For a molecule, decoherence will occur because of entanglement with other molecules, thermal photons, background radiation, or, if the molecule is vibrationally excited, blackbody radiation it emits. Decoherence successfully explains why superpositions are not easily observed in large systems. It is formally expressed by the tendency of the nondiagonal terms of the subsystem's reduced density matrix to become zero with time. For a two-state subsystem, decoherence looks like the process illustrated on the left side of Figure . (Decoherence can also be formulated without referring to reduced density matrices in the frame of environment-assisted invariance (envariance). )