer is another theoretically interesting mechanical computing scheme. Analog computing An analog computer is a type of computer that uses analog signals, which are continuous physical quantities, to model and solve problems. These signals can be electrical, mechanical, or hydraulic in nature. Analog computers were widely used in scientific and industrial applications, and were often faster than digital computers at the time. However, they started to become obsolete in the 1950s and 1960s and are now mostly used in specific applications such as aircraft flight simulators and teaching control systems in universities. Examples of analog computing devices include slide rules, nomograms, and complex mechanisms for process control and protective relays. The Antikythera mechanism, a mechanical device that calculates the positions of planets and the Moon, and the planimeter, a mechanical integrator for calculating the area of an arbitrary 2D shape, are also examples