A two-dimensional gas is a collection of objects constrained to move in a planar or other two-dimensional space in a gaseous state. The objects can be: classical ideal gas elements such as rigid disks undergoing elastic collisions; elementary particles, or any ensemble of individual objects in physics which obeys laws of motion without binding interactions. The concept of a two-dimensional gas is used either because:
(a) the issue being studied actually takes place in two dimensions (as certain surface molecular phenomena); or,
(b) the two-dimensional form of the problem is more tractable than the analogous mathematically more complex three-dimensional problem.While physicists have studied simple two body interactions on a plane for centuries, the attention given to the two-dimensional gas (having many bodies in motion) is a 20th-century pursuit. Applications have led to better understanding of superconductivity, gas thermodynamics, certain solid state problems and several questions in quantum mechanics.