We believe that the computer simulation of physics should retain the elegance of physics itself. This book is about one approach towards this objective: lattice-gas cellular automata models of fluids.

Cellular automata are fully discrete models of physical and other systems. Lattice gases are just what the name says: a model of a gas on a grid. Thus lattice-gas cellular automata are a special kind of gas in which identical particles hop from site to site on a lattice at each tick of a clock. When particles meet they collide, but they always stay on the grid and appropriate physical quantities are always conserved.

Our subject is interesting because it provides a new way of thinking about the simulation of fluids. It also provides an instructive link between the microscopic world of molecular dynamics and the macroscopic world of fluid mechanics. Lastly, it allows us to create new computational tools that can be usefully applied to solve certain problems.

There are, broadly speaking, two ways to use computers to make progress in physics. The first approach is to use computers to compute a number, say the result of a certain integral or the hydrodynamic drag past a certain body. The second is to use computers as a kind of experimental laboratory, to explore the phenomena of interest much as an experimentalist would do him or herself. In the former case, realism is essential—if you do not solve the right equations, then you will not compute the right drag.