Since the purpose of heavy ion collisions is to study the properties of Quantum Chromodynamics at extreme temperature and energy density, any successful phenomenology must ultimately be based on QCD. However, as discussed in Chapter 2, heavy ion phenomenology requires strong coupling techniques not only for bulk thermodynamic quantities like the QCD equation of state, but also for many dynamical quantities at nonzero temperature, such as transport coefficients, relaxation times and quantities accessed by probes propagating through a plasma. By now, lattice-regularized QCD calculations provide well controlled results for the former, but progress on all the latter quantities is likely to be slow since one needs to overcome both conceptual limitations and limitations in computing power. Alternative strong coupling tools are therefore desirable. The gauge/string duality provides one such tool for performing nonperturbative calculations for a wide class of non-Abelian plasmas.

In this book we have mostly focused on results obtained within one particular example of a gauge/string duality, namely the case in which the gauge theory is N = 4 SYM or a small deformation thereof. One reason for this is pedagogical: N = 4 SYM is arguably the simplest and best understood case of a gauge/string duality. By now many examples are known of more sophisticated string duals of non-supersymmetric, nonconformal QCD-like theories that exhibit confinement, spontaneous chiral symmetry breaking, thermal phase transitions, etc.