Effective mass is a property used in solid-state physics to describe the mass that a particle seems to have when responding to forces or interacting with other identical particles in a thermal distribution). It is a mathematical contraption and has nothing to do with a real mass. The effective mass is usually stated as a factor multiplying the rest mass of an electron, me (9.11 × 10−31 kg), and is usually in the range 0.01 to 10, but can be lower or higher, depending on the material). The value of effective mass depends on the purpose for which it is used and can vary depending on a number of factors). For electrons or electron holes in a solid, the effective mass is used in transport calculations, such as transport of electrons under the influence of fields or carrier gradients, but it also is used to calculate the carrier density of states). The effective mass can be anisotropic, and it may even be negative.