21 Chapter 2: Motivation 2 ↔ 3 processes; utilizing this promising new code they have found, using perturbative ideas alone, η/s ∼ . 1 [140] and also hope to include both bulk and jet dynamics within a single theoretical framework [141]. There are plenty of difficulties, though, as including gluon splitting and merging makes numerical acausal artifacts more pronounced (their work has yet to be confirmed by other groups) and the gluon radiation from jets does not model coherence effects well. As noted previously it is hard to overemphasize the importance of initial conditions (IC) when considering hydrodynamics output. On the crucial issue of viscosity the two are necessarily coupled: consistency with data with more diffuse IC requires a more ideal fluid; sharper edges a more viscous one. Hopefully by independently experimentally testing the IC—see [115] for a proposed discerning observable—and by theoretically motivating the fluid viscosity we can arrive at a self-consistent picture of the bulk dynamics in heavy ion collisions. The exciting first evidence of jets came from Isr in 1972 [142–146]. Orders of magnitude more high- p$_{T}$ pions were observed than were expected from a low- p$_{T}$ extrapolation; the production spectrum had turned over from exponen- tial to power law. Soon afterward, two jet events were explicitly seen at e $^{+}$e − colliders [41, 42]. While Bjorken was the first to suggest using jet suppression to learn about a QCD medium [147], the precision pQCD predictions of pro- 2.2 High- p$_{T}$ Observables