14 Chapter 2: Motivation from these previous experiments in exciting new ways. The most basic bulk observable is the energy deposited by all particles. Sur- prisingly, this elementary quantity provides a qualitative estimate of the energy density created in heavy ion collisions [77]. More differential measurements have the potential to provide much more information on the medium. Sim- ply taking ratios of particle species gives insight into the thermal properties of their creation [88–95]. Single particle spectra and their distribution over the reaction plane, through the use of hydrodynamics, hold the hope of deter- mining the bulk evolution and its equation of state. Two particle correlations, often quoted as Hanbury Brown-Twiss (HBT) radii [96, 97], measure the phys- ical dimensions of the fireball at freezeout and provide a consistency check for hydrodynamics, a test which it has consistently failed (for a review see [98]). A thermalized medium with a small mean free path compared to the system size can be understood using (relativistic) hydrodynamics (see [99–102] for reviews). One then might hope to learn about the physics of a heavy ion collision by measuring bulk observables and comparing them to results from hydrodynamic evolution. Schematically, hydrodynamics takes a given set of initial conditions and evolves them according to the known conservation laws of the system and a set of externally specified equations, for instance the equation 2.1.1 Bulk Evolution 2.1 Bulk Observables