Tampan, Paminggir people. Lampung region of Sumatra, 19th century, 87 x 70 cm. From the library of Darwin Sjamsudin, Jakarta. Photograph by D Dunlop. |

Consider a particle P described using a frame of reference F. We presume that a few quarks are always being exchanged between P and F. And since these interactions are occurring around P we focus our attention on some subset of the quarks in F called; the surroundings of P, the neighborhood of P, the medium around P, the environment of P, the vicinity of P, and so on. We use the symbol $\mathbb{S}$ to represent these **surrounding** quarks, and we assume that $\mathbb{S}$ is a small subset of the quarks in F so that P's environment may be quite different from the larger frame of reference. Then to be more precise, the hypothesis of temporal homogeneity asserts that in addition to any specific explosions or collisions, the momentum of P is incessantly altered by haphazardly emitting and absorbing quarks in exchanges with $\mathbb{S}$. The general term *dispersion* is associated with these interactions. We characterize $\mathbb{S}$ using the wavevector $\ \overline{\kappa} \:$. Definition: if

$\overline{\kappa} ^ { \mathbb{S} } = (0, 0, 0)$

then we say that $\mathbb{S}$ is a **non-dispersive** medium.