Topological quantum number

In physics, a topological quantum number (also called topological charge) is any quantity, in a physical theory, that takes on only one of a discrete set of values, due to topological considerations. Most commonly, topological quantum numbers are topological invariants associated with topological defects or soliton-type solutions of some set of differential equations modeling a physical system, as the solitons themselves owe their stability to topological considerations. The specific "topological considerations" are usually due to the appearance of the fundamental group or a higher-dimensional homotopy group in the description of the problem, quite often because the boundary, on which the boundary conditions are specified, has a non-trivial homotopy group that is preserved by the differential equations. The topological quantum number of a solution is sometimes called the winding number of the solution, or, more precisely, it is the degree of a continuous mapping. Recent ideas about the nature of phase transitions indicates that topological quantum numbers, and their associated solutions, can be created or destroyed during a phase transition. In particle physics, an example is given by the Skyrmion, for which the baryon number is a topological quantum number. The origin comes from the fact that the isospin is modelled by SU(2), which is isomorphic to the 3-sphere and inherits the group structure of SU(2) through its bijective association, so the isomorphism is in the category of topological groups. By taking real three-dimensional space, and closing it with a point at infinity, one also gets a 3-sphere. Solutions to Skyrme's equations in real three-dimensional space map a point in "real" (physical; Euclidean) space to a point on the 3-manifold SU(2). Topologically distinct solutions "wrap" the one sphere around the other, such that one solution, no matter how it is deformed, cannot be "unwrapped" without creating a discontinuity in the solution. In physics, such discontinuities are associated with infinite energy, and are thus not allowed.