71 Chapter 2: Motivation Figure 2.15: Visualization of a heavy quark propagating through a thermal plasma. The fundamental quark is represented by a string hanging from a D 7 brane (in this case at r = ∞ ) down through the horizon onto the stack of coincident D 3 branes. Its motion causes a metric disturbance on the Mikowski boundary dual to a wake in the SYM plasma and a loss of momentum down the length of the string. Figure adapted from [340]. a number of theoretical tests, although it remains unproven. But maximally supersymmetric Yang-Mills theory is very far from QCD. As symmetries are broken and the field theory dual to string theory becomes more and more like QCD, the further the conjecture must be extended and the less evidence exists for its holding. And while QCD is far from a quantitative description of heavy ion collisions there is a well known procedure for estimating the errors involved. On the other hand, calculating quantities in a theory that is similar but not the same as QCD is an uncontrolled approximation, and it is far from clear how much those results will be modified by convergence to QCD. We are thus motivated to find qualitative features of heavy ion collisions for